The property/properties that show irregularity in first four elements of group-17 is/are
(A) Covalent radius
(B) Electron affinity
(C) Ionic radius
(D) First ionization energy
Choose the correct answer from the options given below :

Solution

Chemistry ma’am · 3 steps

Line up F, Cl, Br, I for each property: only electron affinity breaks its trend.

1. What regular means. Going down group 17 from F to I, the atom gets bigger at every step. So radii should rise steadily and ionization energy should fall steadily. Any property that jumps out of that order is irregular.

   Property	F, Cl, Br, I	Trend
   (A) Covalent radius (pm)	64, 99, 114, 133	rises steadily
   (B) Electron affinity (kJ mol⁻¹)	328, 349, 325, 295	Cl > F > Br > I, irregular
   (C) Ionic radius of X⁻ (pm)	133, 184, 196, 220	rises steadily
   (D) First ionization energy (kJ mol⁻¹)	1681, 1251, 1140, 1008	falls steadily

2. Why fluorine dips. Fluorine is very small, so its 2p subshell is crowded. An incoming electron is strongly repelled by the electrons already there, so less energy is released than for chlorine, whose larger 3p subshell has more room.
3. So. Only electron affinity (B) breaks the order. The radii and the ionization energy all follow smooth trends, so the answer is (c), B only.

The idea to keep. Down a group, size-based properties change smoothly. Electron affinity is the odd one out: Cl beats F, and S beats O, because the tiny period 2 atom repels the added electron.

Answer from the NTA final answer key. Solution by PrepWiser.