chemistry

Democritus

442 BCE

held that everything is composed of "atoms", which are physically, but not geometrically, indivisible

Dalton

1803

Elements are made of extremely small particles called atoms

Mendeleev

1869

Mendeleev realized that the physical and chemical properties of elements were related to their atomic mass in a 'periodic' way, and arranged them so that groups of elements with similar properties fell into vertical columns in his table.

J.J Thomson

1897

Thomson believed electrons to be two thousand times lighter that a proton. He assumed that an atom is composed of a cloud of negative charge in a sphere of positive charges

Rutherford

1911

The model described the atom as a tiny, dense, positively charged core called a nucleus, in which nearly all the mass is concentrated, around which the light, negative constituents, called electrons, circulate at some distance, much like planets revolving around the Sun.

Bohr

1913

The Bohr model and all of its successors describe the properties of atomic electrons in terms of a set of allowed (possible) values.

Cloud

1926

It consisted of a dense nucleus surrounded by a cloud of electrons at various levels in orbitals.