JEE Main 2024 · 29 January · evening · Single correct · +4 / −1
Given below are two statements:
Statement I : Fluorine has most negative electron gain enthalpy in its group.
Statement II : Oxygen has least negative electron gain enthalpy in its group.
In the light of the above statements, choose the most appropriate from the options given below
- (a) Both Statement I and Statement II are true
- (b) Both Statement I and Statement II are false
- (c) Statement I is false but Statement II is true
- (d) Statement I is true but Statement II is false
Answer and solution
Answer (c) Statement I is false but Statement II is true
Solution
Chemistry ma’am · 3 steps
F and O are so small and crowded that an incoming electron gets pushed back, so they lose out to Cl and S.
- Expected trend. Electron gain enthalpy usually becomes less negative down a group, because the added electron sits farther from the nucleus. So you would expect the period 2 element to be the most negative.
- Period 2 exception. F and O are very small, and their 2p subshell is already packed with electrons. An incoming electron feels strong repulsion from them, so less energy is released than for Cl and S, which have roomier 3p orbitals.
Period Group 17 (kJ mol⁻¹) Group 16 (kJ mol⁻¹) 2 F: -328 O: -141, least negative in group 16 3 Cl: -349, most negative in group 17 S: -200 4 Br: -325 Se: -195 5 I: -295 Te: -190 - So. Statement I is false, because chlorine, not fluorine, has the most negative value in group 17. Statement II is true, because oxygen has the least negative value in group 16. The answer is (c).
The idea to keep. For groups 16 and 17, the period 3 element (S, Cl) has the most negative electron gain enthalpy. The period 2 element is too small, and electron repulsion there cuts the energy released.
Answer from the NTA final answer key. Solution by PrepWiser.