Though ionisation energy, generally increases from left to right in a period, but elements of II A and V A groups have higher ionisation than corresponding III A and VI A group elements (true upto IVth period). The reason is their comparatively stabler electronic configuration ( $ n{{s}^{2}} $ and $ n{{s}^{2}}n{{p}^{3}} $ respectively.) Hence, in the light of above discussion, we can conclude that ionisation potential of phosphorus should be the highest among given element and it is found so $ I{{E}_{1}} $ for P= 1012 kJ $ \text{mo}{{\text{l}}^{-1}} $ $ I{{E}_{1}} $ for S = 999 kJ $ \text{mo}{{\text{l}}^{-1}} $