What do you understand by iso-electronic ions? In iso-electronic ions, the value of ionic radius decreases with the increase in atomic number. Explain.

Isoelectronic ions are those which have a same number of electrons and hence same electronic configuration but the different magnitude of nuclear charge. A set of species (cations or anions) having the same number of electrons is known as iso-electronic series, e.g. N^3-, O^2-, F^-, Na⁺, Mg⁺, Al³⁺ are isoelectronic species.
Within the series of isoelectronic ions, as the atomic number or nuclear charge increases, the attractive force between the nucleus and a same number of electrons also increases. This results in the decrease of ionic radius. 

For example,

 Ion                          =N^3-    O^2-    F^-     Na⁺    Mg⁺       Al³⁺

No. of electrons   =  10      10     10      10       10      10

At. No                      =  7        8      9       11       12      13

Since, the size of the iso-electronic ion decreases with the increase in atomic number, the order of decreasing size is

N^3- > O^2- > F^- > Na⁺ > Mg²⁺ > Al³⁺