(i) The respective electronic configurations are : (a) $\mathrm{H}^{-}$ → $1s^{2}$ (b) $\mathrm{Na}^{+}$ → $1s^{2}\,2^{s2}\,2p^{6}$ (c) $\mathrm{O}^{2-}$ → $1s^{2}\,2s^{2}\,2p^{6}$ (d) $\mathrm{F}^{-}$ → $1s^{2}\,2s^{2}\,2p^{6}$ (ii) (a) $3s^{1}$ → $1s^{2}\,2s^{2}\,2p^{6}\,3s^{1}$ ∴ The atomic number of element = 11 (b) $2p^{3}$ → $1s^{2}\,2s^{2}\,2p^{3}$ ∴ The atomic number of element = 7 (c) $3p^{5}$ → $1s^{2}\,2s^{2}\,2p^{6}\,3s^{2}\,3p^{5}$ ∴ The atomic number of element = 17 (iii) (a) $[\mathrm{He}]\,2s^{1}$ → Lithium, Li (b) $[\mathrm{Ne}]\,3s^{2}\,3p^{3}$ → Phosphorus, P (c) $[\mathrm{Ar}]\,4s^{2}\,3d^{1}$ → Scandium, Sc