Task Of Physical
By : Sintya Tifani
Class : 8-A
Number : 18
.~Development of The Atomic Theory~.
"Atomic model" redirects here. For the unrelated term in mathematical logic, see Atomic model
(mathematical logic).
This article focuses on the historical models of the atom. For a history of the study of how
atoms combine to form molecules, see History of the molecule.
In chemistry and physics, atomic theory is a theory of the nature of matter, which states that
matter is composed of discrete units called atoms, as opposed to the obsolete notion that matter
could be divided into any arbitrarily small quantity. It began as a philosophical concept in
ancient Greece and India and entered the scientific mainstream in the early 19th century when
discoveries in the field of chemistry showed that matter did indeed behave as if it were made up
of particles.
The word "atom" (from the Greek atomos, "indivisible") was applied to the basic particle that
constituted a chemical element, because the chemists of the era believed that these were the
fundamental particles of matter. However, around the turn of the 20th century, through various
experiments with electromagnetism and radioactivity, physicists discovered that the so-called
"indivisible atom" was actually a conglomerate of various subatomic particles (chiefly, electrons,
protons and neutrons) which can exist separately from each other. In fact, in certain extreme
environments such as neutron stars, extreme temperature and pressure prevents atoms from
existing at all. Since atoms were found to be actually divisible, physicists later invented the term
"elementary particles" to describe indivisible particles. The field of science which studies
subatomic particles is particle physics, and it is in this field that physicists hope to discover the
true fundamental nature of matter.
T
he concept of atoms, as you will learn, is quite old. The ancient Greeks had an atomic
theory more than 2000 years ago. It is interesting to note that the idea originated from
philosophy, and was based on reason rather than data. It is also interesting to note that,
for many scientists, the final "proof" of the existence of atoms was provided by a, then-unknown
26 year old man named Albert Einstein. The story of the atom makes for interesting reading, and
it involves a huge cast of characters whose lives spanned thousands of years. I have found some
excellent essays on the internet. Follow the links below and enjoy.
I. The Greek Concept of Atomos - by John L. Park
II. Greek Theory and Roman Practice - by James A. Plambeck
III. Middle Ages Through Alchemy - by James A. Plambeck
IV. Two Centuries of Transition - by James A. Plambeck
The Modern Atomic Theory
John Dalton, an English chemist, might be called "the father of the modern atomic theory."
During the early 1800's, Dalton's interests in Meteorology and gases lead him to read the works
of Antoine Lavoisier and Joseph Proust. Lavoisier had stated the law of conservation of mass,
that the mass of materials before a chemical reaction takes place is exactly equal to the mass of
the materials after the reaction is completed. Proust had observed the law of definite
proportions, stating that the proportion by mass of the elements in a given compound is always
the same. Dalton felt that the findings of these men gave strong support to the idea of atoms. He
formulated an atomic theory that would include the observations found by Lavoisier and Proust.
Dalton's Atomic Theory
1) All elements are composed of atoms, which are indivisible and
indestructible particles.
2) All atoms of the same element are exactly alike; in particular, they all
have the same mass.
3) All atoms of different elements are different; in particular, they have
different masses.
4) Compounds are formed by the joining of atoms of two or more
elements. In any compound , the atoms of the different elements in the
compound are joined in a definite whole-number ratio, such as 1 to 1, 2
to 1, 3 to 2, etc.
Much has happened since the time of Dalton, which has made it necessary to update his
atomic theory. We currently believe that all elements are composed of atoms, but we know that
those atoms are not indestructible. Atoms are split in nuclear reactions, and they are made up of
even smaller particles. We also know that atoms of the same element can have different masses,
when they represent different isotopes. Despite these differences, much of Dalton's atomic
theory remains useful to this day.
Greek Theory and Roman Practice
In prehistoric times, earlier than perhaps 1000 BC, no distinction could be, or was, made
between science and technology. Pure science did not exist. Although a great deal of applied
science or technology was known and used by the Assyrian Empire, Pharaonic Egypt, or the
early Greeks no natural philosophy, as distinguished from religious philosophy and from applied
philosophy or technology, existed in the ancient world. In the history of Western civilization and science as it is known today anywhere on earth is clearly of Western origin - the earliest
efforts in natural philosophy or pure science trace back to Greece from about 700 BC onwards,
culminating in the surviving works of Aristotle (384 - 322 BC). This, the first well-defined
conceptual development of science, has had a profound influence on all of the later history and
philosophy of science.
The Greek distinction between natural philosophy and technology is fundamentally different
from our own. This is partially but not completely due to the distinction between scholars, or
teachers, leaders, and philosophers, and craftsmen in the culture of ancient Greece. The
craftsmen, or technologists, would teach their skills to their sons or apprentices but were not
generally able to read and write and so taught by example rather than by book. Moreover, their
skills were generally kept as "trade secrets" within the artisan group or groups, much as the
mediaeval guilds kept their secrets in later centuries. They did no experiments, as such, because
their interest lay not in understanding the nature of clay or gold or lead but in making better,
cheaper, or more beautiful objects out of them. The scholars, who were teachers or philosophers,
were skilled at observation but even more skilled in argument, debate and formal reasoning.
Their skill lay in language and in logical systematic reasoning. The classical logic of hypothesis
and syllogism and the logical beauty of Euclidean plane geometry are examples of this turn of
mind and culture. The idea of resolving a dispute between theories by experiment rather than by
debate would not have occurred to them. Even if it had, the technological ability to do the
experiments was often absent; more important, prehaps, was the conceptual problem, because the
concepts of their science were not clearly enough defined to suggest experiments that they could
have carried out. It would be a mistake, however, to think that the Greeks could not make
accurate measurements- they, or for that matter the priests of Pharaonic Egypt, could and did
make quite accurate measurements of length (as in the construction of the Pyramids, in
establishing boundary markers or surveying of fields, or even in estimating the size of the earth)
or of mass (as in the weighing of silver and gold), or that they were not skilled in observation. It
is the combination of controlled observation for the purpose of advancing understanding which
we call an experiment which is missing from the natural philosophy of the ancient Greeks.
The period of classical history, which to us is this period of Greek theory and Roman practice,
extended roughly from 700 BC to 600 AD. The important Greek thinkers were some of the
philosophers, especially Plato of Athens (427 - 347 BC), who founded the Academy there, and
his student and successor Aristotle of Stagira (384 - 322 BC). Aristotle was a great systematizer
of Greek ideas and his writings had a profound influence on all subsequent Western thought. The
last of the Greeks important to chemistry is Galen (129 - 199 AD), who was the leading
systematizer of medicine in classical times. The works of Galen remained standard medical texts
for centuries.
The nature of the chemical concepts which originated in the Greeks of the classical period
remained, for almost 1500 years, the concepts through which chemistry was understood. We will
take a closer look at two of them: the concept of the four elements, and the atomistic view of
matter, held by two different philosophic schools of Greece.
Concept of the Four Elements
The coherent picture of this concept dates back to Empedocles of Agrigentum (490 - 430 BC).
This picture, somewhat elaborated and elegantly stated by Aristotle, was that all matter was
made up of four elements: earth, air, fire and water. These four elements arise from the working
of the two properties of hotness (and its contrary coldness) and dryness (and its contrary
wetness) upon an original unqualified or primitive matter. The possible combinations of these
two properties of primitive matter give rise to the four elements or elemental forms, as shown in
the Figure below.
