Lithium and magnesium are the hardest among the other elements in their respective groups because:

Identify the ions which form hydrated chlorides from the following:

$\mathrm{a})$ ${\mathrm{Ba}}^{2+}$
$\mathrm{b})$ ${\mathrm{Li}}^{+}$
$\mathrm{c})$ ${\mathrm{Sr}}^{2+}$
$\mathrm{d})$ ${\mathrm{Cs}}^{+}$
$\mathrm{e})$ ${\mathrm{Mg}}^{2+}$
$\mathrm{f})$ ${\mathrm{Rb}}^{+}$
$\mathrm{g})$ ${\mathrm{Ca}}^{2+}$