KD McMahon
Reseda High School
Atoms and The Periodic Table
Aristotle and Democritus
In the fourth century BC two models were presented by Greek philosophers to
explain the nature of matter. Aristotle, perhaps the most important philosopher and
naturalists that ever lived, explained that all matter was composed of four earth elements:
earth, air, fire, and water. Combinations of these four elements made up everything in the
universe. Rearrangement of these elements would result, according to Aristotle, in the
formation of different or new substances. Alchemy, an odd combination of science and
superstition, arose out of this idea. Alchemists attempted to turn base earth elements, such
as lead, into gold or the "philosopher's stone."
Democritus believed that matter was made up of tiny particles which he called
atoms. Individuals that agreed with this "school" of thought were known as "atomists."
According to Democritus, atoms were the smallest thing that exist. Atoms could not be
subdivided into smaller particles, that is, they couldn't be cut in two. That is why he
called them "a tome" (atom) which means "not cut." Democritus further speculated that
atoms came in many sizes, shapes, colors, and weights. Some were smooth while others,
he believed, were bumpy. Democritus said that atoms had existed forever, would
continue to do so, and would never change. Many of Democritus' ideas were fairly close
to the truth. Unfortunately, Aristotle's model of the atom was more popular. For
centuries, alchemists pursued futile dreams of manipulating the "four elements" to
achieve riches and immortal life.
John Dalton and the Atomic Theory
John Dalton (1766- 1844) was a teacher (he started teaching at the age of 12) and
a prolific scientist. His principle interests were meteorology and chemistry. In 1808,
Dalton presented his Atomic Theory. Dalton's atomic theory included the following
ideas:
* All elements are composed of submicroscopic indivisible particles called atoms.
* Atoms of the same element are identical. The atoms of any one element are
different from those of any other element.
* Atoms of different elements can physically mix together or can chemically
combine with one another in simple whole-number ratios to form compounds.
* Chemical reactions occur when atoms are separated, joined, or rearranged.
However, atoms of one element are never changed into atoms of another element
(transmutation) as a result of a chemical reaction.
Dalton developed a system of symbols to depict atoms and the compounds they
formed. He envisioned that compounds were held together by the interlocking of spines
that extended outward from each atom; sort of a modern day Velcro.
Question #1
Why did Democritus call his fundamental particle of matter the “atom?”
Question #2:
Why was Democritus’ theory rejected for so many centuries?
Question #3:
Based on Dalton’s atomic theory what would be wrong with the following formula for
water: HO1/2
Question #4:
According to Dalton’s atomic theory would it have been possible for the alchemists to
have turned the element of lead into the element gold? Explain.
The Experiments of J.J. Thomson
The English chemist, Sir J.J. Thomson (1856-1940) was able to demonstrate that
the atom was not indivisible. He showed that it was composed of smaller particles. From
his experimental results he proposed one of the first models of the atom.
During the 1850's Geissler sealed a positive (anode) and negative (cathode)
electrode in an evacuated tube. When a high voltage was applied to the electrodes a
glowing beam traveled from the cathode to the anode. It was assumed that this beam was
a form of light, hence it was called a "cathode ray." The tubes were known as "cathode
ray tubes."
Thomson was not convinced that these "rays" were a form of energy. He
performed a variety of experiments using special cathode ray tubes that he designed
himself. Thomson devised a cathode ray tube with a paddle wheel built inside. When the
high voltage electricity was turned on the paddle wheel began to rotate and move away
from the cathode and towards the anode.
Thomson concluded that the cathode rays were not a form of light energy, but
were actually tiny particles with mass. When these particles collided with the paddle
wheel they transfered their momentum to the wheel causing it to move.
Thomson devised another cathode ray tube with positive and negatively charged
plates. Thomson observed that the particles were being repelled by the negatively
charged plate and attracted to the positive plate.
Thomson concluded from this evidence and from his previous experiments that
tiny particles were being emitted from the atoms of the cathode. These tiny particles were
negatively charged. He called these particles "electrons."
In 1886, E. Goldstein, using a cathode ray tube in which the cathode had holes,
observed rays traveling in the opposite direction to the cathode ray. These rays emitted
from the anode were called anode or canal rays.
Thomson recognized that these rays must be composed of positive material. He
reasoned that this material must be present along with the negative electrons. This would
explain why matter is not electrically charged.
Given these experimental results, Thomson proposed in 1897 what has since been
referred to as the "plum pudding" model of the atom. This model depicts the atom as a
diffuse cloud of positive charge with the negative electrons embedded randomly in it, like
plums in pudding.
Question #5:
What three facts about the atom did Thomson learn from studying cathode and anode
rays?
Question #6:
Suppose Thomson obtained results like that shown below. What would he have
concluded about the particle he discovered?
