Potassium is a soft, silvery alkali metal. It is highly reactive, tarnishes rapidly in air, and reacts violently with water to form potassium hydroxide and hydrogen.
Potassium has the ground-state configuration \([Ar]4s^1\). The single 4s valence electron is weakly held (low first ionization energy), so potassium readily forms the cation \(\mathrm{K^+}\) by losing that electron. This ease of oxidation explains its vigorous chemical reactivity, typical of Group 1 metals.
Potassium reacts violently with water to yield potassium hydroxide and hydrogen gas; the heat can ignite the hydrogen and the metal, often producing a lilac flame:
\(\mathrm{2\,K(s) + 2\,H_2O(l) \rightarrow 2\,KOH(aq) + H_2(g)}\)
Because the reaction is highly exothermic and K is soft (forming large contact area), even small pieces can explode or spatter caustic solution.
Thermal excitation promotes valence electrons to higher energy levels; when they return to lower levels, photons in the violet–red region are emitted, giving a characteristic lilac flame. Cobalt glass is often used to filter out the intense yellow from sodium impurities.
Modern production is typically by metallothermic reduction of molten potassium salts, especially KCl, with sodium, followed by distillation to separate potassium:
\(\mathrm{KCl(l) + Na(l) \rightleftharpoons NaCl(l) + K(g)}\)
The equilibrium is driven by volatilization of K(g). Historical routes also involved electrolysis of potassium hydroxide under carefully controlled conditions.
On exposure to oxygen, potassium forms a mixture depending on conditions: oxide \(\mathrm{K_2O}\), peroxide \(\mathrm{K_2O_2}\), and under oxygen-rich conditions the superoxide \(\mathrm{KO_2}\):
KO2 is an orange solid that can release O2 upon reaction with CO2/H2O, useful in breathing apparatus.
The larger \(\mathrm{K^+}\) ion lowers lattice enthalpy more strongly for many anions (e.g., carbonates, sulfates), while hydration enthalpy differences are modest. The balance often favors dissolution for potassium salts, making them generally very soluble in water.
In animals, \(\mathrm{K^+}\) is the major intracellular cation, crucial for nerve impulse transmission and muscle function via the Na+/K+ ATPase and voltage-gated channels. In plants, K+ regulates osmotic balance, stomatal opening, and enzyme activation, hence the agricultural importance of potash fertilizers.
Store under dry mineral oil or in an inert atmosphere to prevent reaction with air/moisture. Handle with dry tools and goggles; cut under oil. For fires, use a Class D (dry powder) extinguisher or dry sand—never water, CO2, or foam, which can intensify alkali-metal fires.
Crown ethers are cyclic polyethers (e.g., 18-crown-6) whose cavity size matches \(\mathrm{K^+}\), coordinating it strongly through multiple oxygen lone pairs. This complexation can solubilize K+ in organic solvents and enhance reactivity of paired anions in phase-transfer catalysis.