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.

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(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.

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