Question 7

Analyze each option: (A) The electronic configuration of $B^+$ is $1s^2 2s^2$ (stable fully filled subshell) and $C^+$ is $1s^2 2s^2 2p^1$. Removing an electron from the stable $2s$ subshell of $B^+$ requires more energy than removing it from the $2p$ subshell of $C^+$. Thus, $IE_2(C) < IE_2(B)$. This is correct. (B) $Al^{3+}$, $Mg^{2+}$, and $Na^+$ are isoelectronic species (all have 10 electrons). For isoelectronic species, the ionic radius decreases as the nuclear charge ($Z$) increases. Nuclear charges are $Na (11) < Mg (12) < Al (13)$. Thus, the radii order is $Al^{3+} < Mg^{2+} < Na^+$. This is correct. (C) In Group 1, density generally increases down the group, but potassium is less dense than sodium ($d_{Na} \approx 0.97$ g/cm$^3$, $d_K \approx 0.86$ g/cm$^3$) due to a large increase in atomic volume. This is incorrect. (D) The $H-H$ bond enthalpy is $\approx 436$ kJ/mol, while the $F-F$ bond enthalpy is $\approx 158$ kJ/mol. The $F-F$ bond is weaker due to strong lone pair-lone pair repulsions between the small fluorine atoms. This is incorrect.