Perhaps only a culture whose leaders were involved with logic and geometry would put the
concepts of chemistry in such a logical and geometric form. Fire and Water are obvious
opposites, and so are Earth and Air. These opposites share no common properties. There are four
properties, each shared by two non-opposite elements: fire and air share the property of hotness,
water and air the property of wetness, and so on. Since the four elements are two pairs of
opposite elements, so also are the four properties - hotness being the opposite of coldness and
wetness the opposite of dryness. Each of the four elements was held to exist in an ideal pure
form, which could not actually be found on earth. The real things around us were considered
impure, or mixed, forms of these four ideal elements. Thus the different airs, or gases, were the
form of air mixed with different proportions of the forms of fire or water; smoke was a mixture
of the forms of air and earth with some of the form of fire added. But there was an ideal or pure
form of earth, air, fire, and water, and the real ones which we see and use were not ideal but of
lesser purity. In other words, the real or observed different kinds of the same element are due to
different degrees of the same properties. The elements could be changed into one another by
removal of one property and addition of another. Elements had a natural tendency to separate in
space; fire moved outwards, away from the earth, and earth moved inwards, with air and water
being intermediate. Matter, in whatever form, could be subdivided indefinitely in theory,
although it was recognized that such subdivision might not be practically possible. Aristotle
further held, against the atomistic philosophers, that void, or vacuum, did not exist.
This Greek set of chemical concepts was intended to, and did, explain something of the
relationships between the qualitative properties of substances. It did not attempt to make any
attempt to explain the quantitative results one can obtain in chemical operations. This was not
because the Greeks or other ancients were incapable of making quantitative measurements - in
their geometric work they made many - but because their chemical concepts were qualitative and
they did not think of applying quantitative measurements to chemistry. The idea of making
quantitative measurements of the most important chemical quantity, mass or weight, had to wait
until the time of the French Revolution around 1790, despite the fact that balances capable of
making the measurements necessary had been known, and in use by assay offices, coiners,
jewellers, and merchants, for perhaps 5000 years.
The Greeks used this rather artificial-sounding theory of the four elements to explain many of the
actions of nature. For example, a real fire was impure ideal fire. When a pot was placed over a
fire, the bottom of the pot became black; this happened because the real fire was a mixture of
ideal fire and ideal earth, and therefore when the fire entered the pot to give it more of the
property of hotness some or all of the earth mixed with it was left behind on the bottom of the
pot. When sea water was heated it absorbed the hotness of fire and moved away from water,
becoming air; the impurity in real water, earth, then was left behind on the bottom of the pot as
dry salt when the water finally had absorbed the hotness of enough fire and been completely
converted into air. An air, when cooled, would condense droplets of water, as when cold metal
was placed in contact with the air above a kettle of boiling water or in moist air. This occurred
because the property of coldness, taken from the metal or earth, moved the air toward wetness
and thus partially toward water.
Other Greek concepts of chemistry also reappeared, in slightly modified form, much later in
history. Aristotle added a fifth element called quintaessentia to the four of Empedocles. This
eternal and unchangeable element called the ether, or space, was a sort of pure form in which the
other four elements existed. This concept, expressed as a luminiferous ether in which light waves
were propagated just as water waves as propagated in oceans, rose to haunt physics until well
after 1900 when experiments finally made its retention impossible. Another extension of the
ideas of Empedocles, due to Plato of Athens, was the idea that each of the four elements existed
in a particular geometric form and the properties of the element were therefore related to that
form. So fire particles were believed to exist in the form of tetrahedrons, whose sharp points
gave speed and burning sensations like arrows striking the flesh. Earth particles had the shapes of
cubes, which accounted for their solidity; water particles had the smoother shape of an
icosahedron, while those of air had the shape of an octahedron. Ether, being the highest of the
elements, had the most complex geometry, that of a pentagonal dodecahedron. This idea, that
each of the elements was made up of particles having a single definite shape, recurred again with
considerable impact upon the developing modern definition of a chemical element in the
seventeenth and eighteenth centuries.
Aristotle, like all the Greek scientific writers, paid comparatively little attention to chemical
matters. The classical four elements are not "elements" in the modern, or Boyleian, sense. The
states of earth, water, air and fire might now correspond to four physical states of matter - solid,
liquid, gas,and plasma. Chemical processes involving water such as solution of salts, metabolic
processes, and the release of waters of hydration on heating of minerals were undoubtedly
known but were not distinguished from physical processes such as freezing, melting,
condensation, and boiling. Aristotle comments on Anaxagoras' study of mixtures - the
observation that when a white liquid is mixed drop-by-drop with a black one the color change is
infinitesimally gradual, so these natural processes must be so minute as to escape the senses and
can only be inferred, not observed. But his main concern is with the concept of change, which
expressed in the twin concepts of "coming-into-being" and "passing-away". We may illustrate
this with his comment on the burning of wood: "Now we do not speak of the wood as 'combined'
with fire, not of its burning as a 'combining' either of its particles with one another or of itself
with the fire; what we say is that the fire is coming-to-be but the wood is passing-away."
The Atomistic Philosophers
The concept of atoms appears to arise with Leucippus, of whom we know little, and his follower
Democritus of Abdera (460? - 360 BC); in the form known to us it has become known through
the criticism of Aristotle and the later writings of Epicurus of Samos (341 - 270 BC) and Titus
Lucretius Carus (98 - 54 BC). The ideas of this school, in their fully-developed form, are as
follows.
First, matter is not capable of infinite subdivision. The ultimate and indivisible constituents of
matter are extremely small and imperceptible particles called atoms. These, like matter itself, are
eternal and indestructible. The differences between substancess are due to the elements of which
they are formed, which may differ from each other in the size, shape and arrangements of the
atoms of which they are composed. The properties of a substance which we observe are not the
properties of its atoms but properties which arise from the manner in which the atoms are
combined.
Second, these atoms are constantly in motion and this motion is a property inherent in them, as
motes of dust are seen to dance in a sunbeam. Combinations are due to coalescence of the
particles or atoms as they collide.
Third, these atoms are separated from each other by void, or vacuum, in which the atoms move.
It is tempting to read more into the atomistic philosophers in the light of our modern knowledge
of atoms than they actually did write. Like other Greek thought, theirs was based upon logic and
argument rather than experiment. Nevertheless, the idea of atoms originating with them had, by
1750, been spread through the scientific community and was the foundation stone for the work of
John Dalton.
Practical Chemistry in the Ancient World
Many of the pure substances available in the ancient world were metals, of which seven were
then known: gold, silver, lead, tin, iron, copper, and mercury. The other pure substances known
in the ancient world, in addition to naturally-occurring minerals, constitute rather a mixed group.
One of the earliest pure substances of commerce was common salt (NaCl, sodium chloride). This
was obtained from salt water, either the sea or salt springs, by evaporation. The evaporation
could take place in natural rock hollows or in specially built basins, and both were in use well
before historical records begin. Salt was of great use in food preservation and is a necessary part
of a human diet. Since dry salt is fairly easily transportable, a commerce in it flourished in
ancient times. Another chemical, similar to salt but less useful, known to the ancient world was
soda or natron (Na2CO3, sodium carbonate) which was obtained from natural deposits such as
those of the Wadi Natron in Egypt. It was used for cleansing and medicinal purposes.
Vinegar, used in the ancient world and now in cooking, was also used as a preservative. Wine,
when exposed to air, goes sour and turns to vinegar. The sour taste is due to acetic acid,
CH3COOH, produced by air oxidation of the ethanol, CH3CH2OH, in the wine. A pure vinegar is
a dilute solution of acetic acid.
Lime or quicklime (CaO, calcium oxide) was manufactured in the ancient world for use in
cement; many Roman cement or concrete structures still stand. Lime was obtained by strong
heating of limestone (CaCO3, calcium carbonate) which is abundant in nature.
