Amongst LiCl, RbCl, BeCl, and MgCl₂, the compounds with the greatest and least ionic character respectively are :

According to Fajan's rule, Size of cation $\propto$Ionic character. Recall that size of metal (ion) increases while going down the group, and decreases on crossing the periods from left to right. Thus Rb⁺ (group I) is largest and ${\mathrm{Be}}^{2+}$(group II) is smallest in size. Hence RbCl has greatest, and BeCl₂, has lowest ionic character.