Potassium

Potassium, ₁₉K
	Potassium pearls (in paraffin oil, ~5 mm each)
General properties
Pronunciation	/pəˈtæsiəm/ (pə-TASS-ee-əm)
Appearance	silvery gray
Standard atomic weight (Ar, standard)	39.0983(1)
Potassium in the periodic table
	argon ← potassium → calcium
Atomic number (Z)	19
Group	group 1 (alkali metals)
Period	period 4
Element category	alkali metal
Block	s-block
Electron configuration	[Ar] 4s1
Electrons per shell	2, 8, 8, 1
Physical properties
Phase at STP	solid
Melting point	336.7 K (63.5 °C, 146.3 °F)
Boiling point	1032 K (759 °C, 1398 °F)
Density (near r.t.)	0.862 g/cm3
when liquid (at m.p.)	0.828 g/cm3
Critical point	2223 K, 16 MPa
Heat of fusion	2.33 kJ/mol
Heat of vaporization	76.9 kJ/mol
Molar heat capacity	29.6 J/(mol·K)
Atomic properties
Oxidation states	+1, −1 (a strongly basic oxide)
Electronegativity	Pauling scale: 0.82
Ionization energies	1st: 418.8 kJ/mol; 2nd: 3052 kJ/mol; 3rd: 4420 kJ/mol; (more);
Atomic radius	empirical: 227 pm
Covalent radius	203±12 pm
Van der Waals radius	275 pm
	Spectral lines
	Spectral lines
Miscellanea
Crystal structure	body-centered cubic (bcc)
Speed of sound thin rod	2000 m/s (at 20 °C)
Thermal expansion	83.3 µm/(m·K) (at 25 °C)
Thermal conductivity	102.5 W/(m·K)
Electrical resistivity	72 nΩ·m (at 20 °C)
Magnetic ordering	paramagnetic
Magnetic susceptibility	+20.8·10−6 cm3/mol (298 K)
Young's modulus	3.53 GPa
Shear modulus	1.3 GPa
Bulk modulus	3.1 GPa
Mohs hardness	0.4
Brinell hardness	0.363 MPa
CAS Number	7440-09-7
History
Discovery and first isolation	Humphry Davy (1807)
Main isotopes of potassium
ε	40Ar
β+	40Ar
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Potassium is a chemical element with symbol K (from Neo-Latin kalium) and atomic number 19. 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 the alkali metals have a single valence electron in the outer electron shell, which is easily removed to create an ion with a positive charge – a cation, which combines with anions to form salts. Potassium in nature occurs only in ionic salts. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and 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 sea water (which is 0.04% potassium by weight), and is part of many minerals.

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. That they are different elements that combine with the same anions to make similar salts was suspected in 1702, and was proven in 1807 using electrolysis. Naturally occurring potassium is composed of three isotopes, of which ⁴⁰
K is radioactive. Traces of ⁴⁰
K are found in all potassium, and it is the most common radioisotope in the human body.

Potassium ions are necessary for the function of all living cells. The transfer of potassium ions through nerve cell membranes is necessary for normal nerve transmission; potassium deficiency and excess can each result in numerous abnormalities, including an abnormal heart rhythm and various electrocardiographic (ECG) 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, with a shift of potassium from outside to inside cells and an increase in potassium excretion by the kidneys.

Most industrial applications of potassium exploit the high solubility in water of potassium compounds, such as potassium soaps. 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.