Bitumen, or pitch, was obtained from natural seepages, primarily in the Middle East. It was used
for calking of ships, tarring of roofs, and, in Greek fire, used as a weapon much like a modern
flamethrower. It was not processed, as is done in a modern oil refinery, but it was shipped
overseas in fairly large quantity.
Spices, obtained from plants, were also items of chemical commerce, as were medicinals - which
covered everything used in medicine. Most of these were plant materials, although a few came
from animals and some were naturally occurring minerals. Dye materials such as the blue dye
woad obtained by the Welsh for the coloring of fabrics (and themselves!) were generally of plant
origin. A few dyes such as Tyrian purple obtained from a species of mollusc growing near
modern Lebanon, were of animal origin. One interesting material, used as a burn ointment, a
pigment, and in putty, was white lead (2PbCO3.Pb(OH)2, basic lead carbonate) which was
produced by a chemical manufacturing process.
In this process, lead metal was placed in earthen pots over sharp vinegar and after it had acquired
some thickness of a kind of white rust, or crust, which it usually did in about ten days, the
workers opened the vessels and scraped off this deposit. The lead was returned to the vessel over
the vinegar again and the process was repeated over and over again until the lead was completely
gone. The material scraped off was then beaten to powder and boiled with water for a long time.
What at last settled to the bottom of the vessel was white lead. What happens in this process is
that the lead is attacked by the acetic acid in the vinegar and forms lead acetate, Pb(CH3COO)2.
On boiling in water the impure lead acetate is converted to 2PbCO3.Pb(OH)2, white lead, which
is very insoluble in water and so it is left on the bottom of the vessel.
Few pure substances were known in the ancient world other than those mentioned above, but the
separation of mixtures was understood and employed in metallurgy and in medicine. The theory
of the four elements was interpreted as indicating that all real substances were mixtures in any
case, and separation of complex mixtures into simpler ones made eminent conceptual good sense
even if it was difficult in practice. Only the techniques of smelting of metals and of
crystallization from water were used for the deliberate purification of materials on any large
scale in the ancient world, and it is doubtful that any other methods were understood.
Scientists before Dalton thought about matter
Galileo (1564 - 1642) thought the appearance of new materials in a chemical change was due to
rearrangement of parts too small to be seen.
Francis Bacon (1561 - 1626) seems to be thinking of matter in atomic terms when he says
"Rapid motion of constituent particles is both a necessary and sufficient condition for something
to be hot."
Robert Boyle ( 1627 - 1691) discarded the Aristotelian idea of the elements earth, air, fire and
water as constituents of matter. Instead he used a ‘corpuscular or mechanical hypothesis’,
explaining many physical phenomena in terms of solid bodies moving, colliding, bouncing, and
having their shapes or sizes changed.
Isaac Newton (1642 - 1727) in his book Opticks wrote: "Have not the small Particles of Bodies
certain Powers, Virtues, or Forces, by which they act at a distance, not only upon the Rays of
Light for reflecting, refracting and inflecting them, but also upon one another for producing a
great Part of the Phenomena of Nature?"
What experimental measurements were available to Dalton?
John Dalton was able to supply experimental results to forcefully revive the idea
of the atom.
He was influenced by the experiments of two Frenchmen, Antoine Lavoisier and
Joseph Louis Proust.
Antoine Lavoisier (1743-1794) - Formulated the Law of Conservation of Matter: "Matter is
neither gained nor lost during a chemical reaction." He did this by weighing materials before
and after reactions. For example, the weights of the mercury and oxygen formed by
decomposition of mercuric oxide were compared with the initial weight of the mercuric oxide.
Joseph Louis Proust (1754-1826) - Formulated the Law of Constant Porportions: "In a
compound, the contsitutne elements are always present in a definite proportion by weight." Like
Lavoisier, Proust also conducted quantitative experiments. He showed that regardless of how
copper carbonate was prepared in the laboratory, or how it was isolated from nature, it always
contained the same proportions of copper, oxygen and carbon - 5:4:1 parts by weight.
Read about Proust's research on copper in his own words.
Not all his contemporaries agreed with Proust's conclusions. Berthollet was able to combine
different quantities of copper and tin to produce what seemed to him to be compounds of varying
composition. What is the difference between the combination of carbon and oxygen in carbon
dioxide, and the combination of copper and tin when they are heated together?
John Dalton (1766-1844)- Formulated the Law of Multiple Proportions : "In the formation of
two or more compounds from the same elements, the weights of one element that combine with a
fixed weight of a second element are in a ratio of small whole numbers (integers) such as 2 to 1,
3 to 1, 3 to 2, or 4 to 3." He had made a quantitative study of different compounds made from the
same elements, such as carbon monoxide and carbon dioxide. He found that the weight ratio of
carbon to oxygen in carbon monoxide was 3:4, and the weight ratio of carbon to oxygen in
carbon dioxide was 3:8.
Read a short article about the 'Chemical atom in early 19th century chemistry' which
describes the period including the work of the three scientists mentioned above.
Read John Dalton's own words as he discusses the opinions of some of his contemporaries and
gives his own ideas about how elements combine to form compounds.
Dalton's atomic theory
Here is a summary of Dalton's theory.
1. Elements are composed of tiny, separate, indivisible and indestructible particles. These
particles, called atoms, maintain their identity when the element undergoes physical or chemical
change.
2. All atoms of the same element are identical and different from the atoms of every other
element.
3. Atoms combine in simple whole number ratios to form compounds.
4. Atoms of the same elements can combine in different ratios to form more than one compound.
A simple discussion of this theory and its background can be found here.
Berzelius, contributed significantly to the development of atomic theory. About 1807 he
performed a great number of analyses of chemical compounds, and showed so many examples of
the law of definite proportions that it could no longer be doubted. He also set about determining
atomic weights and his first table, published in 1828, compared favourably with today's accepted
values.
Whereas Dalton represented atoms of elements by circles containing a letter or symbol, Berzelius
chose to omit the circle and just use an initial letter of the Latin name of the element (or two
letters if more than one element began with the same letter). This led to the system we now use
for writing formulae of elements and compounds and writing chemical equations.
Now, 200 years later, how would you modify Dalton's theory?
Think of sub-atomic particles - protons, neutrons, electrons...
spontaneous fission of radioactive atoms, nuclear fission and fusion,
production of radioactive isotopes in an atomic pile... So the first part of the theory is no longer
accepted.
Atoms of isotopes of an element are not identical. So the second part of the theory is no longer
accepted.
Ancient Greeks struggled to understand the nature of matter
Empedocles (around 490 to 444 BC) thought there were four original elements: Earth, Air, Fire,
Water. He thought everything else came about through their combination and/or separation by
the two opposite principles of Love and Strife.
Leucippus (around 460 to 420 BC) and Democritus (around 460 to 370 BC), supposedly a pupil
of Leucippus, are considered the founders of atomism. Leucippus regarded atoms as
imperceptible, individual particles that differ only in shape and position.
Plato (about 427 to 347 BC) in his work, the Timaeus, proposes a mathematical construction of
the elements - earth, air, fire, water. Each of these elements is said to consist of particles or
primary bodies. Each particle is a regular geometrical solid- the cube, tetrahedron, octahedron
and icosahedron. Each of these particles is composed of simple right triangles. The particles are
like the molecules of the theory; the triangles are its atoms.
Plato's beliefs as regards the universe were that the stars, planets, Sun and Moon move round the
Earth in crystalline spheres. The sphere of the Moon was closest to the Earth, then the sphere of
the Sun, then Mercury, Venus, Mars, Jupiter, Saturn and furthest away was the sphere of the
stars. He believed that the Moon shines by reflected sunlight.
