Question

Addition of an electron to ${Na}_{\left(g\right)}$ is slightly exothermic process, whereas addition of electron to ${Mg}_{\left(g\right)}$ is strongly endothermic. Explain.

Answer 1

${}_{11}Na1s^{2}2s^{2}2p^{6}3s^1{}^{}{}_{12}Mg1s^{2}2s^{2}2p^{6}3s^2$

Addition of electrons to $Na$ makes the $3s$ shell completely filled and becomes stable hence it is slightly exothermic.

Addition of electron to $Mg$ is bit difficult because the $3s$ shell is completely filled and is stable and hence, the energy is to be supplied and is endothermic.