Question

why do elements in 3rd period have lower ionization energy than the 2nd period.

Answer 1

Ionisation energy generally decreases down the group and increase across the period. Going down the group, the no of shells increase that decreases the effective nuclear charge and hence the atom can be removed more easily with less energy given. This is the reason why the ionisation energy of 3rd period is less than ionisation energy of 2nd period.