Question

Which of the following species has the largest radius?

• A. ${\mathrm{O}}^{-2}$
• B. ${\mathrm{F}}^{-}$
• C. ${\mathrm{Na}}^{+}$
• D. ${\mathrm{Mg}}^{+2}$

Answer 1

The correct option is A

${\mathrm{O}}^{-2}$

Atomic Radius:

1. The 'Atomic Radius' is the distance between an atom's nucleus and its outermost electron.
2. The precise value of an atom's atomic radius is unknown. Because the atom is not in a free state, the exact position of the electron cannot be determined.
3. The atomic radius falls across a period and grows down a group, in general.

The -ve sign denotes the presence of additional electrons, while the +ve sign denotes the absence of electrons. Because all of these ions contain ten electrons in their shells, they are all isoelectronic.

Explanation for the correct option:

(A) ${\mathrm{O}}^{-2}$ Atomic Number = 8

Thus; 8+2 = 10 electrons

Explanation for the incorrect options:

(B) ${\mathrm{F}}^{-}$ Atomic Number = 9

Thus; 9+1 = 10 electrons

(C) ${\mathrm{Na}}^{+}$ Atomic Number = 11

Thus; 11-1= 10 electrons

(D) ${\mathrm{Mg}}^{+2}$ Atomic Number = 12

Thus; 12-2 = 10 electrons

The ionic radius decreases as the charge become more positive. The -ve sign denotes the addition of electrons. These electrons are deposited in the outermost shells of the atom. In addition, as an atom loses electrons, it clings even tighter to the ones it still has, lowering the ionic radius even more. As a result, the ionic radii will be arranged in the following order:

${\mathrm{Mg}}^{+2}$< ${\mathrm{Na}}^{+}$< ${\mathrm{F}}^{-}$< ${\mathrm{O}}^{-2}$.

Conclusion: Hence, the correct option is (A).