Read the statements carefully and choose the correct alternative from the following: 
Assertion: $\mathrm{Na}, \mathrm{K}, \mathrm{Ca}, \mathrm{Mg}$ are metals because they can lose electrons to form positive ions (cations). 
Reason: $\mathrm{F}, \mathrm{Cl}, \mathrm{N}, \mathrm{P}$ are non-metals because they can gain electrons to form anions.

What is the similarity in the electronic structure of the following set of atoms?

(a) Lithium 
(b) Sodium 
(b) Potassium 

(a) Lithium 
(b) Sodium 
(b) Potassium 
Which of the above elements is most reactive and why? 

Identify the pair of isotopes from the following:

${}_8{}^{16}\mathrm{X}, {}_7{}^{16}\mathrm{X}, {}_8{}^{17}\mathrm{X}$