Question

In increasing order, how would you rank these ions according to ionic radius?
$S{{r}^{+2}} B{{r}^{-}},\,R{{b}^{+}},A{{s}^{-3}},S{{e}^{-2}}$

Answer 1

The given species in the question are isoelectronic species. Isoelectronic species are those ions or atoms which contain the same number of electrons. When an atom loses its electrons, it forms cation and when it gains the electron it forms the anion.

Complete step-by-step answer: We should remember that the size of a cation is always smaller than its parent atom because the cation formed after the loss of electrons has fewer electrons. Though, the nuclear charge remains the same, effective nuclear charge increases.
So, the remaining electrons are more strongly pulled towards the nucleus, thus reducing the size of the cation.
We should also remember that the size of the anion is always larger than the parent atom because the anion formed by the gain of electrons has more electrons. The addition of one or more electrons results in the increased repulsion among the electrons and a decrease in effective nuclear charge.
The decrease in the effective nuclear charge causes a loosening of force of nuclear attraction resulting in the increase of the size of the anion.
The successive loss of electrons from an atom increases the effective nuclear charge whereas the successive gain of electrons decreases the effective nuclear charge.
This is the reason the cation with a greater positive charge has a smaller size because of the greater nuclear attraction of the electrons.
while the anion with a greater negative charge has a larger radius because in this case the net repulsion of the electrons exceeds the nuclear charge and causes for the increase in size.
In the given species, it is now clear to us that anions will be greater than the cations. Also, it is also clear to us that anion with a greater negative charge will have more size and cation with more positive charge will have least size.
Hence, $A{{s}^{-3}}$ with greatest negative charge will have the largest size and $S{{r}^{+2}}$ with greatest positive charge has the least size. The decreasing order of above species will be $A{{s}^{-3}}>S{{e}^{-2}}>B{{r}^{-}}>R{{b}^{+}}>S{{r}^{+2}}$

Note: It should be noted that the above case was of isoelectronic species. So, we only considered the size based on cation and anion. If these were not isoelectronic species, then though the concept of cation and anion would be valid, we need to consider the general periodic trends also.