Some elements exist in several different structural forms, called allotropes. Each allotrope has different physical properties.

For more information on the Visual Elements image see the Uses and properties section below.



A vertical column in the periodic table. Members of a group typically have similar properties and electron configurations in their outer shell.

A horizontal row in the periodic table. The atomic number of each element increases by one, reading from left to right.

Elements are organised into blocks by the orbital type in which the outer electrons are found. These blocks are named for the characteristic spectra they produce: sharp (s), principal (p), diffuse (d), and fundamental (f).

Atomic number
The number of protons in an atom.

Electron configuration
The arrangements of electrons above the last (closed shell) noble gas.

Melting point
The temperature at which the solid–liquid phase change occurs.

Boiling point
The temperature at which the liquid–gas phase change occurs.

The transition of a substance directly from the solid to the gas phase without passing through a liquid phase.

Density (g cm−3)
Density is the mass of a substance that would fill 1 cm3 at room temperature.

Relative atomic mass
The mass of an atom relative to that of carbon-12. This is approximately the sum of the number of protons and neutrons in the nucleus. Where more than one isotope exists, the value given is the abundance weighted average.

Atoms of the same element with different numbers of neutrons.

CAS number
The Chemical Abstracts Service registry number is a unique identifier of a particular chemical, designed to prevent confusion arising from different languages and naming systems.

Fact box

Group Melting point 63.5°C, 146.3°F, 336.7 K 
Period Boiling point 759°C, 1398°F, 1032 K 
Block Density (g cm−3) 0.89 
Atomic number 19  Relative atomic mass 39.098  
State at 20°C Solid  Key isotopes 39
Electron configuration [Ar] 4s1  CAS number 7440-09-7 
ChemSpider ID 4575326 ChemSpider is a free chemical structure database


Image explanation

Murray Robertson is the artist behind the images which make up Visual Elements. This is where the artist explains his interpretation of the element and the science behind the picture.


The description of the element in its natural form.

Biological role

The role of the element in humans, animals and plants.

Natural abundance

Where the element is most commonly found in nature, and how it is sourced commercially.

Uses and properties

Image explanation
The image features the alchemical symbol for potash, from which the element was first isolated.
A soft, silvery metal that tarnishes in air within minutes.
The greatest demand for potassium compounds is in fertilisers. Many other potassium salts are of great importance, including the nitrate, carbonate, chloride, bromide, cyanide and sulfate. Potassium carbonate is used in the manufacture of glass. Potassium hydroxide is used to make detergent and liquid soap. Potassium chloride is used in pharmaceuticals and saline drips.
Biological role
Potassium is essential to life. Potassium ions are found in all cells. It is important for maintaining fluid and electrolyte balance.

Plant cells are particularly rich in potassium, which they get from the soil. Agricultural land, from which harvests are taken every year, needs to have its potassium replenished by adding potassium-based fertilisers.

The average human consumes up to 7 grams of potassium a day, and stores about 140 grams in the body cells. A normal healthy diet contains enough potassium, but some foods such as instant coffee, sardines, nuts, raisins, potatoes and chocolate have above average potassium content.

The naturally occurring isotope potassium-40 is radioactive and, although this radioactivity is mild, it may be one natural cause of genetic mutation in humans.
Natural abundance
Potassium is the seventh most abundant metal in the Earth’s crust. It makes up 2.4% by mass. There are deposits of billions of tonnes of potassium chloride throughout the world. Mining extracts about 35 million tonnes a year.

Most potassium minerals are found in igneous rocks and are sparingly soluble. The metal is difficult to obtain from these minerals. There are, however, other minerals such as sylvite (potassium chloride), sylvinite (a mixture of potassium and sodium chloride) and carnallite (potassium magnesium chloride) that are found in deposits formed by evaporation of old seas or lakes. The potassium salts can be easily recovered from these. Potassium salts are also found in the ocean but in smaller amounts compared with sodium.
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Potassium salts in the form of saltpetre (potassium nitrate, KNO3), alum (potassium aluminium sulfate, KAl(SO4)2), and potash (potassium carbonate, K2CO3) have been known for centuries. They were used in gunpowder, dyeing, and soap making. They were scraped from the walls of latrines, manufactured from clay and sulfuric acid, and collected as wood ash respectively. Reducing them to the element defeated the early chemists and potassium was classed as an ‘earth’ by Antoine Lavoisier. Then in 1807, Humphry Davy exposed moist potash to an electric current and observed the formation of metallic globules of a new metal, potassium. He noted that when they were dropped into water they skimmed around on the surface, burning with a lavender-coloured flame.

