JEE Chemistry 2025 Question with Solution

Given: The listed properties are covalent radius, electron affinity, ionic radius and first ionization energy for the first four elements of group-17: $\text{F, Cl, Br, I}$.

Find: Which property shows an irregular trend.

For the first four halogens, the trends are:

• Covalent radius: $\text{F} < \text{Cl} < \text{Br} < \text{I}$
• Electron affinity: $\text{Cl} > \text{F} > \text{Br} > \text{I}$
• Ionic radius: $\text{F} < \text{Cl} < \text{Br} < \text{I}$
• First ionization energy: $\text{F} > \text{Cl} > \text{Br} > \text{I}$

Covalent radius, ionic radius and first ionization energy follow regular trends down the group.

Electron affinity is irregular because chlorine has higher electron affinity than fluorine.

Therefore, only electron affinity shows irregularity, so the correct option is C.

Given: The question asks which property among the listed ones shows irregularity for the first four halogens: $\text{F, Cl, Br, I}$.

Find: The correct choice among the given options.

Concept used: Down a group, covalent radius and ionic radius generally increase, while first ionization energy generally decreases. Electron affinity usually follows a trend, but fluorine shows an exception.

For covalent radius and ionic radius, size increases down the group because a new shell is added at each step:

This is a regular trend.

For first ionization energy, the outer electron is farther from the nucleus and more shielded down the group, so ionization energy decreases:

This is also a regular trend.

For electron affinity, one may expect fluorine to have the highest value, but due to the very small size of fluorine and greater interelectronic repulsion in its compact shell, chlorine has a more exothermic electron affinity:

So this trend is irregular.

Hence, among the listed properties, only B shows irregularity.

Therefore, the correct option is C.