Millikan and the Mass of the Electron
In addition to theorizing his Plum Pudding Model of the atom, Thomson was able
to calculate the charge-to-mass ratio of the electron:
e/m = -1.76 X 108 C/g
where e represents the charge on the electron in coulombs and m represents the mass in
grams.
In 1909, Robert Millikan (1868-1953), working at the University of Chicago, used
the charge-to-mass ratio to determine the mass of the electron. Millikan designed a
clever experiment using oil drops. He produced very tiny oil droplets using an atomizer.
As these droplets fell through a hole in the chamber they were given a charge by
bombarding them with X-rays. By adjusting the charge on the plates he could halt the fall
of the droplets.
The voltage and the mass of an oil drop could then be used to calculate the charge on the
oil drop. Millikan's experiments showed that the charge on an oil drop is always a wholenumber multiple of the electron charge. Using this information he calculated the mass of
the electron as 9.11 X 10-28 grams.
Question #7:
Thomson determined that the charge to mass ration of the electron (e/m) = -1.76 X 108
C/g (C = Coulombs). Millikan was able to use this information and the results he
obtained from his oil drop experiment to determine mass of the electron. He determined
the mass of the electron to be 9.11 X 10-28 grams. Use this information to calculate the
charge of the electron in coulombs.
Rutherford and the Planetary Atom
In the first few years of the 20th century, Ernest Rutherford, a student of J.J.
Thomson, began some experiments on the nature of radiation that would eventually lead
him to propose his own model of the atom.
In 1896, the French scientist, Henri Becquerel, found that a piece of a mineral
containing uranium could produce its image on a photographic plate in the absence of
light. He attributed this phenomenon to a spontaneous emission of something which he
called "radiation" which originated from the uranium.
Rutherford performed experiments on
uranium, and other radioactive elements, to
determine the nature of radiation. Rutherford
placed a piece of uranium in a lead box. A
beam of radioactivity was allowed to pass
through an opening in the box. This beam
passed through two charged plates. A
fluorescent screen would detect radiation by
flashing whenever it was struck by the
radiation. Rutherford observed that the
radiation was diffracted into three beams by the charged plates.
He concluded that the beam attracted to the negative plate consisted of positively
charged particles. He called these particles "alpha particles" Likewise, he concluded that
those particles attracted to the positively charged plate had to be negatively charged. He
called these particles "beta particles." He recognized that the radiation that was not
diffracted by the charged plates had to be a form of electromagnetic radiation. He called
this "gamma radiation." Rutherford used his newly discovered knowledge of the nature
of radiation to test Thomson's model of the atom.
Rutherford set up the apparatus which would bombard thin gold foil with alpha
particles; these particles would then be detected by the fluorescent screen. He anticipated
that all of the alpha particle detection would occur on the screen directly behind the foil.
This would be consistent with Thomson's model in which the positive charge is diffusely
distributed throughout the atom. The positively charged alpha particles would easily pass
through the foil and its diffusely charged gold atoms and strike the screen behind it.
Rutherford got surprisingly different
results. He observed alpha particle detection
even in front of the foil. This meant that the
particles were being deflected by the gold
foil atoms. Rutherford observed, "It was
about as credible as if you had fired a 15inch shell at a piece of tissue paper and it
came back and hit you!" Rutherford
suggested that the atom was mostly empty
space with a highly charged center. Most of
the particles pass through the atom
undisturbed, but a few get too close to the
center and are deflected.
To account for these results Rutherford proposed a new
model of the atom in 1913. This model had the following
characteristics: The atom is mostly empty space with the
majority of its mass concentrated in the center of the atom
which he called the "nucleus." This nucleus was composed of
charged particles called "protons." Protons have an equal
amount of charge as an electron, but they are positive, not
negative. Protons have a mass nearly 2000 times greater than an electron. The electrons
rotate around the nucleus like planets around the sun. This is why Rutherford's model is
often referred to as "Planetary Model."
Question #8:
Discuss the role of serendipity in Becquerel’s discovery of radiation.
Question #9:
Suppose Rutherford had obtained the results
illustrated here. Draw a model of an atom that would
be consistent with these results. Explain your model
and why it is consistent with the experimental results.
Question #10:
Discuss the results Rutherford actually obtained .
Explain how this evidence led him to propose the
Planetary model of the atom. Draw and label his atomic model.
Moseley and Atomic Number
During this same time, Henry Moseley (18871915) was a lecturer in physics at Ernest Rutherford's
laboratory at the University of Manchester. He did
important research on the role of the proton in the
atom which led to a better understanding of the
elements.
Just before the turn of the century, Roentgen
discovered X-rays. Shortly after this discovery a
British scientist, Charles Barkla, found that each
element, on being struck by cathode rays, emitted their
own characteristic X-rays. This led Moseley to make a
systematic study of the elements using X-rays.
