One of the most curious properties of the alkali metals—lithium, sodium, potassium, rubidium, and cesium—is that they are soluble in certain inorganic solvents, especially liquid ammonia. Ammonia is a gas at standard temperature and pressure but its boiling point, about -33 °C, is high enough to permit working with ammonia as a liquid. It's polar like water but a much weaker acid, hence the alkali metals do not react with ammonia to abstract hydrogen, the way they do with water.1 Instead the ammonia strips electrons from the metal, surrounding them in a protective shell of ammonia molecules. These solvated electrons exhibit strong absorption of visible light, turning the solution dark blue. Amines of low molecular weight, like liquid methylamine and ethylenediamine, behave similarly to ammonia.
- Certain catalysts, like iron salts, induce the chemical reaction of alkali metals with liquid ammonia, releasing hydrogen just like what happens in water, and that's one way to get "sodamide", NaNH2.
At a high enough concentration, some physical change occurs which I don't fully understand and the mass of solvated electrons in solution begins to act like a metal; I guess there's now a mobile population of free electrons moving through a mass of ammonia molecules. Electrical conductivity goes way up and the solution exhibits specular reflection like a metal, turning brassy yellow. Weird, huh?
Unbound electrons may be confined within other chemical structures, producing a general type of substance called an "electride", an ionic compound in which electrons function as counteranions. There's a calcium aluminate with a cagelike structure, known to nature as the mineral mayenite, which is capable of trapping small anions like Cl- and O= in its structure; it's also capable of holding free electrons, and this mayenite electride has remarkable physical properties, like low-temperature superconductivity. Now that would be some cool stuff to play with but apparently it's not exactly easy to make.
~Alyx Woodward