(solved)Question 2.22 of NCERT Class XI Chemistry Chapter 2

Calculate the electrons for each species:

Z = 11 for Na $\displaystyle \therefore $ Na+ has 11 - 1 = 10 electrons

Z = 19 for K $\displaystyle \therefore $ K+ has 19 - 1 = 18 electrons

Z = 12 for Mg $\displaystyle \therefore $ Mg2+ has 12 - 2 = 10 electrons

Z = 20 for Ca $\displaystyle \therefore $ Ca2+ has 20 - 2 = 18 electrons

Z = 16 for S $\displaystyle \therefore $ S2- has 16 + 2 = 18 electrons

Z = 18 for Ar $\displaystyle \therefore $ Ar 18 electrons

$\displaystyle \therefore $ (1) isolectronics are Na+, Mg2+

and (2) isolectronics are Ar, K+, Ca2+, S2-