Looking for classes? Ksquare Career Institute, Bengaluru →
In which of the following options the order of arrangement does not agree with the variation of property indicated against it?
A
B < C < N < O (increasing first ionisation enthalpy).
B
I < Br < Cl < F (increasing electron gain enthalpy).
C
Li < Na < K < Rb (increasing metallic radius).
D
$Al^{3+} < Mg^{2+} < Na^+ < F^-$ (increasing ionic size).
Explanation
N > O in IE (half-filled) and Cl > F in electron gain enthalpy.
Detailed Solution
First ionisation energy: half-filled orbitals are more stable than partly filled ones, so the correct order is B < C < O < N; the order B < C < N < O does not agree.
Electron gain enthalpy: though F is more electronegative than Cl, inter-electronic repulsion in F is more than in Cl, so the order is I < Br < F < Cl; the order I < Br < Cl < F does not agree.
Down the group, atomic radius increases due to addition of an extra shell: Li < Na < K < Rb (correct).
For isoelectronic species, ionic size: $Al^{3+} < Mg^{2+} < Na^+ < F^-$ (correct).
Note: the source key accepts both the ionisation enthalpy order and the electron gain enthalpy order as answers.
Electron gain enthalpy: though F is more electronegative than Cl, inter-electronic repulsion in F is more than in Cl, so the order is I < Br < F < Cl; the order I < Br < Cl < F does not agree.
Down the group, atomic radius increases due to addition of an extra shell: Li < Na < K < Rb (correct).
For isoelectronic species, ionic size: $Al^{3+} < Mg^{2+} < Na^+ < F^-$ (correct).
Note: the source key accepts both the ionisation enthalpy order and the electron gain enthalpy order as answers.