(a) Bohr Model of atom: 

An atom is made up of three particles: Electrons, neutrons and protons. 

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

2. The electrons revolve rapidly around the nucleus at the centre of the atom 

3. There is a limit to the number of electrons that each energy level can hold 

4. Each energy level is associated with a fixed amount of energy. 

5. There is no change in the energy of electrons as long as they keep revolving in the same energy level. 

Bohr explained the stability through the concept of revolution of electrons in different energy levels. The change in the energy of an electron occurs when it jumps from lower to higher energy levels. When it gains energy, it excite from lower to higher and vice versa. Thus energy is not lost and the atom remains stable. 

(b) Given: 

Atomic no. = 11 

Mass no. = 23 

Then, electronic configuration – (2,8,1) 

Nuclear composition is – 11 protons and 12 neutrons