A solvated electron is a free electron in (solvated in) a solution, and is the smallest possible anion. Solvated electrons occur widely, although it is difficult to observe them directly because their lifetimes are so short. The deep color of solutions of alkali metals in liquid ammonia arises from the presence of solvated electrons: blue when dilute and copper-colored when more concentrated (> 3 molar). Classically, discussions of solvated electrons focus on their solutions in ammonia, which are stable for days, but solvated electrons also occur in water and other solvents – in fact, in any solvent that mediates outer-sphere electron transfer. The real hydration energy of the solvated electron can be estimated by using the hydration energy of a proton in water combined with kinetic data f