The discovery of the atom as we know it today was a progression, like the discovery of DNA or any other major scientific discovery. Many scientists contributed tothe development of present day atomic theory. Each proposed model of the atom was based on the models developed prior to it. With each new discovery dealing with the nature of theatom, a new atomic model was constructed. The first person to believe in the existence of the atom was Democritus in 400 B.C.
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 somedistance, much like planets revolving around the Sun. The Rutherford atomic model has been alternatively called the nuclear atom, or the planetary model of the atom.
TheBohr model of the atom, a radical departure from earlier, classical descriptions, was the first that incorporated quantum theory and was the predecessor of whollyquantum-mechanical models. The Bohr model and all of its successors describe the properties of atomic electrons in terms of a set of allowed (possible) values. Atoms absorb or emit radiation only whenthe electrons abruptly jump between allowed, or stationary, states.
Joseph John Thomson, who had discovered (1897) the electron, a negatively charged part of everyatom. Though several alternative models were advanced in the 1900s by Lord Kelvin and others, Thomson held that atoms are uniform spheres of positively charged matter in whichelectrons are embedded. Popularly known as the plum-pudding model, it had to be abandoned (1911) on both theoretical and experimental grounds in favour of the Rutherford atomic model