Aristotle (384-322 BC) said that Earth was both the centre of the universe and one of the four
primordial elements. He saw the universe as a series of concentric spheres, with earth at the
centre, followed by water, air, fire. The harmonious relationships and interworkings of these
spheres could be heard as celestial music: the music of the spheres. Above fire was the Moon,
and this sphere delimited matter of a different kind. Beyond the Moon were spheres for the Sun,
the planets, and the stars, which were carried around the Earth in daily, complicated orbits. All
matter inside of the Moon’s orbit was different in kind from matter above the Moon.
Reminiscent of Plato’s ideas, Aristotle’s theory said that terrestrial matter decays and is
ephemeral, while celestial matter, the aether, is unchanging and eternal.
.~Modern Atomic Theory~.
N
ear the end of the 18th century, two laws about chemical reactions emerged without
referring to the notion of an atomic theory. The first was the law of conservation of
mass, formulated by Antoine Lavoisier in 1789, which states that the total mass in a
chemical reaction remains constant (that is, the reactants have the same mass as the products).
The second was the law of definite proportions. First proven by the French chemist Joseph Louis
Proust in 1799, this law states that if a compound is broken down into its constituent elements,
then the masses of the constituents will always have the same proportions, regardless of the
quantity or source of the original substance. Proust had synthesized copper carbonate through
numerous methods and found that in each case the ingredients combined in the same proportions
as they were produced when he broke down natural copper carbonate.
Various atoms and molecules as depicted in John Dalton's A New System of Chemical
Philosophy (1808).
In the early years of the 19th century, John Dalton developed an atomic theory in which he
proposed that each chemical element is composed of atoms of a single, unique type, and that
though they are both immutable and indestructible, they can combine to form more complex
structures (chemical compounds). The conservation of mass suggested to Dalton that the atoms
of matter are indestructible. His theory allowed him to explain various new discoveries in
chemistry that he and his contemporaries made. This marked the first truly scientific theory of
the atom, since Dalton reached his conclusions by experimentation and examination of the
results in an empirical fashion. It is unclear to what extent his atomic theory might have been
inspired by earlier such theories.
Dalton studied and expanded upon Proust's work to develop the law of multiple proportions: if
two elements form more than one compound between them, then the ratios of the masses of the
second element which combine with a fixed mass of the first element will be ratios of small
integers. One pair of reactions Dalton studied involved the combinations of "nitrous air", or what
we now call nitric oxide (NO), and oxygen (O2). Under certain conditions, these gases formed an
unknown product at a certain combining ratio (now known to be nitrogen dioxide (NO2)), but
when he repeated the reaction under other conditions, exactly twice the amount of nitric oxide (a
ratio of 1:2—small integers) reacted completely with oxygen to form a different product—now
known as dinitrogen trioxide (N2O3).
2NO + O2 → 2NO2
4NO + O2 → 2N2O3
Dalton also believed atomic theory could explain why water absorbed different gases in different
proportions: for example, he found that water absorbed carbon dioxide far better than it absorbed
nitrogen. Dalton hypothesized this was due to the differences in mass and complexity of the
gases' respective particles. Indeed, carbon dioxide molecules (CO2) are heavier and larger than
nitrogen molecules (N2).
In 1803 Dalton orally presented his first list of relative atomic weights for a number of
substances. This paper was published in 1805, but he did not discuss there exactly how he
obtained these figures. The method was first revealed in 1807 by his acquaintance Thomas
Thomson, in the third edition of Thomson's textbook, A System of Chemistry. Finally, Dalton
published a full account in his own textbook, A New System of Chemical Philosophy, 1808 and
1810.
Dalton estimated the atomic weights according to the mass ratios in which they combined, with
hydrogen being the basic unit. However, Dalton did not conceive that with some elements atoms
exist in molecules – e.g. pure oxygen exists as O2. He also mistakenly believed that the simplest
compound between any two elements is always one atom of each (so he thought water was HO,
not H2O). This, in addition to the crudity of his equipment, resulted in his table being highly
flawed. For instance, he believed oxygen atoms were 5.5 times heavier than hydrogen atoms,
because in water he measured 5.5 grams of oxygen for every 1 gram of hydrogen and believed
the formula for water was HO (an oxygen atom is actually 16 times heavier than a hydrogen
atom).
The flaw in Dalton's theory was corrected in 1811 by Amedeo Avogadro. Avogadro had
proposed that equal volumes of any two gases, at equal temperature and pressure, contain equal
numbers of molecules (in other words, the mass of a gas's particles does not affect its volume).
Avogadro's law allowed him to deduce the diatomic nature of numerous gases by studying the
volumes at which they reacted. For instance: since two liters of hydrogen will react with just one
liter of oxygen to produce two liters of water vapor (at constant pressure and temperature), it
meant a single oxygen molecule splits in two in order to form two particles of water. Thus,
Avogadro was able to offer more accurate estimates of the atomic mass of oxygen and various
other elements, and firmly established the distinction between molecules and atoms.
In 1815 the English chemist William Prout observed that the atomic weights that had been
measured for the elements known at that time appeared to be whole multiples of the atomic
weight of hydrogen. Prout hypothesized that the hydrogen atom was the only truly fundamental
object, and that the atoms of other elements were actually groupings of various numbers of
hydrogen atoms. Prout's hypothesis was confirmed in essence by Ernest Rutherford a century
later.
In 1827, the British botanist Robert Brown observed that pollen particles floating in water
constantly jiggled about for no apparent reason. In 1905, Albert Einstein theorized that this
Brownian motion was caused by the water molecules continuously knocking the grains about,
and developed a hypothetical mathematical model to describe it. This model was validated
experimentally in 1908 by French physicist Jean Perrin, thus providing additional validation for
particle theory (and by extension atomic theory).
Discovery of subatomic particles
Thomson's Crookes tube in which he observed the deflection of cathode rays by an electric field.
The purple line represents the deflected electron stream.
Atoms were thought to be the smallest possible division of matter until 1897 when J.J. Thomson
discovered the electron through his work on cathode rays. A Crookes tube is a sealed glass
container in which two electrodes are separated by a vacuum. When a voltage is applied across
the electrodes, cathode rays are generated, creating a glowing patch where they strike the glass at
the opposite end of the tube. Through experimentation, Thomson discovered that the rays could
be deflected by an electric field (in addition to magnetic fields, which was already known). He
concluded that these rays, rather than being waves, were composed of negatively charged
particles he called "corpuscles" (they would later be renamed electrons by other scientists).
Thomson believed that the corpuscles emerged from the very atoms of the electrode. He thus
concluded that atoms were divisible, and that the corpuscles were their building blocks. To
explain the overall neutral charge of the atom, he proposed that the corpuscles were distributed in
a uniform sea or cloud of positive charge; this was the plum pudding model as the electrons were
embedded in the positive charge like plums in a plum pudding.
Discovery of the nucleus
The gold foil experiment
Top: Expected results: alpha particles passing through the plum pudding model of the atom with
negligible deflection.
Bottom: Observed results: a small portion of the particles were deflected, indicating a small,
concentrated positive charge.
Thomson's plum pudding model was disproved in 1909 by one of his former students, Ernest
Rutherford, who discovered that most of the mass and positive charge of an atom is concentrated
in a very small fraction of its volume, which he assumed to be at the very center.
