Which of the following will have the most negative electron gain enthalpy and which the least negative?
$P, S, Cl, F$
Explain your answer.

(A) Electron gain enthalpy generally becomes more negative across a period as we move from left to right.
Within a group,electron gain enthalpy becomes less negative down a group.
However,adding an electron to the $2p$-orbital (as in $F$) leads to greater inter-electronic repulsion than adding an electron to the larger $3p$-orbital (as in $Cl$).
Therefore,$Cl$ has the most negative electron gain enthalpy.
Among the given elements,$P$ has the least negative electron gain enthalpy due to its stable half-filled $3p^3$ electronic configuration.