State all the exceptions to the general trend of Ionization enthalpy with reasons.

Dear Student,

Ionisation enthalapy increases on going from left to right across a period as the effective nuclear charge increases on going from left to right. 
It decreases on going from top to bottom along a group due to increasing atomic radius and so less effective nuclear charge is experienced by the last electron.

Exception:

1. Boron has lower ionisation enthalpy than beryllium and oxygen has lower ionisation enthalpy than nitrogen. This is due to their electronic configuration. Beryllium has full filled electronic configuration and nitrogen has half filled electronic configuration and so they are more stable and hence require higher energy to remove an electron, hence high ionisation enthalpy. 
2. The first series of transition elements show irregular behaviour of ionisation energy along the periods due to shielding effect of d-electrons.