Arrange the following ions in the decreasing order of their size’s

${\mathrm{Be}}^{+},{\mathrm{Cl}}^{-},{\mathrm{S}}^{-2},{\mathrm{Na}}^{+},{\mathrm{Mg}}^{+2},{\mathrm{Br}}^{-}.$

By loss or gain of electrons by an atom the atomic radius is changed.

${\mathrm{Be}}^{+2}\left(\mathrm{He}\right)\to 1^{\mathrm{st}} \mathrm{periods}$

${\mathrm{Na}}^{+},{\mathrm{Mg}}^{+2}\left(\mathrm{Ne}\right)\to 2^{\mathrm{nd} }\mathrm{periods}$

${\mathrm{Cl}}^{-},{\mathrm{S}}^{-2}\left(\mathrm{Ar}\right)\to 3^{\mathrm{rd}} \mathrm{periods}$

${\mathrm{Br}}^{-}\left(\mathrm{Br}\right)\to 4^{\mathrm{th}} \mathrm{periods}$

In isoelectronic species, for cations if nuclear charge (+ve) increases and then size decreases, for anions if  the negative charge (-Ve) increases and then size increases.  The size of ions increasing order in different periods as per their electronic configuration is shown below

${\mathrm{Br}}^{-}>{\mathrm{S}}^{-2}>\mathrm{Cl}>{\mathrm{Na}}^{+}>{\mathrm{Mg}}^{+2}>{\mathrm{Be}}^{+2}$