The photoelectric effect is maximum in which of the following elements?

Match the following:

Metals	Ore
$1$. Barium	$d$. Witherite
$2$. Magnesium	$a$. Carnallite
$3$. Potassium	$b$. Sylvine
$4$. Strontium	$c$. Celestine

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.
$(c)$ $E^{\circ}$ for $M^{2+}_{(aq)} + 2e^{-} \longrightarrow M_{(s)}$ (where $M = Ca, Sr$ or $Ba$) is nearly constant.

Which of the following reactions does not liberate a gaseous product?

Which metal forms amide with $NH_3$ at $300\ ^oC$?

Give uses of sodium carbonate $(Na_2CO_3 \cdot 10H_2O)$.