Which important property did Mendeleev use to classify the elements in his periodic table?

Name a species that will be isoelectronic with the following atom/ion:

• Ne

Name a species that will be isoelectronic with the following atom/ion:

• ${\mathrm{Cl}}^{-}$

Name a species that will be isoelectronic with the following atom/ion:

• ${\mathrm{Ca}}^{2+}$

Name a species that will be isoelectronic with the following atom/ion:

• $\mathrm{Rb}$

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.

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