Answer:
The alkali metal atoms on
dissolution in liquid ammonia ionise according to following equation:
\[M+(x+y)N{{H}_{3}}\to
{{M}^{+}}{{(N{{H}_{3}})}_{x}}+{{e}^{-}}{{(N{{H}_{3}})}_{y}}\]
The
solutions thus contain ammoniated cations and ammoniated electrons. When light
falls on this solution, the ammoniated electrons are excited by absorbing energy
corresponding to red light and the light transmitted is blue. This imparts blue
colour to the solution. This colour is observed in dilute solutions. However,
when concentration increases above 3M the colour changes to bronze and solution
acquires metallic lustre due to the formation of metal ion clusters.
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