Middle School Physical Science: Atoms - History of the Atom — Flashcards

The matter is made of building blocks that are not divisible. This idea was considered in early the 5th century BCE by Leucippus and Democritus. These particles were called “atomos” by the Greeks which means indivisible and in modern times the word “atom” is derived from this term.
 

Rutherford's Gold Foil Experiments    
Ernest Rutherford discovered that some elements give off positively charged particles, which he named alpha particles (a). He used alpha particles to study atoms. Rutherford aimed a beam of alpha particles at a very thin sheet of gold foil. Outside the foil, he placed a screen of material that glowed when alpha particles struck it.    
If Thomson's plum pudding model were correct, the alpha particles should be deflected a little as they passed through the foil. Why? The positive “pudding” part of gold atoms would slightly repel the positive alpha particles. This would cause the alpha particles to deflect a little from their original path. But Rutherford got a surprise. Most of the alpha particles passed straight through the foil as though they were moving through empty space. Even more surprising, a few of the alpha particles bounced back from the foil as though they had struck a wall. This is called back scattering. It happened only in very small areas at the centers of the gold atoms.    
Based on his results, Rutherford concluded that all the positive charge of an atom is concentrated in a small central area. He called this area the nucleus. Rutherford later discovered that the nucleus contains positively charged particles, which he named protons.