Question

The increasing order of the first ionisation enthalpies of the elements B, P, S and F (lowest first) is

(a) F < S < P < B 

(b) P < S < B < F

(c) B < P < S < F 

(d) B < S < P < F.

Answer 1

(d) : Element:    B        S      P         F
         I.E. (eV): 8.3    10.4  11.0    17.4
In general as we move from left to right in a period, the ionisation enthalpy increases with increasing atomic number. The ionisation enthalpy decreases as we move down a group. P(1s² 2s² 2p6 3s² 3p³ ) has a stable half filled electronic configuration than S (1s² 2s² 2p⁶ 3s² 3p⁴ ). For this reason, ionisation enthalpy of P is higher than S.