The radius of the  $C{l}^{-}$  ion is 38 per cent larger than that of the $F^{-}$ ion but the radius of the $B{r}^{-}$  ion is only 6.5 per cent larger than that of the $C{l}^{-}$  ion. The relatively small difference in size between $C{l}^{-}$  and  $B{r}^{-}$  ions is due to the fact that 

The $B{r}^{-}$  ion contains six 4p electrons, which shield the nuclear charge effectively.

The  $B{r}^{-}$  ion contains ten 3d electrons and six 3p electrons, and together they shield the nuclear charge effectively. 

The  $B{r}^{-}$ ion contains ten 3d electrons, which fail to shield the nuclear charge effectively 

The  $B{r}^{-}$ ion contains ten 3d electrons, which shield the nuclear charge effectively