Potassium
Pronunciation/pəˈtæsiəm/(pə-TAS-ee-əm)
Appearancesilvery gray
Standard atomic weightAr, std(K)39.0983(1)
Atomic number (Z)19
Groupgroup 1: H and alkali metals
Periodperiod 4
Blocks-block
Element category Alkali metal
Electron configuration[Ar] 4s1
Electrons per shell
2, 8, 8, 1
Phase at STPsolid
Melting point336.7 K ​(63.5 °C, ​146.3 °F)
Boiling point1032 K ​(759 °C, ​1398 °F)
Density (near r.t.)0.862 g/cm3
when liquid (at m.p.)0.828 g/cm3
Critical point2223 K, 16 MPa
Heat of fusion2.33 kJ/mol
Heat of vaporization76.9 kJ/mol
Molar heat capacity29.6 J/(mol·K)
Oxidation states−1, +1 (a strongly basic oxide)
ElectronegativityPauling scale: 0.82
Ionization energies
  • 1st: 418.8 kJ/mol
  • 2nd: 3052 kJ/mol
  • 3rd: 4420 kJ/mol
  • (more)
Atomic radiusempirical: 227 pm
Covalent radius203±12 pm
Van der Waals radius275 pm
Natural occurrenceprimordial
Speed of sound thin rod2000 m/s (at 20 °C)
Thermal expansion83.3 µm/(m·K) (at 25 °C)
Thermal conductivity102.5 W/(m·K)
Electrical resistivity72 nΩ·m (at 20 °C)
Magnetic orderingparamagnetic
Magnetic susceptibility+20.8·10−6 cm3/mol (298 K)
Young's modulus3.53 GPa
Shear modulus1.3 GPa
Bulk modulus3.1 GPa
Mohs hardness0.4
Brinell hardness0.363 MPa
CAS Number7440-09-7
Discovery and first isolationHumphry Davy (1807)