On dissolving a moderate amount of sodium metal in liquid $NH_3$ at low temperature,which one of the following does not occur?

Correct order of density is

The valence shell electronic configuration of alkali metals is

Which of the following reacts with a solution of caustic soda upon heating to produce a combustible gas?

Which of the following compounds is obtained as a by-product in the synthesis of sodium carbonate by the Solvay process?

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: $LiF$ is sparingly soluble in water.
Reason $R$: The ionic radius of $Li^{+}$ ion is smallest among its group members,hence it has the least hydration enthalpy.
In the light of the above statements,choose the most appropriate answer from the options given below.