Which of the elements Na, Mg, Si and P would have the greatest difference between the first and second ionisation enthalpies. Briefly explain your answer.
Among Na, Mg, Si and P, Na being alkali metals has only one electron in the valence shell, therefore has very low ∆ iH1. However, after the removal of one electron, it acquires nearest inert gas or neon gas configuration, i. e., Na+ (${{1s}^{2}}$, ${{2s}^{2}}$, ${{2p}^{6}}$). Therefore, its ∆i H2 is expected to be very high. Consequently, the difference in first and second ionisation enthalpies would be greatest in case of Na.
However, it may be noted here that in case of Mg, Si and P, although their ∆ iH1 will be much higher than that of Na but their ∆i H2 will be much lower than that of Na. As a result, the ” difference in their respectively, ∆ iH1 and ∆i H2 would be much lower than that of Na.