In an atom first four shells (K, L, M and N) are completely filled, then what is the total number of electrons in that atom?

Identify the isotopic pair(s) out of the following species.

${}_8\mathrm{C}^{16}, {}_6\mathrm{C}^{14}, {}_7\mathrm{N}^{14}, {}_8\mathrm{O}^{18}, {}_7\mathrm{N}^{16}$

Write the electronic configuration of the ions of the electrovalent products of the following reaction.

$2\mathrm{Na} + {\mathrm{Cl}}_2 \to 2{\mathrm{Na}}^{+}{\mathrm{CI}}^{-}$