History and Models of the Atom

Haley Spencer

Idea of Atoms

The idea of the modern atom came from two Greek philosophers, Democritus and Leucippus, in the fifth century B.C. Although the thought has been varied many times since then, we now know that atoms are made of a positively charged nucleus in the center surrounded by negatively charged electrons.

Dalton Model

John Dalton proposed that all matter is composed of very small things called atoms. This was not a new idea since the ancient Greeks had a concept that all matter was composed of very small, indivisible objects. When Dalton proposed his model nucleus and electorns were unknown.

Dalton Model

Thompson Model

When the electron was discovered by J.J Thompson in 1897 people realized that atoms where made of even smaller particles than we had already thought. When this discovery was made the "Plum Pudding" model was put forward in 1904. In this model, the atom is made up of negative electrons that float in a “soup” of positive charge, much like plums in a pudding or raisins in a fruit cake.

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _

I
I
I
I
I
I
I
I

= Electron

Thompson Model

'Soup' of

+ Charge

Rutherford Model

Rutherford experimented with some theories which led to a change in ideas around the atom.  His new model described the atom as a small, dense, positively charged core, called a nucleus, surrounded by lighter, negatively charged electrons.

Nucleus

= Electrons

Bohr Model

Niels Bohr proposed that the electrons could only orbit the nucleus in certain special orbits at different energy levels around the nucleus.

Electron

Orbit

Nucleus

Sources

http://www.nobeliefs.com/atom.htm