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

The correct option is D B < S < P < F

$\begin{array}{ccccc}Element:& B& S& P& F\\ I.E.\left(eV\right):& 8.3& 10.4& 11.0& 17.4\end{array}$
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. $P\left(1s^{2}2s^{2}2p^{6}3s^{2}3p^3\right)$ has a stable half filled electronic configuration than $S\left(1s^{2}2s^{2}2p^{6}3s^{2}3p^4\right).$ For this reason, ionisation enthalpy of P is higher than S.