The History of... 

A story of matter

 By Lisa Carrol and Anh-Thu Nguyen (HPS 2002-030

once upon a time...

There lived a Greek philosopher named Decmocritus two thousand years ago. He pondered whether matter could be divided into smaller and smaller pieces forever. He came to the conclusion that there must be some end to the division. He called this piece the atom, which comes from the Greek word "atomo," meaning "indivisible," or "not the be cut." His theories consisted of beliefs such as the properties of matter are determined by the shapes and sizes of the atom.

For example, the water atom is round, which causes the substance to be slippery or fire would consist of sharp, pointed atoms that is responsible for fire's sting or burn.

 

In the 1800s, John Dalton, an English chemist, formed four basic ideas which he called The Atomic Theory. They consisted of the following:
~All elements are composed of atoms. Atoms are indivisible and indestructible particles.
~Atoms of the same element are exactly alike.
~Atoms of different element are different.
~Compounds are formed by the joining of atoms of two or more elements.

Dalton's Atom

Dalton believed that the atom was a plain sphere and was the smallest particle.

 

In 1897, Joseph John Thomson, an English scientist, proposed that the atom was made of smaller particles. The particles are negatively charged, lying inside a positively charged material, causing the atom to be neutral. He concluded this by studying the passage of an electric current through a gas. The gas was thought to be made of uncharged atoms, but gave off rays that were made of negatively charged particles. Afterwards, Thomson called this the plum-pudding model.
This is Thomson's plum pudding model. The black dots are negatively charged particles, which he called corpuscles, now known as elctrons. The yellow is the positively charged material.

 

 In 1911, a British physicist named Ernest Rutherford made a new discovery. He performed an experiment using a lead box, particle emitter, florescent screen, thin gold sheet, and another screen. He fired tiny positively charged "bullets" through the gold atoms in the sheet of foil and found that they didn't change course at all, which meant that the gold atoms were mostly empty space. This proved J.J. Thomson's theory to be wrong, and that atoms were not a pudding filled with positively charged material. However, not all of the bullets went through the gold sheet; some of them had bounced back, as though as it had hit a solid. This indicated that since positive charges repel positive charges.

He then proposed that atoms have small, dense, positively charged centers. He called this center the nucleus, which is very tiny in comparion to the atom as a whole. An exmple of this is a bee in a football stadium! While the positively charged particles were all contained in the nucleus, the negetive electrons were scattered along the atom's edge.

 

An improvement on Rutherford's model was made by Neils Bohr in 1913. Being an intellectual Danish scientist, he believed the electrons moved in definite orbits around the nucleus. The orbits, also called energy levels, can be found at fixed distances from the nucleus.

 

 

Researchers have now found that the atom is structured differently than the above scientists have predicted.

 

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