The Evolution of the Atomic Theory

The Evolution of the Atomic Theory

Rob Congrove

B4 10/23/00

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

1803.

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

indestructable particles.

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

different masses.

4. Compounds are formed by the joining of atoms of two or more

elements." 1

All of Dalton's ideas account for the laws of definite

and multiple

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