The Evolution Of The Atomic Theory

The Evolution of the Atomic Theory

The five atomic theorys of the past two centuries represent the sudden

advancement of science in modern times. Begining 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 belived that

everything was made up of invisible particles called atmos. Since then the

theory of atoms did not progress untill 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 definate 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 origional 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 peice 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 belive 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 definate 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.