Question

Which of the following orders is not correct about the property shown against it?

• A. $N>O>P>S$ - First ionisation enthalpy
• B. $N>O>P>S$ - Negative electron gain enthalpy
• C. $F>Cl>O>S$ - Negative electron gain enthalpy
• D. $Fe^{3+}<Fe^{2+}<Fe$ - Size

Hint:

Electron gain enthalpy trends may differ from ionisation trends due to electronic configuration effects.

Answer 1

The Correct Option is B

% Option (1) First ionisation enthalpy order $N>O>P>S$ is correct since N has a half-filled p-orbital making it stable with higher ionisation energy.
% Option (2) Negative electron gain enthalpy does not follow this order because O has more negative electron affinity than N. So $N>O$ is incorrect for electron gain enthalpy. Hence this statement is wrong.
% Option (3) Order $F>Cl>O>S$ for negative electron gain enthalpy is valid as halogens have high electron affinity.
% Option (4) Ionic size decreases with increasing positive charge, so $Fe^{3+}<Fe^{2+}<Fe$ is correct.