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

Aug 2, 2017

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:

$A r , {S}^{2 -} , {K}^{+} , C {l}^{-} \mathmr{and} C {a}^{2 +}$

Here, $C {a}^{2 +}$ has smallest size as it is a cation with high magnitude of positive charge.As more electrons are removed from $C a$, the value of ${Z}_{e f f}$ 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:
${S}^{2 -} > C {l}^{-} > A r > {K}^{+} > C {a}^{2 +}$