The five atomic theorys of the past two centuries represent the sudden
advancement of science in modern times. Beginning with a basic theory on the
behavior of atoms to the current model, some changes have been made, and
some ideas are still the same. Ancient Greek philosophers believed that
everything was made up of invisible particles called atmos. Since then the
theory of atoms did not progress until
John Dalton was the first scientist to compose a theory of matter based
on atoms. Dalton's atomic theory is based on four concepts. He stated:
"1. All elements are composed of atoms, which are indivisable and
2. All atoms of the same element are exactly alike; in particular, they
have the same mass.
3. Atoms of different elements are different; in particular, they have
4. Compounds are formed by the joining of atoms of two or more
All of Dalton's ideas account for the laws of definite
proportions and the law of conservation of mass. Some of Dalton's points are
still thought to be true, but over time this original
theory has been modifyed.
The first of these modifications came in 1897 when J.J. Thomson discovered
the electron. Based on the work of William Crookes and his "Crookes tube"
(Cathode-ray tube), Thomson discovered a negative charged particle was the
cause of the light produced by the cathode-ray tube. He also discovered that
these particles are present in all elements. These cathode-ray particles are
now known as electrons. Soon after the discovery of electrons the proton
was discovered. This led Thomson to conclude that ther were an equal
number of both particles present in the atom.
Twelve years later Lord Ernest Rutherford was experimenting with
alpha particles. He shot a stream of them at a piece
of gold foil surrounded
by zinc-sulfide. When an alpha particle strikes ZnS it produces a flash of
light. The particles mostly stayed in a constant stream through the foil, but a
few were deflected. This led Rutherford to believe
that there must be a small,
dense cluster of protons in the middle of the atoms to deflect the small
number of particles.
Neils Bohr was the next physicist to advance the atomic theory. He
explained what Rutherford could not about how the electron could stay in
orbit around the nucleus. When the electron has little energy it is closer to the
nucleus, when it absorbs more energy it travels farther from the nucleus.
There is a definite
number of electrons that can be in the same orbit. When
the orbits closest to the nucleus are filled the atom is at a ground state. When
the electrons become charged they move into a higher orbit and are then at an
exited state. When the electrons move into a closer orbit they relese a photon
This model for the atom became insufficient. The charged-cloud model
was born. It was the research of many scintists that led to this. Instead of
showing the position and orbit of the electron, it shows the most probable
locations. The orbits from the Bohr model are divided into sub-orbits, but
ther is no way of showing the infinite amount of sub-orbits or which orbit the
elecron would be in so the most dense place in the cloud is the most probable
location of an electron.
The Atomic theory has gone through many changes since Dalton's time,
but two of his ideas are still true. What we know about atoms today could be
proved entirely wrong tomorrow
just like Dalton. The atomic theory is still
just a theory and some day it might be proved or disproved.