Question

Addition of an electron to $N{a_{(g)}}$ is a slightly exothermic process, whereas addition of electron to $M{g_{(g)}}$ is strongly endothermic. Explain:

Answer 1

First of all we have to check the electronic configuration of both Na and Mg. If the subshell is completely filled or half filled, it will be more stable due to its symmetry and exchange energy. If an orbital is partially filled, then it will have some tendency to gain electrons to attain stability. But the energy for this depends on the ease of addition or removal of electrons in that particular subshell.
Exothermic process is the process that releases energy and an endothermic process is the process in which energy has to be supplied or the process that absorbs energy.

Complete step by step answer:
Na is a S block element having atomic number 11. So its electronic configuration is $1{s^2}2{s^2}2{p^6}3{s^1}$ . Here last subshell 3s have only 1 electron. We know that the maximum number of electrons that can accommodate in the s subshell is 2. So by the addition of an electron to it, the outer subshell becomes completely filled and hence attains stability. Actually, Stability relates with decrease in energy. So some energy will get slightly released during this addition. Then we can call it an exothermic process. That is for a stable system the energy will be less when compared to an unstable system.
Atomic number of Mg is 12. So its electronic configuration is $1{s^2}2{s^2}2{p^6}3{s^2}$ The system is stable since its valence subshell 3s contains 2 electrons.
So the system does not favour the addition of an electron. Not only that the added electron goes to the next subshell 3p breaking its stable state. So additional energy is to be supplied to add one electron to Mg. Hence the process is strongly endothermic.
Additional information:
Maximum number of electrons that can be accommodated by s,p,d,f orbitals are 2,6,10,14 respectively. These subshells belong to different shells. Every atom tries to attain stability. If the addition of an electron is accompanied with more release of energy , i.e exothermic. This is because of the attraction between electrons and nuclei. Then the system attains more stability. Then it will be strongly exothermic. The formation of ionic compounds is extremely exothermic in nature.

Note:
Addition of an electron to Na is slightly exothermic instead of strongly exothermic. This is due to the fact that Na has more tendency to lose the one electron in the valence subshell to attain octet electron configuration (completely filled shell) state. Generally it shows only less tendency to gain electrons. Since it attains stability to a small extent, addition of electrons is slightly exothermic.