In the gold foil experiment, Hans Geiger and Ernest Marsden (colleagues of Rutherford working
at his behest) shot alpha particles through a thin sheet of gold, striking a fluorescent screen that
surrounded the sheet. Given the very small mass of the electrons, the high momentum of the
alpha particles and the unconcentrated distribution of positive charge of the plum pudding model,
the experimenters expected all the alpha particles to either pass through without significant
deflection or be absorbed. To their astonishment, a small fraction of the alpha particles
experienced heavy deflection.
This led Rutherford to propose a model of the atom (the planetary model or Rutherford model) to
explain the experimental results. In this model, the atom was made up of a nucleus of
approximately 10-15 m in diameter, surrounded by an electron cloud of approximately 10-10 m in
diameter. The pointlike electrons orbited in the space around the massive, compact nucleus like
planets orbiting the Sun.
Following this discovery, the study of the atom split into two distinct fields, nuclear physics,
which studies the properties and structure of the nucleus of atoms, and atomic physics, which
examines the properties of the electrons surrounding the nucleus.
First steps towards a quantum physical model of the atom
The planetary model of the atom had two significant shortcomings. The first is that, unlike the
planets orbiting the sun, electrons are charged particles. An accelerating electric charge is known
to emit electromagnetic waves according to the Larmor formula in classical electromagnetism;
an orbiting charge would steadily lose energy and spiral towards the nucleus, colliding with it in
a small fraction of a second. The second problem was that the planetary model could not explain
the highly peaked emission and absorption spectra of atoms that were observed.
The Bohr model of the atom
Quantum theory revolutionized physics at the beginning of the 20th century, when Max Planck
and Albert Einstein postulated that light energy is emitted or absorbed in discrete amounts
known as quanta (singular, quantum). In 1913, Niels Bohr incorporated this idea into his Bohr
model of the atom, in which the electrons could only orbit the nucleus in particular circular orbits
with fixed angular momentum and energy, their distances from the nucleus (i.e., their radii)
being proportional to their respective energies. Under this model electrons could not spiral into
the nucleus because they could not lose energy in a continuous manner; instead, they could only
make instantaneous "quantum leaps" between the fixed energy levels. When this occurred, light
was emitted or absorbed at a frequency proportional to the change in energy (hence the
absorption and emission of light in discrete spectra).
Bohr's model was not perfect. It could only predict the spectral lines of hydrogen; it couldn't
predict those of multielectron atoms. Worse still, as spectrographic technology improved,
additional spectral lines in hydrogen were observed which Bohr's model couldn't explain. In
1916, Arnold Sommerfeld added elliptical orbits to the Bohr model to explain the extra emission
lines, but this made the model very difficult to use, and it still couldn't explain more complex
atoms.
Discovery of isotopes
While experimenting with the products of radioactive decay, in 1913 radiochemist Frederick
Soddy discovered that there appeared to be more than one element at each position on the
periodic table. The term isotope was coined by Margaret Todd as a suitable name for these
elements.
That same year, J.J. Thomson conducted an experiment in which he channeled a stream of neon
ions through magnetic and electric fields, striking a photographic plate at the other end. He
observed two glowing patches on the plate, which suggested two different deflection trajectories.
Thomson concluded this was because some of the neon ions had a different mass. The nature of
this differing mass would later be explained by the discovery of neutrons in 1932.
Discovery of nuclear particles
In 1918, Rutherford bombarded nitrogen gas with alpha particles and observed hydrogen nuclei
being emitted from the gas. Rutherford concluded that the hydrogen nuclei emerged from the
nuclei of the nitrogen atoms themselves (in effect, he split the atom). He later found that the
positive charge of any atom could always be equated to that of an integer number of hydrogen
nuclei. This, coupled with the facts that hydrogen was the lightest element known and that the
atomic mass of every other element was roughly equivalent to an integer number of hydrogen
atoms, led him to conclude hydrogen nuclei were singular particles and a basic constituent of all
atomic nuclei: the proton. Further experimentation by Rutherford found that the nuclear mass of
most atoms exceeded that of the protons it possessed; he speculated that this surplus mass was
composed of hitherto unknown neutrally charged particles, which were tentatively dubbed
"neutrons".
In 1928, Walter Bothe observed that beryllium emitted a highly penetrating, electrically neutral
radiation when bombarded with alpha particles. It was later discovered that this radiation could
knock hydrogen atoms out of paraffin wax. Initially it was thought to be high-energy gamma
radiation, since gamma radiation had a similar effect on electrons in metals, but James Chadwick
found that the ionisation effect was too strong for it to be due to electromagnetic radiation. In
1932, he exposed various elements, such as hydrogen and nitrogen, to the mysterious "beryllium
radiation", and by measuring the energies of the recoiling charged particles, he deduced that the
radiation was actually composed of electrically neutral particles with a mass similar to that of a
proton. For his discovery of the neutron, Chadwick received the Nobel Prize in 1935.
Quantum physical models of the atom
In 1924, Louis de Broglie proposed that all moving particles–particularly subatomic particles
such as electrons–exhibit a degree of wave-like behavior. Erwin Schrödinger, fascinated by this
idea, explored whether or not the movement of an electron in an atom could be better explained
as a wave rather than as a particle. Schrödinger's equation, published in 1926, describes an
electron as a wavefunction instead of as a point particle. This approach elegantly predicted many
of the spectral phenomena that Bohr's model failed to explain. Although this concept was
mathematically convenient, it was difficult to visualize, and faced opposition. One of its critics,
Max Born, proposed instead that Schrödinger's wavefunction described not the electron but
rather all its possible states, and thus could be used to calculate the probability of finding an
electron at any given location around the nucleus.
The five filled atomic orbitals of a neon atom, separated and arranged in order of increasing
energy from left to right, with the last three orbitals being equal in energy. Each orbital holds up
to two electrons, which exist for most of the time in the zones represented by the colored
bubbles. Each electron is equally in both orbital zones, shown here by color only to highlight the
different wave phase.
A consequence of describing electrons as waveforms is that it is mathematically impossible to
simultaneously derive the position and momentum of an electron; this became known as the
Heisenberg uncertainty principle. This invalidated Bohr's model, with its neat, clearly defined
circular orbits. The modern model of the atom describes the positions of electrons in an atom in
terms of probabilities. An electron can potentially be found at any distance from the nucleus,
but—depending on its energy level—tends to exist more frequently in certain regions around the
nucleus than others; this pattern is referred to as its atomic orbital.
.~Atomic Theory~.
About 400 B.C. the Greek philosopher Democritus proposed that matter consisted of various
types of tiny discrete particles and that the properties of matter were determined by the properties
of these particles. This theory was later elaborated in a work by Lucretius. These philosophers
did not have a method to verify the theory and it was not pursued for many centuries. The theory
reappeared in the early 19th century as an explanation for several laws that had been established
in the previous century when chemists had begun carefully measuring the mass of reactants and
products. One of these was the law of conservation of mass, first stated by the French chemist
Lavoisier, which says that there is no change in mass with a chemical reaction. Another was the
observation that compounds always contain the same elements in the same proportions, the law
of constant composition. Today it is known that some compounds, particularly metal oxides and
sulfides, exist in ratios that vary slightly from simple whole number ratios. Some of these have
the property of superconductivity. These are known as nonstoichiometric compounds.
A third law is the law of multiple proportions which states that a given mass of one element can
combine with various masses of another element (or elements) but always in small whole
number ratios. John Dalton revived the atomic theory in order to explain these observations. In
1808 he proposed that a chemical element (which could not be decomposed into two or more
components) consisted of tiny particles (atoms), all of which had the same chemical properties.
Further, the atoms of a given element have different properties than the atoms of other elements
and that these atoms are not changed during ordinary chemical reactions. Compounds are formed
by combination of atoms of different elements in certain simple whole number ratios.
