Question

For an isoelectronic series of ions, what is the ion that is always the smallest?

Answer 1

For an isoelectronic series of ions, the smallest ion is always a cation with high magnitude positive charge or an anion with low magnitude of negetive charge.

Explanation:

Let us understand this by an example. Consider the following ions:

`Ar,S^(2-),K^(+),Cl^(-) and Ca^(2+)`

Here, `Ca^(2+)` has smallest size as it is a cation with high magnitude of positive charge.As more electrons are removed from `Ca`, the value of `Z_(eff)` increases(number of protons in nucleus increases).Thus, the electron cloud is pulled very close to the nucleus.

The largest ion will be `S^(2-)` as more electrons are added.The effect of nucleus decreases and the electron cloud expands.

Thus the order of radii is:
`S^(2-)>Cl^(-)>Ar>K^(+)>Ca^(2+)`