Potassium,  ₁₉K

Potassium pearls (in paraffin oil, ~5 mm each)
General properties
Pronunciation	/pəˈæsiəm/ ​(pə-TAS-ee-əm)
Appearance	silvery gray
Standard atomic weight (Ar, standard)	39.0983(1)
Potassium in the periodic table
Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson Na ↑ K ↓ Rb argon ← potassium → calcium
Atomic number (Z)	19
Group	group 1 (alkali metals)
Period	period 4
Block	s-block
Element category	alkali metal
Electron configuration	[Ar] 4s1
Electrons per shell	2, 8, 8, 1
Physical properties
Phase at STP	K: 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 of potassium
Other properties
Natural occurrence	K: Primordial
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
Iso­tope Abun­dance Half-life (1/2) Decay mode Pro­duct 39K 93.258% stable 40K 0.012% 1.248×109 y β− 1.311 40Ca ε 1.505 40Ar β+ 1.505 40Ar 41K 6.730% stable
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Potassium metal

Potassium is a chemical element in the periodic table. It has the symbol K. This symbol is taken from the Latin word kalium. Potassium's atomic number is 19. It has 19 protons and electrons. Potassium is not found as an element in nature, because it is so reactive.

Potassium has two stable isotopes, with 20 or 22 neutrons. Its atomic mass is 39.098. The unstable isotope with 21 neutrons is one of the most common radioactive materials.