John Dalton proposed that all matter is composed of very small things which he called atoms. This was not a completely new concept as the ancient Greeks (notably Democritus) had proposed that all matter is composed of small, indivisible (cannot be divided) objects. When Dalton proposed his model electrons and the nucleus were unknown.
After the electron was discovered by J.J. Thomson in 1897, people realized that atoms were made up of even smaller particles than they had previously thought. However, the atomic nucleus had not been discovered yet and so the “plum pudding model” was put forward in 1904. In this model, the atom is made up of negative electrons that float in a “soup” of positive charge, much like plums in a pudding or raisins in a fruit cake .In 1906, Thomson was awarded the Nobel Prize for his work in this field. However, even with the Plum Pudding Model, there was still no understanding of how these electrons in the atom were arranged.
Chemist and physicist Ernest Rutherford was born August 30, 1871, in Spring Grove, New Zealand. A pioneer of nuclear physics and the first to split the atom, Rutherford was awarded the 1908 Nobel Prize in Chemistry for his theory of atomic structure. Dubbed the “Father of the Nuclear Age,” Rutherford died in Cambridge, England, on October 19, 1937 of a strangulated hernia.
Bohr developed the Bohr model of the atom with the atomic nucleus at the centre and electrons in orbit around it, which he compared to the planets orbiting the Sun. He helped develop quantum mechanics, in which electrons move from one energy level to another in discrete steps, instead of continuously.