History of the Atomic Theory

Who first thought of the atom?

Democritus Leucippus

Democritus and Leucippus~440 BC

• They were the first philosophers that believed that atoms existed.

• They thought that you could cut something in half , over and over, until you eventually get to a particle so small that it can no longer be cut In half.

• They called this particle “atomos” which is Greek for indivisible.

They thought what?!?

• They reasoned that the solidness of the material corresponded to the shape of the atoms involved.

• Thus, iron atoms are solid and strong with hooks that lock them into a solid;

• water atoms are smooth and slippery;• salt atoms, because of their taste, are sharp and pointed;• and air atoms are light and whirling, • Using analogies from his sense experiences, he gave a

picture or an image of an atom that distinguished them from each other by their shape, their size, and the arrangement of their parts.

Their Atom:

John Dalton

So what did this guy do?

• Dalton proposed that each chemical element is composed of atoms of a single, unique type, and though they cannot be altered or destroyed by chemical means, they can combine to form more complex structures (chemical compounds).

• This marked the first truly scientific theory of the atom, since Dalton reached his conclusions by experimentation and examination of the results in an empirical fashion.

His Atom:

Element 1Element 2

JJ Thomson

So what did this guy do?

• Thomson, in 1897, was the first to suggest that the fundamental unit was more than 1,000 times smaller than an atom, suggesting the subatomic particle now known as the electron.

• He discovered this during his cathode ray tube experiments

• He believed that the smaller sub atomic particles were spread evenly through out the atom. He called these particles “corpuscles”

• His theory is known as the Plumb Pudding model.

His Atom:

Ernst Rutherford

So what did this guy do?

• Along with Hans Geiger and Ernest Marsden in 1909, he carried out the Geiger–Marsden experiment, which demonstrated the nuclear nature of atoms by deflecting alpha particles passing through a thin gold foil.

• Much of an atom's charge (specifically, its positive charge) is concentrated in a relatively tiny volume at the center of the atom, known today as the nucleus

His Atom:

Neils Bohr

So what did this guy do?

• He expanded on Rutherford's theory by using Einstein's quantum theory of light to state that electrons orbit around the condensed nucleus. (which would explain what when energy is released from an atom light is emitted)

• He proposed that electrons orbited the nucleus at different levels.

His Atom:

Erwin Schrödinger

So what did this guy do?

• He predicted many of the phenomena that Bohr's model failed to explain.

• He proposed the wave model of the atom (or cloud model) He proposed a mathematical equation that would help scientists figure out the probable location of an electron within an atom.

• Although this concept was mathematically convenient, it was difficult to visualize, and faced opposition.

His Atom:

James Chadwick

So what did this guy do?

• He discovered a particle with in the atom that had no charge at all.

• He called it the neutron.• He discovered the neutron was in the nucleus

and had nearly the same mass as the protons.

His Atom: