How do atomic sizes vary in a group and in a period? Give reasons for the variations.

Arrange the following ions in the order of their increasing size :

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

Select from each group the species which has the smallest radius, stating the appropriate reason.

• $\mathrm{O}, {\mathrm{O}}^{-}, {\mathrm{O}}^{2-}$

Select from each group the species which has the smallest radius, stating the appropriate reason.

• ${\mathrm{K}}^{+}, {\mathrm{Sr}}^{2+}, \mathrm{Ar}$

Select from each group the species which has the smallest radius, stating the appropriate reason.

• $\mathrm{Si}, \mathrm{P}, \mathrm{Cl}$

Choose from the following :

${}_4\mathrm{Be}$, ${}_9\mathrm{F}$, ${}_{19}\mathrm{K}$, ${}_{20}\mathrm{Ca}$

The element having one electron in its outermost shell.

Choose from the following :

${}_4\mathrm{Be}$, ${}_9\mathrm{F}$, ${}_{19}\mathrm{K}$, ${}_{20}\mathrm{Ca}$

Two elements of the same group.