JEE Main 2020 · 7 January · evening · Single correct · +4 / −1
Within each pair of elements F & Cl, S & Se, and Li & Na, respectively, the elements that release more energy upon and electron gain are :
- (a) F, S and Li
- (b) Cl, Se and Na
- (c) Cl, S and Li
- (d) F, Se and Na
Answer and solution
Answer (c) Cl, S and Li
Solution
Chemistry ma’am · 3 steps
Down a group, less energy is released as atoms grow, except that tiny, crowded F loses out to Cl.
- General trend. Going down a group, the atom gets bigger. The incoming electron sits farther from the nucleus and feels less pull, so less energy is released. That is why S beats Se and Li beats Na.
- The F exception. Fluorine is very small, and its compact 2p subshell is already crowded with electrons. The added electron is repelled strongly, so F releases less energy than Cl, the larger atom with more room in 3p.
Pair Electron gain enthalpy (kJ mol⁻¹) Releases more energy F and Cl F: -328, Cl: -349 Cl (the exception) S and Se S: -200, Se: -195 S Li and Na Li: -60, Na: -53 Li - So. The elements that release more energy are Cl, S and Li, which is option (c).
The idea to keep. Electron gain enthalpy usually becomes less negative down a group, but the second-period elements F and O are so small that electron repulsion makes Cl and S release more.
Answer from the NTA final answer key. Solution by PrepWiser.