It took many years for the concept to become widely accepted. Of course nowadays the atomic
theory is fundamental to the physical sciences.
H-flux
Atomic Hydrogen Source
Thermal hydrogen cracker
H-flux Atomic Hydrogen Source with optional shutter
The new H-flux Atomic Hydrogen Source* works by thermally dissociating hydrogen in an electron
bombardment heated fine tungsten capillary (thermal hydrogen cracker). By bouncing along the hot walls
the molecular hydrogen is cracked to atomic hydrogen. This is e.g. very useful for in-situ, damage free
cleaning of residual oxygen prior to sample coating or analysis. Other applications are in surface
modifications appreciating the high reactivity of atomic species.
The H-flux Atomic Hydrogen Source is UHV compatible and mounted on a NW35CF (2.75"OD) flange,
making the source an easy retrofit to existing vacuum systems.
Applications:
Atomic Hydrogen can be used in surface science and thin film technolgy (MBE, GSMBE) mainly for the
following applications:
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damage free in situ cleaning e.g GaAs, InP, Ge and Si. Removal of residual oxygen and carbon.
Low temperature cleaning

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Surfactant - improvement of layer properties during growth
post growth surface treatment/improvement
chemical passivation and surface reconstruction
annealing of amorphous silicon
Key Features:
Key features of the H-flux Atomic Hydrogen Source are the zero residual ion current and almost 100%
cracking efficiency, due to the superior and unique Bertel design. Integral watercooling is included as
standard to minimise heat load in the system and outgassing. A manual shutter is available as option.
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Zero residual ion current
Almost 100% cracking efficiency
Integral water-cooling as standard to minimise heat load in the system and outgassing
Capable of operation in UHV without differential pumping
Thermocouple and PID controller as standard for reproducible and constant operation
Filament made of standard Tungsten wire for inexpensive on-site replacement
Specifications:
Typical operating conditions: the flux of Hydrogen atoms at 10-9 mbar in a chamber with typical pumps at
a distance of 10cm is ca. 5x1013 atoms/cm².
Source Properties:
Gas Flow:
10-5 sccm to 1sccm
Operating Pressure: <10-10 to 10-5 mbar
Hydrogen Flux:
~5x1013 atoms/cm² @ 10cm distance and 10-9 mbar
Beam Divergence:
~15° half angle
Source Hardware:
High Voltage:
1,2kV (max) adjustable
Beam current:
50mA (max) adjustable
Power:
60W (max) adjustable via voltage and current
In vacuum length:
190mm (without shutter)
In vacuum diameter: 34mm (max)
Mounting Flange:
NW40CF (2.75")
Leak valve:
required (see option)
Integral cooling:
water
Bakeable:
200°C
Power Supply:
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HV connection via over-specified high voltage feedthrough with matching connector potted onto
the cable and screw-on retaining sleeve for high-level user safety
All feedthroughs are user demountable for on-site service/repair
Filaments are easily replaced and user fabricated from 0.3mm tungsten wire of near zero running
costs
Water-cooled copper jacket to minimise radiated heat and outgassing
Water connection via 1/4” Swagelok fittings
Gas connection via mini-Conflat (NW16CF) flange
Additional spare NW 16CF ports on the multiport for future upgrade with the options listed above
19" rack mount, 3U high, 230VAC/50Hz (115VAC/60Hz optional)
Options:
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Manual shutter: can be operated and closed within the ID of an DN40CF port.
Shutter actuator: for remote/computer controlled shutter operation.
References:
"Simple source of atomic hydrogen for ultrahigh vacuum applications", U. Bischler and E. Bertel; J. Vc.
Sci. Technol. A 11(2), Mar/Apr 1993
"Quantitative characterisation of a highly effective atomic hydrogen doser", C. Eibl, G. Lackner, and A.
Winkler. J.; Vac. Sci. Technol. A 16(5), Sep/Oct 1998.
Atomic Hydrogen Source (front view)
* In cooperation with Prof. Dr. E. Bertel of the University of Innsbruck
In the interests of continuous product development, specifications are subject to change without notice.
Rutherford's Atomic Theory was a revolutionary theory regarding the nature of atomic structure
that varied significantly from past theories on the same subject matter. In fact, although the
Rutherford Atomic Theory was first posited in 1911, many facets of it are still accepted by the
majority of the scientific community.
History of
1. In the early 1900s the predominant theory regarding how atoms were physically
structured was called the Plum Pudding Model. The Plum Pudding Model had been
theorized and established by J. J. Thomson. In 1911, physicist Ernest Rutherford used his
experimental data from several different experiments to conclude that the basic atomic
structure was very different than that proposed by the Plum Pudding Model. Rutherford's
work eventually led to the coherent theory which we refer to today as Rutherford's
Atomic Theory.
Features
2. The basic difference between Rutherford's Atomic Theory and the Plum Pudding Model
has to do with the fact that the Plum Pudding Model theorized that an atom was made up
electrons (the plums) surrounded by a positively charged mass (the pudding). Rutherford
later proved that this wasn't the case and theorized that atoms were comprised of a very
small nucleus surrounded by electrons. The basic tenets of that statement are still held to
be true in the modern era.
Misconceptions
3. Many people believe that Rutherford's Atomic Theory is essentially the same as Bohr's
Atomic Model, which is untrue. Bohr's Model was inspired by Rutherford's work and
built upon it, but the Bohr Atomic Model contains certain elements that were definitely
missing from Rutherford's Atomic Theory. The chief difference is the fact that Niels
Bohr, when theorizing the design of the atom, included a thesis for the forces that help to
hold the atomic structure together. The Bohr model is still widely accepted by
contemporary scientists.
Time Frame
4. The Bohr model of the atom was established in 1913 as the leading theory of atomic
structure. Because of this, it can be truthfully said that Rutherford's Atomic Theory was
an extremely short-lived phenomenon. Despite its brief time frame, however,
Rutherford's theory was extremely important because Niels Bohr wouldn't have been able
to develop his own model without the established background that Rutherford's Atomic
Theory laid out for him.
Effects
5. The effects of Rutherford's Atomic Theory are truly awe inspiring and the shockwaves
are still being felt today. As mentioned, Rutherford's work paved the way for the Bohr
Model of the atom, which is still widely accepted. Much of modern science and medicine
has atomic theory at its very root. Without Rutherford's previous work, then, and without
the later polishing work done by Niels Bohr, the face of contemporary science would be
unimaginably different.
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Rutherford
Early Atomic Theory
The atomic theory, which holds that matter is composed of tiny, indivisible particles in constant
motion, was proposed in the 5th cent. B.C. by the Greek philosophers Leucippus and Democritus
and was adopted by the Roman Lucretius. However, Aristotle did not accept the theory, and it
was ignored for many centuries. Interest in the atomic theory was revived during the 18th cent.
following work on the nature and behavior of gases (see gas laws).
From Dalton to the Periodic Table
Modern atomic theory begins with the work of John Dalton, published in 1808. He held that all
the atoms of an element are of exactly the same size and weight (see atomic weight) and are in
these two respects unlike the atoms of any other element. He stated that atoms of the elements
unite chemically in simple numerical ratios to form compounds. The best evidence for his theory
was the experimentally verified law of simple multiple proportions, which gives a relation
between the weights of two elements that combine to form different compounds.
Evidence for Dalton's theory also came from Michael Faraday's law of electrolysis. A major
development was the periodic table, devised simultaneously by Dmitri Mendeleev and J. L.
Meyer, which arranged atoms of different elements in order of increasing atomic weight so that
elements with similar chemical properties fell into groups. By the end of the 19th cent. it was
generally accepted that matter is composed of atoms that combine to form molecules.
