Question 10.10 When an alkali metal dissolves in liquid ammonia the solution can acquire different colours. Explain the reasons for this type of colour change.
Class XI The s-Block Elements Page 305
Class XI The s-Block Elements Page 305
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When an alkali metal dissolve in liquid ammonia, the solution can acquire different colors,I mean dilute solutions of alkali metals in liquid ammonia exhibit dark blue color this is because of ammoniated electrons absorb energy in the visible region of light.e.g., M + (x + y)NH3--->[M(NH3)x]^+ + [e-(NH3)y]{ammoniated electrons}
However, if the concentration increases above 3M, the color changes to copper -bronze and it becomes diamagnetic in nature.
The alkali metals like Na,K etc. dissolved in liquid Ammonia giving deep blue coloured solution.This solution is highly conducting in nature.It contains a large amount of Ammoniated captions as well as electrons as shown below in the reaction:-M + ( a + b )NH3 => M* ( NH3 )a + e(NH3)bIt has been found that the blue colour of solution is mainly due to the presence of the Ammoniated electrons in the solution.These electrons absorb energy corresponding to the red region of the visible light spectrum for its excitation to the higher energy levels.The transmitted light is blue coloured, thus it imparts blue colour to the solution.The solution is paramagnetic due to the presence and in the absence of catalysts and impurities decomposes the blue coloured solution and it gives formation to "Metal amide" and liberation of hydrogen gas.