The different forms of atomic models are :
Thomson’s Model of Atom
An atom is electrically neutral. It contains positive charges (due to the presence of protons), as well as, negative charges (due to the presence of electrons). Hence, J J Thomson assumed that an atom is a uniform sphere of positive charges with electrons embedded into it.
Thus, the atom may be visualized as a pudding or cake of positive charge with raisins (electrons) embedded into it. Hence, this model of atom later became known as the ‘raisin pudding’ model.
This model of atom explains the electrical neutrality of atom, but it failed to stand for a long time as it was not consistent with the experimental results obtained by Rutherford.
Rutherford’s Model of Atom (Discovery of Nucleus)
The speculation as to how protons and electrons are placed inside the atom was a matter of debate for a long time. However, the a-particle scattering experiment carried out by Rutherford in 1911 gave evidence in Support of the existence of nucleus in an atom.
Alpha-particles are doubly charged helium atoms (He++). Thus, alpha-particles are positively charged particles having two units of positive charge and 4 units of mass.
Drawbacks of Rutherford’s model :
The Rutherford’s nuclear model of atom suffers from the following defects:
(i) The electron revolving around the nucleus will be accelerated. According to the electromagnetic theory, if a charged particle (electron) is accelerated around an oppositely charged particle (nucleus), the electron would continuously radiate or lose energy. So, the speed of the election will decrease and hence it would gradually come closer to the nucleus. Thus, the electron would fall into the nucleus after following a spiral path. This will make the atom unstable. But, in reality, this does not happen. Thus, Rutherford’s model does not explain the stability of the atom.
(ii) Rutherford proposed that electrons revolve around the nucleus in fixed orbits. However, the number of electrons in each orbit was not specified.