Discovery of the Atom's Structure
In 1911, Ernest Rutherford developed the first coherent explanation of the structure of an atom.
Using alpha particles emitted by radioactive atoms, he showed that the atom consists of a central,
positively charged core, the nucleus, and negatively charged particles called electrons that orbit
the nucleus. There was one serious obstacle to acceptance of the nuclear atom, however.
According to classical theory, as the electrons orbit about the nucleus, they are continuously
being accelerated (see acceleration), and all accelerated charges radiate electromagnetic energy.
Thus, they should lose their energy and spiral into the nucleus.
This difficulty was solved by Niels Bohr (1913), who applied the quantum theory developed by
Max Planck and Albert Einstein to the problem of atomic structure. Bohr proposed that electrons
could circle a nucleus without radiating energy only in orbits for which their orbital angular
momentum was an integral multiple of Planck's constant h divided by 2π. The discrete spectral
lines (see spectrum) emitted by each element were produced by electrons dropping from allowed
orbits of higher energy to those of lower energy, the frequency of the photon of light emitted
being proportional to the energy difference between the orbits.
Around the same time, experiments on x-ray spectra (see X ray) by H. G. J. Moseley showed that
each nucleus was characterized by an atomic number, equal to the number of unit positive
charges associated with it. By rearranging the periodic table according to atomic number rather
than atomic weight, a more systematic arrangement was obtained. The development of quantum
mechanics during the 1920s resulted in a satisfactory explanation for all phenomena related to
the role of electrons in atoms and all aspects of their associated spectra. With the discovery of the
neutron in 1932 the modern picture of the atom was complete.
Contemporary Studies of the Atom
With many of the problems of individual atomic structure and behavior now solved, attention has
turned to both smaller and larger scales. On a smaller scale the atomic nucleus is being studied in
order to determine the details of its structure and to develop sources of energy from nuclear
fission and fusion (see nuclear energy), for the atom is not at all indivisible, as the ancient
philosophers thought, but can undergo a number of possible changes. On a larger scale new
discoveries about the behavior of large groups of atoms have been made (see solid-state physics).
The question of the basic nature of matter has been carried beyond the atom and now centers on
the nature of and relations between the hundreds of elementary particles that have been
discovered in addition to the proton, neutron, and electron. Some of these particles have been
used to make new types of exotic “atoms” such as positronium (see antiparticle) and muonium
(see muon).
Atomic Theory:
The ancient philosopher, Heraclitus, maintained that everything is in a state of flux.
Nothing escapes change of some sort (it is impossible to step into the same river). On
the other hand, Parmenides argued that everything is what it is, so that it cannot
become what is not (change is impossible because a substance would have to
transition through nothing to become something else, which is a logical
contradiction). Thus, change is incompatible with being so that only the permanent
aspects of the Universe could be considered real.
An ingenious escape was proposed in the fifth century B.C. by Democritus. He
hypothesized that all matter is composed of tiny indestructible units, called atoms. The
atoms themselves remain unchanged, but move about in space to combine in various
ways to form all macroscopic objects. Early atomic theory stated that the
characteristics of an object are determined by the shape of its atoms. So, for example,
sweet things are made of smooth atoms, bitter things are made of sharp atoms.
In this manner permanence and flux are reconciled and the field of atomic physics was
born. Although Democritus' ideas were to solve a philosophical dilemma, the fact that
there is some underlying, elemental substance to the Universe is a primary driver in
modern physics, the search for the ultimate subatomic particle.
It was John Dalton, in the early 1800's, who determined that each chemical element is
composed of a unique type of atom, and that the atoms differed by their masses. He
devised a system of chemical symbols and, having ascertained the relative weights of
atoms, arranged them into a table. In addition, he formulated the theory that a
chemical combination of different elements occurs in simple numerical ratios by
weight, which led to the development of the laws of definite and multiple proportions.
He then determined that compounds are made of molecules, and that molecules are
composed of atoms in definite proportions. Thus, atoms determine the composition of
matter, and compounds can be broken down into their individual elements.
The first estimates for the sizes of atoms and the number of atoms per unit volume
where made by Joesph Loschmidt in 1865. Using the ideas of kinetic theory, the idea
that the properties of a gas are due to the motion of the atoms that compose it,
Loschmidt calculated the mean free path of an atom based on diffusion rates. His
result was that there are 6.022x1023 atoms per 12 grams of carbon. And that the typical
diameter of an atom is 10-8 centimeters.
By the 19th century, it was determined that atoms bind together to form substances
through electromagnetic forces. That atoms are very small and matter is mostly empty
space, but 'feels' solid because the atoms of your hand are repelled by the
electromagnetic forces between your atoms and an objects atoms (like a table).
Matter:
Matter exists in four states: solid, liquid, gas and plasma. Plasmas are only found in
the coronae and cores of stars. The state of matter is determined by the strength of the
bonds between the atoms that makes up matter. Thus, is proportional to the
temperature or the amount of energy contained by the matter.
The change from one state of matter to another is called a phase transition. For
example, ice (solid water) converts (melts) into liquid water as energy is added.
Continue adding energy and the water boils to steam (gaseous water) then, at several
million degrees, breaks down into its component atoms.
>>c15
The key point to note about atomic theory is the relationship between the macroscopic
world (us) and the microscopic world of atoms. For example, the macroscopic world
deals with concepts such as temperature and pressure to describe matter. The
microscopic world of atomic theory deals with the kinetic motion of atoms to explain
macroscopic quantities.
Temperature is explained in atomic theory as the motion of the atoms (faster = hotter).
Pressure is explained as the momentum transfer of those moving atoms on the walls of
the container (faster atoms = higher temperature = more momentum/hits = higher
pressure).
Ideal Gas Law:
Macroscopic properties of matter are governed by the Ideal Gas Law of chemistry.
An ideal gas is a gas that conforms, in physical behavior, to a particular, idealized
relation between pressure, volume, and temperature. The ideal gas law states that for a
specified quantity of gas, the product of the volume, V, and pressure, P, is
proportional to the absolute temperature T and the number or density of particles, n,;
i.e., in equation form, PV = nkT, in which k is a constant. Such a relation for a
substance is called its equation of state and is sufficient to describe its gross behavior.
Although no gas is perfectly described by the above law, the behavior of real gases is
described quite closely by the ideal gas law at sufficiently high temperatures and low
pressures (such as air pressure at sea level), when relatively large distances between
molecules and their high speeds overcome any interaction. A gas does not obey the
equation when conditions are such that the gas, or any of the component gases in a
mixture, is near its triple point.
The ideal gas law can be derived from the kinetic theory of gases and relies on the
assumptions that (1) the gas consists of a large number of molecules, which are in
random motion and obey Newton's deterministic laws of motion; (2) the volume of
the molecules is negligibly small compared to the volume occupied by the gas; and (3)
no forces act on the molecules except during elastic collisions of negligible duration.
Thermodynamics:
The study of the relationship between heat, work, temperature, and energy,
encompassing the general behavior of physical system is called thermodynamics.
The first law of thermodynamics is often called the law of the conservation of energy
(actually mass-energy) because it says, in effect, that when a system undergoes a
process, the sum of all the energy transferred across the system boundary--either as
heat or as work--is equal to the net change in the energy of the system. For example, if
you perform physical work on a system (e.g. stir some water), some of the energy
goes into motion, the rest goes into raising the temperature of the system.
The second law of thermodynamics states that, in a closed system, the entropy
increases. Cars rust, dead trees decay, buildings collapse; all these things are examples
of entropy in action, the spontaneous movement from order to disorder.
