Potassium is the chemical element with the symbol K and atomic number 19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to form flaky white potassium peroxide in only seconds of exposure. It was first isolated from potash, the ashes of plants, from which its name derives. In the periodic table, potassium is one of the alkali metals, all of which have a single valence electron in the outer electron shell, which is easily removed to create an ion with a positive charge (which combines with anions to form salts). In nature, potassium occurs only in ionic salts. Elemental potassium reacts vigorously with water, generating sufficient heat to ignite hydrogen emitted in the reaction, and burning with a lilac-colored flame. It is found dissolved in seawater (which is 0. 04% potassium by weight), and occurs in many minerals such as orthoclase, a common constituent of granites and other igneous rocks. Potassium is chemically very similar to sodium, the previous element in group 1 of the periodic table. They have a similar first ionization energy, which allows for each atom to give up its sole outer electron. It was suspected in 1702 that they were distinct elements that combine with the same anions to make similar salts, and this was proven in 1807 through using electrolysis. Naturally occurring potassium is composed of three isotopes, of which 40K is radioactive. Traces of 40K are found in all potassium, and it is the most common radioisotope in the human body.

Potassium ions are vital for the functioning of all living cells. The transfer of potassium ions across nerve cell membranes is necessary for normal nerve transmission; potassium deficiency and excess can each result in numerous signs and symptoms, including an abnormal heart rhythm and various electrocardiographic abnormalities. Fresh fruits and vegetables are good dietary sources of potassium. The body responds to the influx of dietary potassium, which raises serum potassium levels, by shifting potassium from outside to inside cells and increasing potassium excretion by the kidneys.

Most industrial applications of potassium exploit the high solubility of its compounds in water, such as saltwater soap. Heavy crop production rapidly depletes the soil of potassium, and this can be remedied with agricultural fertilizers containing potassium, accounting for 95% of global potassium chemical production.

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Atomic properties

Standard atomic weight39.0983 ±1·10-4
Atomic mass39.0983 u

Atomic radii

Radius (empirical)227 pm
Radius (calculated)243 pm
Covalent radius203 ±12 pm
Van der Waals radius275 pm

Atomic shell

Electron configurationAr 4s1
Ionization energy(1st) 4.3406636 eV
(2nd) 31.625 eV
(3rd) 45.8031 eV
(4th) 60.917 eV
(5th) 82.66 eV
Shell model

Physical properties

Density0.856 g·cm−3 (293.1 K)
Molar volume4.594·10-5 m3·mol−1
Speed of sound2,000 m·s−1 (293.1 K)


Melting point336.7 K
Boiling point1,030 K
Liquid range694 K
Critical point2,223 K @ 16 MPa


Melting enthalpy2.33 kJ·mol-1
Enthalpy of vaporization76.9 kJ·mol-1
Binding energy89 kJ·mol-1

Heat and conductivity

Specific heat capacity757.8 J·kg−1·K−1
Thermal conductivity100 W·m-1·K-1
Work function2.3 eV


Mohs hardness0.4
Brinell hardness0.363 NM·m-2

Elastic properties

Shear modulus1.3 GPa
Bulk modulus3.1 GPa

Electrical properties

Electrical conductivity1.43·107 S·m-1
Resistance7.499·10-8 Ωm


Magnetic susceptibility2.08·10-5 cm3·mol−1 (298 K)

Chemical properties

Basicitystrongly basic
Oxidation state-1, +1
Standard potential-2.931 V (K+ + e- → K)


Pauling scale0.82
Sanderson scale0.45
Allred-Rochow scale0.91
Mulliken scale0.73
Allen scale0.734
Ghosh-Gupta scale2.002 eV
Boyd-Edgecombe scale0.99
Nagle scale0.84
Pearson absolute negativity2.42 eV

Other properties

Natural occurrenceprimordial
Crystal structureBody-centered cubic
Goldschmidt Classificationlithophile
Superconductorwithout transition tempperature
Price/kg12.85 ±1.5 USD

Natural abundances

3,000 ppb ≈ 3.01·1013 M☉
4,000 ppb ≈ 7.95·1012 Mt
710,000 ppb ≈ 710 g
Earth’s crust
1.5·107 ppb ≈ 4.15·108 Mt
416,000 ppb ≈ 569 Mt
Flowing water
2,300 ppb ≈ 36.8 Gt
Human body
2·106 ppb ≈ 140 g