Potassium
Pronunciation | /p ə ˈ t æ s i ə m/​(pə-TAS-ee-əm)
Appearance | silvery gray
Standard atomic weight Ar, std(K) | 39.0983(1)
Atomic number (Z) | 19
Group | group 1: H and alkali metals
Period | period 4
Block | s-block
Element category | Alkali metal
Electron configuration | [Ar] 4s1
Electrons per shell | 2, 8, 8, 1
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)
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
Natural occurrence | primordial
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
Discovery and first isolation | Humphry Davy (1807)