Ranking the ions in each set in order of decreasing size -
a) Se²⁻> S²⁻ > O²
Ionic radius is the distance from the nucleus of an ion up to which it has an influence on its electron cloud. Ions are formed when an atom loses or gains electrons. When an atom loses an electron it forms a cation and when it gains an electron it becomes an anion.
In general, ionic radius decreases with increasing positive charge.
As the charge on the ion becomes more positive, there are fewer electrons.
The ion has a smaller radius. In general, ionic radius increases with increasing negative charge.
For ions of the same charge (e.g. in the same group) the size increases as we go down a group in the periodic table.
Se²⁻> S²⁻ > O²
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