• # question_answer 85)   How would you explain the fact that first ionisation enthalpy of sodium is lower than that of magnesium but its second ionisation enthalpy is higher than that of magnesium?

The first ionisation enthalpy of sodium is lower than that of magnesium as in both the atoms, the electron is to be removed from 3s-orbital but in sodium the nuclear attraction is less than magnesium as atomic number of sodium is 11 and that of magnesium is 12 (or atomic radius of Na is more than atomic radius of Mg). After the removal of one electron from each atom, the ions are formed. $Na:1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{1}};$  $N{{a}^{+}}:1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}$ $Mg:1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}};$ $M{{g}^{+}}:1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{1}};$ Sodium ion acquires the configuration of an inert gas which is a stable configuration, i.e., all the 2p-orbitals are completely filled. It is difficult to remove the second electron from this ion as it requires very high energy, i.e., second ionisation energy is very high. In the magnesium ion, the second electron is to be removed from 3s-orbital which is bit easier. Thus, second ionisation enthalpy of Na is much higher than the second ionisation energy of Mg.
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