Question

The correct order of radii is

• A. $ N < Be < B $
• B. $ {{F}^{-}}<{{O}^{2-}}<{{N}^{3-}} $
• C. $ Na < Li < K $
• D. $ F{{e}^{3+}} < F{{e}^{2+}} < F{{e}^{4+}} $

Answer 1

Answer: (B)

These are isoelectronic species and their radii decreases with increasing their atomic number due to increasing effective nuclear charge $\left(Z_{e f f}\right)$.
$Z_{e f f}=Z-\sigma$
where, $Z_{e f f}=$ effective nuclear charge, $Z=$ atomic number and $\sigma=$ screening constant. For $F^{-}, O^{2-}$ and $N^{3-}$, the value ofcis constant due to equal number of electrons. So order of $Z_{\text {eff }}$ is $F^{-}>O^{2-}>N^{3-}$ hence, order of radii
$=F^{-} < O^{2-} < N^{3-}\left(\operatorname{radii} \propto \frac{1}{Z_{e f f}}\right) .$