Daniel Golland
Chemistry
Dr. J
November 24, 2008
Evolution of The Atom
It all started long ago when a Greek philosopher thought, “I bet you anything that
everything that exists is made of small, indestructible particles.” His name was
Democritus and he shall forever be remembered as the first one to come up with the idea
that matter is made up of atoms. However, he did not draw that conclusion through
scientific experiments; he merely pondered. It was Dalton, around the year 1800, who
truly proved Democritus’s idea. And so, the first model of the atom was born from his
research. After his big step into the field of chemistry, many more followed (most
notably: J.J. Thompson, Ernest Rutherford, James Chadwick, and Niels Bohr). They built
on Dalton’s initial atomic model and made it into an intensely complex diagram that only
few can understand.
Dalton’s model of the atom was very simple. It was an extremely miniscule chunk
of matter. However, Dalton’s claim to fame is not the simplicity of his model of atomic
structure, but rather what that meant. From the idea that all matter is made of atoms, he
made 5 conclusions:
1. Elements are made of tiny particles called atoms.
2. All atoms of a given element are identical.
3. The atoms of a given element are different from those of any other element; the
atoms of different elements can be distinguished from one another by their
respective relative weights
4. Atoms of one element can combine with atoms of other elements to form
chemical compounds; a given compound always has the same relative numbers of
types of atoms.
5. Atoms cannot be created, divided into smaller particles, nor destroyed in the
chemical process; a chemical reaction simply changes the way atoms are grouped
together.
Those 5 conclusions are the basis for chemistry as we know it. However, Dalton’s model
of the atom did not explain everything. And, two of his rules have been proven wrong.
His second rule was proven wrong by J.J. Thompson
J.J. Thompson’s model was the first to have “electrons,” these new, negatively
charged particles. He conducted a series of experiments using a cathode ray tube, in
which, electrons jump between two metal plates in a vacuum to complete a circuit. He
concluded that the atom was like plum pudding where the negative electrons were
floating in positive cloud like plums are in pudding. With this new model, interactions
between elements could be explained because this model had magnetic charges.
However, this model was disproved by Ernest Rutherford.
Rutherford directed his famous “gold foil experiment,” in which, he shot alpha
rays at a very thin sheet of gold foil. The expected result, based on the plum pudding
model, was that the rays would go through the foil without any trouble. However,
Rutherford found that some rays were deflected a bit to the side and some rays were even
deflected backwards. He concluded, therefore, that the atom must have a dense, positive
core with electrons orbiting outside of it, to balance the charge. He also concluded that
most of the mass of an atom was in the nucleus. Rutherford, however, did not do much
research on electrons. His model, however, said nothing about neutrons. Those were
discovered by James Chadwick.
Although the neutron has no charge, it drastically affects the mass of an atom
(since it has about the same mass as a proton) and it can affect an atom’s properties.
Isotopes are atoms with the same number of protons, but a different number of electrons.
Different isotopes have different properties. For example, carbon-12 is stable and found
everywhere in nature. Carbon-14, on the other hand is radioactive and is not abundant in
nature. Neutrons, though, were discovered after Niels Bohr added to Rutherford’s model
of the atom.
Bohr did research focused mainly on the electrons and their organization in the
atom. He laid the basis for quantum mechanics by saying that electrons are organized in
definite energy levels around the nucleus, and not in a continuous fashion. This model
explained the emission of light when atoms were energized. More importantly, though, it
allowed for the valence electron idea to exist. All interactions between elements are
caused by valence electrons. Essentially, Bohr’s model explained a lot of the
unexplainable because it was quantum based.
As one can see, the atom began as a simple chunk of matter to a complex machine
similar to that of a solar system. Dalton’s idea was simple. J.J. Thompson’s was a little
more complicated, Rutherford’s was even more complicated, Chadwick’s likewise, and
Bohr’s was almost as wild as Democritus’s idea must’ve been to the Greeks. The atom
has evolved and has grown more complex. Through each new model, phenomena were
explained. And, the atomic model will continue to evolve to explain phenomena not yet
known.