JEE Main 2025 Apr 3 Shift 1, Chemistry Q9: Periodicity In Atomic Properties

A. Electron gain is not universally exothermic. For example, noble gases and group 2 elements have positive (endothermic) electron gain enthalpies. ✗

B. Removing an electron from an isolated gaseous atom requires energy input in every case, so ionisation is always endothermic. ✓

C. Be has the configuration $1s^{2}2s^2$ (filled $2s$, extra stability), while B is $1s^{2}2s^{2}2p^1$ (loosely held $2p$ electron). Hence ${\text{IE}}_1(\text{B})<{\text{IE}}_1(\text{Be})$. ✓

D. Electronegativity is a property of the atom in its bonding environment. The effective electronegativity of C is not the same in ${\text{CH}}_4$ and ${\text{CCl}}_4$ because the Cl substituents withdraw electron density and raise $Z_{\text{eff}}$ on C. ✗

E. Among group 1, Cs (not Li) is the most electropositive. ✗

$\therefore$ Only B and C are correct.