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Which of the following represents the correct order of increasing electron gain enthalpy with negative sign for the elements O, S, F and Cl?
A
S < O < Cl < F
B
Cl < F < O < S
C
O < S < F < Cl
D
F < S < O < Cl
Detailed Solution
Halogens (group 17) have more negative electron gain enthalpies than the chalcogens (group 16) of the same period, since they need only one electron to reach a noble gas configuration.
Within a group the value normally becomes less negative down the group, but the second period elements O and F are exceptions: their very small size causes strong electron–electron repulsion in the compact 2p subshell, so the added electron is held less strongly.
Hence Cl is more negative than F, and S is more negative than O.
Values (kJ $mol^{-1}$): O = −141, S = −200, F = −328, Cl = −349.
Increasing order of negative electron gain enthalpy: O < S < F < Cl
Within a group the value normally becomes less negative down the group, but the second period elements O and F are exceptions: their very small size causes strong electron–electron repulsion in the compact 2p subshell, so the added electron is held less strongly.
Hence Cl is more negative than F, and S is more negative than O.
Values (kJ $mol^{-1}$): O = −141, S = −200, F = −328, Cl = −349.
Increasing order of negative electron gain enthalpy: O < S < F < Cl