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 $\left(Z_{eff}\right)$.
$Z_{eff}=Z-\sigma$
where, $Z_{eff}=$ 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
$=F^{-}<O^{2-}<N^{3-}\left(\mathrm{radii}\propto \frac{1}{Z_{eff}}\right).$