The present concept of atom was given by Neils Bohr. The bohr’s model of atom can be described as follows:

1) An atom is made up of three particles, called as electrons, protons and neutrons.

2) The protons and neutrons are located in a small nucleus at the centre of the atom.

3) The electrons revolve rapidly round the nucleus in fixed circular paths called energy levels or shells.

4) There is a limit to the number of electrons which each energy level (or shell) can hold.

5) Each energy level (or shell) is associated with a fixed amount of energy.

6) There is no change in the energy of electrons as long the keep revolving in the same energy level, and the atom remains stable.