Question

When an alkali metal dissolves in liquid ammonia the solution can acquire different colours. Explain the reasons for this type of colour change.

Answer 1

$\text{Solution of alkali metal in liquid ammonia}:$

All alkali metals dissolve in liquid ammonia giving deep blue solution, which are conducting in nature.

$M+(x+y)N{H}_3\to \left[M\right(N{H}_3{)}_x{]}^{+}+\left[e\right(N{H}_3{)}_y{]}^{-}$

The blue colour of the solution is due to the ammoniated electron which absorbs energy in the visible region of light and thus imparts blue colour to the solution.


$\text{General Characteristics}:$

1. (Noble gas) $n{s}^2$ is the electronic configuration of alkali metal.

2. To occupy the nearest inert gas configuration, these metals lose one of their electrons and show its oxidation state $+1.$

3. In alkali metal when we move down the group, there is an increase in ionic radii and atomic radii due increment in number of shells.

The solutions are paramagnetic and on standing slowly liberate hydrogen resulting in the formation of amide.

$M_{am}^{+}+e^{-}+N{H}_3\left(l\right)\to MN{H}_{2\left(am\right)}+\frac{1}{2}{H}_2\left(g\right)$

where ${}^{\prime }a{m}^{\prime }$ denotes solution in ammonia. In concentrated solution $\left(3M\right)$, the blue colour changes to bronze, due to formation of metal ion clusters and become diamagnetic.