Call

a) electronic configuration of Mo

1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d5 5s1

b) The ionization energy increases due to the increase in the nuclear charge with an atomic number at the beginning of the series. Gradually, the shielding effect of the added electrons also increases. ... These two opposing factors lead to a rather gradual increase in the ionization energies in any transition series.