Development of the Atomic Model

Democritus

400 BCE

Democritus stated atoms were 'miniscule quantities of matter'. Democritus theorised that on earth there were the atoms and the void, and the atoms moved within the void. He called them "atomos", Greek for 'undivisible'. Meaning that was the smallest size they could be.

John Dalton

1804

Dalton's atomic theory was that
- Atoms are indivisible and indestructable
- All atoms of a given element are identical in mass and properties
- A chemical reaction is a rearrangement of atoms
- Compounds are formed by a combination of atoms

JJ Thomson

1897

In 1897, JJ Thomson was investigating cathode rays and discovered atoms were made of pieces with negative and positive charges. And that the negative charged "corpsucles" (electrons) were much smaller in comparison to the positive electrons.
He put the atoms in a model known as the "plum pudding", where the positive electrons are the pudding and the negative were plums on top.

Ernest Rutherford

1911

Rutherford directed the Geiger-Marsden experiment in 1909, which disproved Thomson's atomic model theory.
He created the design of the positive electrons being together, called a nucleus, surrounded by negative electrons.
Much like a solar system

Niels Bohr

1913

Bohr developed Rutherford's model and discovered that electrons travel in separate orbits around the nucleus. And that the number of electrons determines the properties of an element.

James Chadwick

1932

Chadwick discovered the neutron of an atom. Before this discovery, people believed atoms were composed of a nucleus of positive neutrons, orbited by negative electrons. Neutrons were neither positive nor negative, but contributed to the weight of the atom.