Q3 / 125 · BITSAT 2011 · Chemistry

The correct order of radii is

  • A
    $\quad \mathrm{N}<\mathrm{Be}<\mathrm{B}$
  • Correct answer
    B
    $\mathrm{F}^{-}<\mathrm{O}^{2-}<\mathrm{N}^{3-}$
  • C
    $\quad \mathrm{Na}<\mathrm{L}_{1}<\mathrm{K}$
  • D
    $\mathrm{Fe}^{3+}<\mathrm{Fe}^{2+}<\mathrm{Fe}^{4+}$

Solution

Effective nuclear charge (i,e. Z/e ratio) decreases from $\mathrm{F}^{-}$ to $\mathrm{N}^{3-}$ hence the radii follows the order: $\mathrm{F}^{-}<\mathrm{O}^{2-}<\mathrm{N}^{3-} . \mathrm{Z} / \mathrm{e}$ for $\mathrm{F}^{-}=9 / 10=0.9,$ for $\mathrm{O}^{2-}=8 / 10=.8,$ for $\mathrm{N}^{3-}=0.7$

Hence, the correct answer is option: 'B'.

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