The correct values of ionization enthalpies (in kJ mol-1 ) of Si, P, Cl and S respectively are :

I. E. gradually increases due to increase in Zeff on valence shell on moving left to right across the period but with some irregularities. Half-filled and full-filled electronic configuration have higher I.E. along a period. Correct increasing order of I.E. is Si < S < P < Cl.