In which of the following options the order of arrangement does NOT agree with the variation of property indicated against it?

Solution & Explanation

\textbf{Answer:} (B) $\text{B}<\text{C}<\text{N}<\text{O}$ (increasing first ionization enthalpy) is the order that does NOT agree. \textbf{Solution:} Across a period the first ionization enthalpy generally increases, but $\text{N}$ has a stable half-filled $2p^3$ configuration, giving it a higher first ionization enthalpy than $\text{O}$. Hence the correct order is $\text{B}<\text{C}<\text{O}<\text{N}$, so the arrangement $\text{B}<\text{C}<\text{N}<\text{O}$ in option (B) is wrong. Option (A) is correct: for the isoelectronic species $\mathrm{Al}{}^{3+},\mathrm{Mg}{}^{2+},\mathrm{Na}{}^{+},\mathrm{F}{}^{-}$, more nuclear charge means a smaller size, so size increases as $\mathrm{Al}{}^{3+}<\mathrm{Mg}{}^{2+}<\mathrm{Na}{}^{+}<\mathrm{F}{}^{-}$. Option (D) is correct: metallic (atomic) radius increases down group 1.