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Which one of the following arrangements represents the correct order of electron gain enthalpy (with negative sign) of the given atomic species :-
A
F < Cl < O < S
B
S < O < Cl < F
C
O < S < F < Cl
D
Cl < F < S < O
Detailed Solution
The negative electron gain enthalpy generally increases across a period and decreases down a group.
However, O and F, the second period elements, have less negative electron gain enthalpies than S and Cl, the elements just below them.
This is because O and F are very small; the added electron goes into the compact 2p subshell and suffers strong repulsion from the electrons already present.
In S and Cl the added electron enters the larger 3p subshell, where the repulsion is much smaller.
Approximate values (kJ/mol, with negative sign): O = 141, S = 200, F = 328, Cl = 349.
So the order is O < S < F < Cl.
However, O and F, the second period elements, have less negative electron gain enthalpies than S and Cl, the elements just below them.
This is because O and F are very small; the added electron goes into the compact 2p subshell and suffers strong repulsion from the electrons already present.
In S and Cl the added electron enters the larger 3p subshell, where the repulsion is much smaller.
Approximate values (kJ/mol, with negative sign): O = 141, S = 200, F = 328, Cl = 349.
So the order is O < S < F < Cl.