Question

Select the correct order of ionic radii
A. ${{O}^{2-}}>{{F}^{-}}>M{{g}^{2+}}>N{{a}^{+}}$
B. $N{{a}^{+}}>M{{g}^{2+}}>{{O}^{2-}}>{{F}^{-}}$
C. ${{O}^{2-}}>{{F}^{-}}>N{{a}^{+}}>M{{g}^{2+}}$
D. $M{{g}^{2+}}>N{{a}^{+}}>{{F}^{-}}>{{O}^{2-}}$

Answer 1

Hint: When an atom loses or gains an electron then it forms ions. When an atom loses an electron it forms a cation and when it gains an electron it becomes an anion. The Ionic radius can be described as the distance between the nucleus of an ion and the outermost shell of the ion.

Complete Solution :
- Examples given in the equation are isoelectronic with each other isoelectronic word is composed of two words named as iso and electronics where iso means same and electronic refers to electronic configuration hence these are those compounds which have same electronic configuration as all the examples given have the configuration which can be shown as: $M{{g}^{2+}},N{{a}^{+}},{{F}^{-}},{{O}^{2-}}=1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}$
- And we know in case of isoelectronic species cations which lose electrons are smaller than neutral atoms which are in turn smaller than anions which gain electrons rather than this ionic radius generally dependent on the factor called effective nuclear charge which can be defined as charge carried by the outermost valence electron. Also the electron or multi-electron takes into account the number of shielding electrons that surrounds the nucleus and greater the effective nuclear charge smaller will be the ionic radius.
Hence we can say that the correct order of ionic radii will be ${{O}^{2-}}>{{F}^{-}}>N{{a}^{+}}>M{{g}^{2+}}$.

So, the correct answer is “Option C”.

Note: In a periodic table while moving down in a group ionic radii increases because atoms will be added in an extra shell while in periods it first decreases and then suddenly increases and then again it slowly decreases.