Here, we will discuss how ionic radii varies in the group and period. Let’s see the comparison of ionic radii with corresponding atomic radii.

What is ionic radii

The effective distance from the centre of the nucleus of the ion up to where it exerts its influence on its electron cloud. Or we can say that ionic radii corresponds to the radii of the ions in the ionic crystal.

Why cation is smaller than its parent atom

1. A cation has one or more electrons less than of its parent atom.
2. In cation electrons are less but nuclear charge remains same as that of parent atom.
3. Therefore, effective nuclear charge Zeff increases in the cation.
4. This results in decrease in the size of cation than of its parent atom.
5. Na ( 1s^2,, 2s², 2p⁶, 3s¹ ) ➡️ Na⁺ ( 1s^2,, 2s², 2p⁶ ) + e^–

There is one more reason of cation being smaller than parent atom. Generally a full valence shell removes in the making of a cation. Therefore, cation has smaller ionic radii than the neutral gaseous atom.

Why anion is larger than its parent atom

1. A anion has one or more electrons more than of its parent atom.
2. In anion electrons are more but nuclear charge remains same as that of parent atom.
3. Therefore, effective nuclear charge Zeff decreases in the anion.
4. This results in increase in the size of anion than of its parent atom.
5. Cl ( 1s^2,, 2s², 2p⁶, 3s²,3p⁵ ) + e- ➡️ Cl^– ( 1s^2,, 2s², 2p⁶, 3s²,3p⁶ )

Addition of one or more electrons increases the repulsion among the electrons. The electron cloud of the atom expands. This results in increase in the size of anion.

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