After the discoveries of electron, proton and neutron, scientists made efforts to understand their arrangement inside an atom. The arrangement of subatomic particles inside an atom is called an atomic model.
Thomson model : The British physicist J J Thomson presented his atomic model in 1898 which is also called watermelon model or raisin pudding model. According to this model :
1. The positive charge is uniformly distributed inside an atom.
2. The negative charge i. e. electrons are distributed in such a way that the atom becomes neutral.
Limitations of Thomson model : Thomson model was able to describe the neutrality of an atom but it didn’t answer many other questions like –
Why do only electrons involve in a chemical reaction?
Why are there electric and magnetic fields of an atom?
Rutherford’s nuclear atomic model : Rutherford presented his model in 1911 on the basis of his alpha scattering experiment.
Experiment : In this experiment a very thin (about 100 nm) gold foil was taken and it was bombarded with α particles. Gold foil was surrounded by a fluorescent screen of ZnS. A tiny flash of light is observed when α particles strike the screen.
Observations : In the experiment Rutherford made following observations :
I. Maximum α-particles pass through the foil undeflected.
II. A few α-particles are deflected with small angles.
III. A very few α-particles (1 in 20000) bounced back.
Conclusions : On the basis of above observations, Rutherford made some conclusions given below :
I. Most of α-particles pass without any deflection. It means that maximum space inside an atom is vacant.
II. Few α-particles are deflected. It means that there is a small positive charge in the centre of an atom and α-particles face repulsion.
Bohr’s atomic model for Hydrogen: In 1913, Neils Bohr presented his model for Hydrogen atom on the basis of his research and experiments. The postulates of his atomic model are as follows:
1. The electron moves on a circular path called an orbit. Orbits can also be called stationary states or energy states because they have fixed energy. All orbits are concentric and have fixed radius with nucleus as the centre.
2. In an orbit, the energy of an electron is fixed. But, when it jumps from the lower energy state to the higher one, it absorbs energy. It loses energy when it falls from the higher energy state to the lower one.
Quantum mechnical model : To answer many questions which were not entertained by Bohr’s model, another model was presnted by Shrodinger which is called Quantum mechanical model. The postulates of this model were as follows :
1. Electrons show dual nature i.e. particle and wave nature.
2. To find the correct location and velocity of an electron simultaneously is not possible.