Blue Color Origin in Alkali Metal-Ammonia Solutions

When alkali metals (like Sodium, Potassium) are dissolved in liquid ammonia, they undergo ionization.

Dissociation and Solvation

• The alkali metal atom donates its valence electron, forming a metal cation ($M^+$) and a free electron ($e^-$).
• Both the cation and the electron become solvated by ammonia molecules ($NH_3$).
• The solvated cation is called an ammoniated cation ($M^+_{(sol)}$).
• The solvated electron is called an ammoniated electron ($e^-_{(sol)}$).

The process can be represented as:

$M_{(s)} + excess \ NH_3 \rightarrow M^+_{(sol)} + e^-_{(sol)}$

Color Determination

• The ammoniated electrons are responsible for the characteristic deep blue color observed in these solutions.
• These electrons absorb energy from the visible light spectrum, promoting them to higher energy levels. This absorption is responsible for the observed color.
• Ammoniated cations do not absorb visible light and are essentially colorless.

Therefore, the blue color is specifically due to the presence and light absorption properties of ammoniated electrons.