Why atomic size increases on going down the group? Arrange the following in decreasing order of their atomic size- Na, Li, Rb, Cs, K.

Potassium, bromine and krypton are elements in period $4$ of the Periodic Table. 

Bromine exists as a molecule. Draw a 'dot-and-cross' diagram to show the bonding in a molecule of bromine.

Potassium, bromine and krypton are elements in period $4$ of the Periodic Table. 

Krypton does not react with either potassium or bromine. Explain the unreactive nature of krypton.