How would you explain the fact that the first ionization enthalpy of sodium is lower than that of magnesium but its second ionization enthalpy is higher than that of magnesium?
The electronic configurations of Na and Mg are:
11Na ; 1s22s22p63s1.
12Mg : 1s22s22p63s2
The first ionization enthalpy of sodium is lower than that of magnesium because the nuclear charge of Na (+11) is lower than that of Mg (+12) and size of Na is lower than Mg. After the loss of first electron, the electronic configuration of Na+ is 1s22s12p6 (has noble gas configuration) and Mg+ is 1s22s22p63s1. Hence removal of the second electron from Na+ is very difficult as compared to Mg+ (electron to be removed from 3s). Thus second ionisation enthalpy of sodium is higher than that of magnesium.
