As the atomic number increases,which of the following properties of alkali metals increases?

$A$ metal $M$ on heating in nitrogen gas gives $Y$. $Y$ on treatment with $H_2O$ gives a colourless gas which when passed through $CuSO_4$ solution gives a blue colour. $Y$ is

The atomic radii of alkali metals $(M)$ lie in the order $Li < Na < K < Rb$,but the radii of $M^{+}$ ions in aqueous solution lie in the reverse order $Li^{+} > Na^{+} > K^{+} > Rb^{+}$. What is the reason for this reverse order (on going from $Li$ to $Rb$)?

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.

Which of the following salts does not produce nitrogen dioxide $(NO_2)$ upon heating?

Which of the nitrates on strong heating leaves the metal as the residue?