Moseley observed that as he
went across a row of elements the
frequency of the X-rays emitted
became progressively higher as the
atomic mass increased. This
suggested to Moseley that there
might be another "number" in the
atom in which there is a truely
linear relationship with the X-rays
emitted.
He decided that the
frequency emitted reflected the size
of the electron's orbit around the nucleus. He reasoned that the closer the electrons were
to the nucleus, the smaller would be their orbits, and the higher the frequency of the Xrays they emitted. Since the frequency increased with the weight of the atom, then in
heavier atoms the electrons must be drawn closer to the nucleus. What was the force that
drew them closer?
Moseley suggested that
what drew the electrons closer in
was the increase in the nucleus'
positive charge which attracted the
negative electrons. He further
reasoned the nuclear charge must
increase from element to element
right through the periodic table. The
most reasonable way to account for
this is to suppose that each element
has one more unit of positive
charge, one more proton, than the
one before. This number of protons in an atom is known as that element's "atomic
number." When Moseley plotted the frequency of X-rays emitted versus the element's
atomic number he obtained the true linear relationship he was seeking. Using this graph
he was able to predict the existence of unknown elements, and the frequency of X-rays
they would emit.
Chemists now know that it is the number of protons in an atom's nucleus which
makes one element different from another. In addition, the number of protons and
electrons are equal in neutral atoms. Other scientists had to complete Moseley's work
when his life was tragically cut short during World War I.
Question #11:
When Moseley plotted the square root of the frequency of X-rays emitted by elements
against their atomic mass he obtained nearly a straight line. What did this suggest to
Moseley? This eventually led to the discovery of another “number” that could be used to
describe the atom. What is this number and what does it tell us about the atom?
Aston and Isotopes
While studying can rays, Aston, an English scientist working in Thomson's lab,
made an important discovery that would eventually lead to the discovery of another
subatomic particle.
Aston accelerated canal rays
consisting of Neon nuclei through a
specialized anode ray tube. As the neon
nuclei passed by charged plates and a
magnetic field they were deflected. The
extent of their deflection was detected on a
fluorescent screen at the end of the tube.
To Aston's surprise three different
types of neon atoms were detected on the
screen. The neon atoms differed in mass. The
more massive neon was deflected least because of its greater momentum. The least
massive neon was deflected the most. Analysis of these three types of neon atoms
demonstrated that they had masses of 20, 21, and 22 amu (atomic mass units). These
values are known as "mass numbers." Aston called these atoms that were the same
element, but that had different masses, "isotopes." Isotope means "same place;"
indicating that isotopes are atoms of the same element and would all be in the "same
place" on the Periodic Table. Continued research by Aston and others showed that all of
the elements have isotopes, ie: atoms that have the same number of protons (and
therefore atomic number), but have different masses.
Question #12
Identify the isotopes of Neon in the diagram as
either 20, 21, or 22. How do you know that
this is the correct pattern?
Question #13
Why did Aston’s discovery suggest that there was another fundamental particle that made
up the atom?
Chadwick and the Neutron
Aston and his colleagues knew that the proton and electrons could not account for
the existence of isotopes. In 1920, Rutherford suggested that there must be an
undiscovered particle in the atom. In 1932, his student, John Chadwick, discovered the
particle. Chadwick bombarded beryllium atoms with alpha particles. The resulting
collisions broke the beryllium nuclei apart sending subatomic particles in all directions.
The subatomic particles passed by charged plates. Some of the particles where deflected,
as expected, towards the negative electrode. These particles were protons that formerly
made up the beryllium nuclei.
Some of the particles
however, were not deflected by the
charged plates. Chadwich concluded
that these heretofore unknown
particles were uncharged and with
further analysis he determined that
they had a mass equal to that of the
proton. He called these particles
"neutrons."
Mass Number and Atomic Mass
By the early 1930's the major subatomic particles had been discovered and their
physical properties had been described.
Particle
Proton
Electron
Neutron
Mass (grams)
1.67262 X 10-27
9.10939 X 10-31
1.67493 X 10-27
Relative Mass (g)
1.007
5.486 X 10-4 (~ 0)
1.009
Relative Charge
+1
-1
0
The mass numbers that Aston had observed could now be explained. Neon has an
atomic number of 10; each neon atom has 10 protons in the nucleus. This accounts for 10
amu of mass. The isotope of neon with a mass of 20 must have 10 protons and 10
neutrons. Neon-21 must have 10 protons and 11 neutrons. Neon-22 has 10 protons and 12
neutrons.
As can be seen from the table below the mass numbers of an element's isotopes
and the element's atomic mass are close in value, but different.
