What is the primary purpose of the Dow's process?

Important ore of $Mg$ is

The valence shell electronic configuration of alkali metals is

Comment on each of the following observations:
$(a)$ The mobilities of the alkali metal ions in aqueous solution are $Li^{+} < Na^{+} < K^{+} < Rb^{+} < Cs^{+}$
$(b)$ Lithium is the only alkali metal to form a nitride directly.
$(c)$ The standard electrode potential $E^{\circ}$ for the reaction $M^{2+}_{(aq)} + 2e^{-} \to M_{(s)}$ (where $M = Ca, Sr$ or $Ba$) is nearly constant.

$A$ piece of magnesium ribbon is heated to redness in an atmosphere of nitrogen and on cooling with water,the evolved gas is

Calcium is obtained by