p-Block Elements (Group 15–18) · Question 14

The property/properties that show irregularity in first four elements of group-17 is/are Choose the correct answer from the options given below :

1. A, B, C and D
2. A and C only
3. B only
4. B and D only

Answer

(3) B only

Solution

For F, Cl, Br, I:

A. Covalent radius increases regularly down the group (F < Cl < Br < I) regular.

B. Electron gain enthalpy (electron affinity): Cl > F > Br > I. Fluorine's small size causes strong electron–electron repulsion in its compact 2p subshell, so its value is less negative than that of Cl irregular.

C. Ionic radius (F- < Cl- < Br- < I-) increases regularly regular.

D. First ionisation enthalpy decreases regularly F > Cl > Br > I regular.

Only B shows irregularity.

Answer: (3)