>>c16
Classical or Newtonian physics is incomplete because it does not include irreversible
processes associated with the increase of entropy. The entropy of the whole Universe
always increased with time. We are simply a local spot of low entropy and our destiny
is linked to the unstoppable increase of disorder in our world => stars will burn out,
civilizations will die from lack of power.
The approach to equilibrium is therefore an irreversible process. The tendency toward
equilibrium is so fundamental to physics that the second law is probably the most
universal regulator of natural activity known to science.
The concept of temperature enters into thermodynamics as a precise mathematical
quantity that relates heat to entropy. The interplay of these three quantities is further
constrained by the third law of thermodynamics, which deals with the absolute zero of
temperature and its theoretical unattainability.
Absolute zero (approximately -273 C) would correspond to a condition in which a
system had achieved its lowest energy state. The third law states that, as this minimum
temperature is approached, the further extraction of energy becomes more and more
difficult.
Rutherford Atom :
Ernest Rutherford is considered the father of nuclear physics. Indeed, it could be said
that Rutherford invented the very language to describe the theoretical concepts of the
atom and the phenomenon of radioactivity. Particles named and characterized by him
include the alpha particle, beta particle and proton. Rutherford overturned Thomson's
atom model in 1911 with his well-known gold foil experiment in which he
demonstrated that the atom has a tiny, massive nucleus.
His results can best explained by a model for the atom as a tiny, dense, positively
charged core called a nucleus, in which nearly all the mass is concentrated, around
which the light, negative constituents, called electrons, circulate at some distance,
much like planets revolving around the Sun.
The Rutherford atomic model has been alternatively called the nuclear atom, or the
planetary model of the atom.
.~ Discoverers Of Atomic Theory ~.
1803 - John Dalton - Atomic Theory
1.
2.
3.
4.
Matter is made up of indivisible atoms.
All atoms of an element are identical.
Atoms are neither created nor destroyed.
Atoms of different elements have different weights and chemical
properties.
5. Atoms of different elements combine in simple whole numbers to
form compounds.
1830 - Michael Faraday
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Set up a pair of metal plates sealed in a glass tube. The tube was filled with a gas, and
the metal plates were connected to a series of batteries.
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As the pressure of the gas decreased, the gas began to glow.
Julius Plucker (1858) noticed that only one end emitted light.
o He also changed the position of the patch of glass that glowed by
bringing
a magnet close to the tube.
Conclusion: The effect of the magnetic field as evidence that
whatever
produced this glow was electrically charged.
Cathode - metal plate connected to the negative end
Anode - metal plate connected to the positive end
1869 - Johannes Hittorf
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Found that when a solid object was placed between the cathode and anode, a shadow was
cast on the end of the tube across from the cathode.
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Conclusion: Some beam or ray is given off by the cathode - subsequently called the
tubes cathode-ray tubes.
1879 - William Crookes
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Developed a better vacuum pump that allowed him to produce
cathode-ray
tubes with a smaller residual gas pressure.
Conclusion: Cathode r0ays are negatively charged by studying
deflection
of cathode rays by magnetic fields.
1897 - J.J. Thompson
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Found that cathode rays could be deflected by an electric field
Showed that cathode "rays" were actually particles
Found the charge to mass ratio of the particles to be
approximately
108 Coulomb (C) per gram.
Same charge to mass ratio regardless of metal used for
cathode/anode
or gas used to fill the tube.
Conclusion: Particles were a universal component of matter.
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Electron - (originally called corpuscles by Thompson) particles
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Raisin Pudding
Model
given off by the cathode; fundamental unit of negative electricity
Raisin Pudding Model o Matter is electrically neutral and electrons are much lighter
than atoms.
o Conclusion: There must be positively charged
particles which also must carry the mass of the
atom. Dalton's model is now incorrect because
atoms are divisible.
1895 - William Conrad Roentgen
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Discovered x-rays while using cathode-ray tubes. Found that x-rays
could pass
through solid objects.
1899 - Ernst Rutherford
Studied absorption of radioactivity.
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
Alpha radiation - positive charge - absorbed by a few hundredths of a
cm or metal foil
Beta radiation - negative charge - could pass through 100x as much foil
before it was absorbed
Gamma rays - no charge - could penetrate several cm of lead
1907-1911 - Rutherford updated Thomson's Raisin Pudding Model of the
atom.

Studied the deflection of alpha particles as they were
targeted
at thin gold foil sheets.
o Most of the alpha particles penetrated straight
through.
o However few were deflected at slight angles.
o Even fewer (only about 1 in 20,000) were
deflected at
angles over 90 .


Conclusion: The positive charge and mass of an atom were concentrated in the
center and only made up a small fraction of the total volume. He named this
concentrated center the nucleus (Latin for little nut).
Rutherford was also able to estimate the charge of an atom by studying the deflection of
alpha particles. He found that the positive charge on the atom was approximately half of
the atomic weight.
1908-1917 - Robert Millikan - Oil-drop experiment


J.J. Thomson had previously hypothesized that the mass of a single
electron
was at least 1000 times smaller than that of the smallest atom.
Millikan measured the charge on an electron with his oil-drop apparatus.

An "atomizer" from a
perfume bottle sprayed
oil or water droplets into
the sample chamber.
Some of the droplets fell
through the pinhole
into an area between two
plates (one positive
and one negative). This
middle chamber was
ionized by x-rays.
Particles that did not
capture any electrons fell
to the bottom plate
due to gravity. Particles
that did capture one
or more electrons were
attracted to the
positive upper plate and
either floated upward
or fell more slowly.

Conclusion: The charge on a drop was always a multiple of 1.59 x 10-19 Coulombs.
He proved Thomson's hypothesis that the mass of an electron is at least 1000 times
smaller than the smallest atom.
1913 - A. van den Broek

Suggested that the positive charge on atoms should be compared to their atomic numbers,
not their atomic weights.
o At the time, atomic number (Z) only specified an element's location on the
periodic table. Today, the atomic number is, by definition, the number of protons
in an atom.
1914 - H. G. J. Moseley



Studied the frequencies of the x-rays given off by cathode-ray tubes
when electrons strike the anode.
Found that there was a relationship between the frequencies (v) of the
x-rays given off by the cathode-ray tube and the atomic number of the
metal used to form the anode:
Conclusion: He argued that the frequencies of the x-rays should depend on the
charge on the nucleus emitting these x-rays. Therefore, the atomic number was
equal to the positive charge (charge on the nucleus) of an atom.
1920 - Rutherford proposed the name "proton" for the positively charged particles in the
nucleus of an atom. At the same time, he also proposed that the nucleus also contained
electrically neutral particles which accounted for the remaining mass of the atom. He called this
yet unknown particle the "neutron".
1932 - James Chadwick

Proved that neutrons, neutral particles in the nucleus that made up
approximately
half the mass of an atom, did exist.
Summary of Subatomic Particles
Particle Symbol Charge
Mass
Electron
e-
-1
0.0005486 amu
+
Proton
p
+1
1.007276 amu
Neutron
n
0
1.008665 amu
Atomic Rules




The number of protons in the nucleus of an atom is equal to the atomic number (Z).
In a neutral atom, the number of electrons is equal to the number of protons.
The mass number (M) of an atom is equal to the sum of the number of
protons and neutrons in the nucleus.
The number of neutrons is equal to the difference between the mass number (M)
and the atomic number (Z).
Atomic number: protons (and electrons if neutral)
Mass number: protons + neutrons (neutrons = mass number - atomic number)
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