Question

How would you account for the irregular variation of ionization enthalpies (first and second) in the first series of the transition elements?

Answer 1

Ionization enthalpies are found to increase in the given series due to a continuous filling of the inner$-d$ orbitals. The irregular variation of ionization enthalpies can be attributed to the extra stability of configuration such as $d^0,d^5,d^{10}$. Since these states are exceptionally stable, their ionization enthalpy are very high.