Calculate the number of states per cubic metre of sodium is 3s band. The density of sodium is 1013 kg m–3. How many of them are empty?
Electronic configuration of Sodium atom is → 1s22s22p63s1
Each 3s orbit will have two state and one of each would be occupied,
∴ in N atoms of sodium there will be 2N states possible states and N states will be empty.
Given:
mass of the sodium in 1 m3 = 1013 kgm-3
∴ the total number of atoms
= 
Therefore, total number of states
And total unoccupied states are 
1