Periodic Table


Atomic number: 19
Atomic weight: 39.0983
Symbol: K
Group number: 1
Electronic configuration: [Ar].4s1


The metal is the seventh most abundant and makes up about 2.4% by weight of the earth's crust. Potassium is an essential constituent for plant growth and it is found in most soils. It is also a necessary element in the human diet.

Potassium is never found free in nature, but is obtained by electrolysis of the chloride or hydroxide, much in the same manner as prepared by Davy. It is one of the most reactive and electropositive of metals and, except for lithium, it is the lightest known metal. It is soft, easily cut with a knife, and is silvery in appearance immediately after a fresh surface is exposed.

It oxidises very rapidly in air and must be stored under argon or under a suitable mineral oil. As with other metals of the alkali group, it decomposes in water with the evolution of hydrogen. It catches fire spontaneously on water. Potassium and its salts impart a lilac colour to flames.

General information

Discoveror: Sir Humphrey Davy
Date discovered: 1807
Discovered at: England
Meaning of name: From the English word "potash/EM>" (pot ashes) and the Arabic word "qali meaning alkali (the origin of the symbol K comes from the Latin word "kalium")

Physical data

Standard state: solid at 298 K
Colour: silvery white
Density of solid at ambient temperature/kg m-3: 856
Molar volume/cm3: 45.94

Radii /pm

Atomic: 243
Covalent (single bond):
Pauling radius for the ion [K]-: no data

Valence shell orbital radius maxima (Rmax)
orbital s p d f
radius230.0no datano datano data


Both values are quoted on the Pauling scale.

Pauling: 0.82
Allred Rochow: 0.91

Crystal Structure

structure: bcc (body-centred cubic)

The following CrystalMaker image represents the solid state structure. For most elements, the most stable allotrope is illustrated. Try WebElements version 2 for interactive virtual reality and CHIME images.


Temperatures (/K)

melting point: 336.53
boiling point: 1032

Enthalpies /kJ mol-1

fusion: 2.33
vaporization: 76.9

single bond enthalpies:
K-F K-Cl K-Br K-I K-K
490 423 379 326 57.3

Ionization enthalpies /kJ mol-1

Number Enthalpy


This section gives some data for naturally occurring isotopes.
Nominal mass Accurate mass % natural abundance
39K38.9637074 (12)93.2581 (30)
40K39.9639992 (12)0.0117 (1)
41K40.9618254 (12)6.7302 (30)

Further Information

Copyright 1997 Mark Winter
Department of Chemistry at the University of Sheffield, Sheffield S3 7HF, England.

The current version of this document is at http://www.shef.ac.uk/~chem/web-elements-I/K.html