How many maximum ionisation energy can boron have? a) 1 b) 3 c) 5 d) none.

Beryllium has two electrons in its outermost orbital which is 2s orbital. The electrons in the 2s orbital are already paired and hence it is quite a stable configuration for the beryllium atom. Hence the first ionization enthalpy of beryllium is higher than that for boron. However, if one would consider the second ionization enthalpy of beryllium and boron, the one for boron would be higher as after the removal of first electron of Boron from 2p1 orbital, the next electron would be in the 2s orbital, where it would be already paired with another electron and removing it would be more difficult.