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...