Isotope
32
S
35
S
34
S
35
S
Mass Number
32
33
34
36
Atomic Number
16
16
16
16
Mass (amu)
31.972
32.971
33.967
35.967
% Abundance
95
0.76
4.22
0.014
The atomic mass of sulfur is 32.064 amu.
How is the atomic mass of an element determined from its isotopes? An element's
atomic mass is a "weighted average" of the mass numbers of all of the isotopes of that
element. Weighting takes into account the varying abundances of each isotope.
Consider the data for sulfur:
(.95)(32) + (.007)(33) + (.042)(34) + (.00014)(36) = 30.4 + 0.23 + 1.43 + 0.005 = 32.065
The data to
calculate the atomic mass
for sulfur and the other
elements is obtained by
"mass spectroscopy." In a
mass spectrometer, atoms
are vaporized and passed
into a beam of high-speed
electrons. The high-speed
electrons knock off
electrons of the atoms
being analyzed and change them into positive ions. An applied electron filed then
accelerates these ions into a magnetic field. Because an accelerating ion produces its own
magnetic field, an interaction with the applied field occurs, which tends to change the
path of the ion. The amount of path deflection for each ion depends on its mass. The most
massive ions are deflected the smallest amount; this causes the ions to separate. A
comparison of the position where the ions hit the detector plate gives very accurate values
of the relative masses.
When C-12 and C-13 are analyzed in a mass spectrometer, the ratio of their
masses is found to be:
mass of C-13 = 1.0836129
mass of C-12
Since the atomic mass unit is defined such that the mass of C-12 is exactly 12 amu, then
on this same scale:
mass C-13 = (1.0836129)(12amu) = 13.0034amu
The masses of other atoms can be determined in a similar fashion.
The mass spectrometer
is also used to determine the
isotopic composition of an
element. For example, when a
sample of natural neon is
injected into a mass
spectrometer, the resultant
graphs are obtained:
The areas of the "peaks" or heights of the bars indicated the relative numbers of Ne-20,
Ne-21, and Ne-22. The average atomic mass =
(0.9092)(20) + (0.00257)(21) + (0.0882)(22) = 20.18 amu
Question #14:
Complete the chart on the following page for the hypothetical elemental samples.
Assume all atoms are neutral.
Element
A
B
C
D
Atomic #
12
Mass #
10
23
30
# of protons
# of neutrons
10
# of electrons
5
11
20
Mendeleev and the Periodic Table
Dmitri Ivanovich Mendeleev (1834-1907) knew that certain elements shared
similar properties. He believed that a careful analysis of these properties would someday
yield a unity and predictability to chemistry that was still absent in the field. One of the
properties he studied was an element's "combining power." The hydrogen atom, for
example, could not take on more than one atom at a time. Oxygen, on the other hand,
could combine with two other atoms.
In 1852, the English chemist, Edward Frankland, coined the term "valence" (from
the Latin word meaning "power") to denote combining power. Hydrogen, therefore, has a
valence of one and oxygen has a valence of two.
Mendeleev then proceeded to make a card for each of the known elements. The
card had the element's name, symbol, valence, and atomic weight (atomic number was
unknown at the time). Mendeleev arranged the cards in various ways looking for any
kind of pattern he might find to the elements.
He discovered that when he arranged the elements be increasing atomic weight
the properties of the elements (as indicated by their valence) would repeat periodically.
He constructed a table consisting of eight columns and nine rows. Elements which
belonged to the same column had similar properties.
Mendeleev's table revealed that the properties of the elements are repeated in an
orderly way when they are arranged by increasing atomic weight. Thus, the properties of
the elements are a periodic function of their atomic weights. This statement is called
"Periodic Law." Mendeleev's table became known as the "Periodic Table of the
Elements."
Mendeleev observed that there were several blanks in his table. he predicted that
these blanks represented elements which had not yet been discovered. Mendeleev
accurately foretold these missing element's atomic weights and chemical properties. In
1874, three years after Mendeleev published his Periodic Table, the first of these
predicted elements was discovered.
The Modern Periodic Table
Mendeleev's Periodic Table was not without its flaws. There were infact several
places in the table in which elements seemed to be in the wrong place. Henry Moseley's
discovery of atomic numbers would eventually lead to a modification of the Periodic
Table into the form that we are familiar with today.
When chemists arranged the elements in the Periodic Table according to
increasing atomic number the problems that were inherent with Mendeleev's table were
eliminated.
Question #15:
Mendeleev arranged his table by increasing…. He discovered that the chemical
properties of the elements would repeat themselves every ______ elements. This periodic
function of the elements became know as ….
Question #16:
After Moseley made his discovery, the Periodic Table was rearranged so that the
elements increased by ….
Question #17:
Label the alkali metals, alkali-earth metals, transition elements, halogens, noble gases,
the lanthanide, and actinides.