ATOM
The notion about splitting or division of matter to be smaller parts has been started
since the Ancient Greek period. Philosophers, such as Aristotle and Democritus proposed their
explanation independently about division of matter and particles composing them. According
to Aristotle, the division of matter can be done continuously or in other words, matter can be
divide until infinitely in the amount. Meanwhile, Democritus proposed an idea that the division
of matter cannot be done continuously, it is because at certain time an indivisible part of the
matter by using a term of atom. This term was derived from Latin, that is “atoms”, meaning
“unable to be cut or split”.
According to Democritus opinion, an atom was defined as a smallest part of matter or
substance. Although, finally the Democritus’ opinion about atoms was proved incorrect, but in
chemistry, the definition of atoms as the smallest particles of an element “it is still acceptable”.
It is because atoms of an element show properties of the element. In other words, atoms of an
element have identical properties with the element. Hence, the symbol of an atom is same as
that of its element. For example, iron atom is symbolized by Fe, and the element of iron is also
symbolized by Fe.
Each element has different atoms to another elements. Nowadays, approximately 118
chemical elements have been found, so there are about 118 kinds of atoms which have been
found by scientists.
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.
Dalton's Atomic Theory
It was in the early 1800s that John Dalton, an observer of weather and discoverer of
color blindness among other things, came up with his atomic theory. Let's set the
stage for Dalton's work. Less than twenty years earlier, in the 1780's, Lavoisier
ushered in a new chemical era by making careful quantitative measurements which
allowed the compositions of compounds to be determined with accuracy. By 1799
enough data had been accumulated for Proust to establish the Law of Constant
Composition ( also called the Law of Definite Proportions). In 1803 Dalton noted that
oxygen and carbon combined to make two compounds. Of course, each had its own
particular weight ratio of oxygen to carbon (1.33:1 and 2.66:1), but also, for the same
amount of carbon, one had exactly twice as much oxygen as the other. This led him to
propose the Law of Simple Multiple Proportions, which was later verified by the
Swedish chemist Berzelius. In an attempt to explain how and why elements would
combine with one another in fixed ratios and sometimes also in multiples of those
ratios, Dalton formulated his atomic theory.
The idea of atoms had been proposed much earlier. The ancient Greek philosophers
had talked about atoms, but Dalton's theory was different in that it had the weight of
careful chemical measurements behind it. It wasn't just a philosophical statement
that there are atoms because there must be atoms. His atomic theory, stated that
elements consisted of tiny particles called atoms. He said that the reason an
element is pure is because all atoms of an element were identical and that in
particular they had the same mass. He also said that the reason elements differed
from one another was that atoms of each element were different from one
another; in particular, they had different masses. He also said that compounds
consisted of atoms of different elements combined together. Compounds are pure
substances (remember they cannot be separated into elements by phase changes)
because the atoms of different elements are bonded to one another somehow,
perhaps by hooks, and are not easily separated from one another. Compounds have
constant composition because they contain a fixed ratio of atoms and each atom
has its own characteristic weight, thus fixing the weight ratio of one element to the
other. In addition he said that chemical reactions involved the rearrangement of
combinations of those atoms.
So that, briefly, is Dalton's theory. With modifications, it has stood up pretty well to
the criteria that we talked about earlier. It did not convince everyone right away
however. Although a number of chemists were quickly convinced of the truth of the
theory, it took about a half century for the opposition to die down, or perhaps I
should say die off.
Let me point out again the difference between a model of atoms and a theory of
atoms. A model focuses on describing what the atoms are like, whereas the theory
not only talks about what the atoms are like but how they interact with one another
and so forth. Dalton's model was that the atoms were tiny, indivisible,
indestructible particles and that each one had a certain mass, size, and chemical
behavior that was determined by what kind of element they were. We will use that
model of an atom for now, but we will modify it considerably in a later lesson.
1897 - J.J. Thompson



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.

Electron - (originally called corpuscles by Thompson) particles


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.
1899 - Ernst Rutherford
Studied absorption of radioactivity.

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 .
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
1885-1962 Niels Bohr
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.[22] 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.[22] 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).[22]
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.
Modern Atomic Theory
According to the development of science and technology, then
the theories about atom also develops. Hence, in the next development
Bohr’s atomic theory also has several weaknesses. The discovery of
electrons, proton, and neutron bases the birth of the modern atomic
theory. However, not all of the prior atomic theories are incorrect, it
may be needed a given repairing and completing.
Based on the modern atomic theory, atom is composed by the
nucleus which is positively charged and electron orbiting the nucleus a
certain path (atomic shell) and the electron has the wave properties.
Basically, the modern atomic nucleus is the development of Bohr’s
atomic theory contributed by many scientists, such as De Broglie,
Wolfgang Pauli, Erwin Schrodinger, and Warner Heisenberg.