In general : (i) First ionisation energy (I.E ‌1 ) or enthalpy, increases along the period from left to right, but due to half-field configuration of P -atom, it has higher I.E ‌1 than of S-atom. (ii) IE1 decreases as we move down the group from top to bottom. (iii) More be the size of atom, less be the IE1. Hence, correct order of IE1 is B<S<P<F. IE1 for (in kJmol−1 ) B=800 S=999.4 P=1012 F=1680.8