Atomic radius, non-bonded
Half of the distance between two unbonded atoms of the same element when the electrostatic forces are balanced. These values were determined using several different methods.

Covalent radius
Half of the distance between two atoms within a single covalent bond. Values are given for typical oxidation number and coordination.

Electron affinity
The energy released when an electron is added to the neutral atom and a negative ion is formed.

Electronegativity (Pauling scale)
The tendency of an atom to attract electrons towards itself, expressed on a relative scale.

First ionisation energy
The minimum energy required to remove an electron from a neutral atom in its ground state.

Atomic data

Atomic radius, non-bonded (Å) 2.75 Covalent radius (Å) 2.00
Electron affinity (kJ mol−1) 48.385 Electronegativity
(Pauling scale)
Ionisation energies
(kJ mol−1)


Common oxidation states

The oxidation state of an atom is a measure of the degree of oxidation of an atom. It is defined as being the charge that an atom would have if all bonds were ionic. Uncombined elements have an oxidation state of 0. The sum of the oxidation states within a compound or ion must equal the overall charge.


Atoms of the same element with different numbers of neutrons.

Key for isotopes

Half life
  y years
  d days
  h hours
  m minutes
  s seconds
Mode of decay
  α alpha particle emission
  β negative beta (electron) emission
  β+ positron emission
  EC orbital electron capture
  sf spontaneous fission
  ββ double beta emission
  ECEC double orbital electron capture

Oxidation states and isotopes

Common oxidation states 1
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  39K 38.964 93.2581
  40K 39.964 0.0117 1.248 x 109 β- 
  41K 40.962 6.7302


Data for this section been provided by the British Geological Survey.

Relative supply risk

An integrated supply risk index from 1 (very low risk) to 10 (very high risk). This is calculated by combining the scores for crustal abundance, reserve distribution, production concentration, substitutability, recycling rate and political stability scores.

Crustal abundance (ppm)

The number of atoms of the element per 1 million atoms of the Earth’s crust.

Recycling rate

The percentage of a commodity which is recycled. A higher recycling rate may reduce risk to supply.


The availability of suitable substitutes for a given commodity.
High = substitution not possible or very difficult.
Medium = substitution is possible but there may be an economic and/or performance impact
Low = substitution is possible with little or no economic and/or performance impact

Production concentration

The percentage of an element produced in the top producing country. The higher the value, the larger risk there is to supply.

Reserve distribution

The percentage of the world reserves located in the country with the largest reserves. The higher the value, the larger risk there is to supply.

Political stability of top producer

A percentile rank for the political stability of the top producing country, derived from World Bank governance indicators.

Political stability of top reserve holder

A percentile rank for the political stability of the country with the largest reserves, derived from World Bank governance indicators.

Supply risk

Relative supply risk 4.5
Crustal abundance (ppm) 22774
Recycling rate (%) Unknown
Substitutability Unknown
Production concentration (%) 20.9
Reserve distribution (%) 61.1
Top 3 producers
  • 1) Canada
  • 2) Russia
  • 3) Belarus
Top 3 reserve holders
  • 1) Canada
  • 2) Russia
  • 3) Belarus
Political stability of top producer 81.1
Political stability of top reserve holder 81.1


Specific heat capacity (J kg−1 K−1)

Specific heat capacity is the amount of energy needed to change the temperature of a kilogram of a substance by 1 K.

Young's modulus

A measure of the stiffness of a substance. It provides a measure of how difficult it is to extend a material, with a value given by the ratio of tensile strength to tensile strain.

Shear modulus

A measure of how difficult it is to deform a material. It is given by the ratio of the shear stress to the shear strain.

Bulk modulus

A measure of how difficult it is to compress a substance. It is given by the ratio of the pressure on a body to the fractional decrease in volume.

Vapour pressure

A measure of the propensity of a substance to evaporate. It is defined as the equilibrium pressure exerted by the gas produced above a substance in a closed system.

Pressure and temperature data – advanced

Specific heat capacity
(J kg−1 K−1)
757 Young's modulus (GPa) Unknown
Shear modulus (GPa) Unknown Bulk modulus (GPa) 3.1
Vapour pressure  
Temperature (K)
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
Pressure (Pa)
0.0188 96.9 - - - - - - - - -
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Listen to Potassium Podcast
Transcript :

Chemistry in its element: potassium


You're listening to Chemistry in its element brought to you by Chemistry World, the magazine of the Royal Society of Chemistry.

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Chris Smith

Hello, this week the story of the first alkaline metal ever isolated, why it's an alkaline metal at all and why its symbol begins with the letter K. Here's Peter Wothers.

Peter Wothers

Potassium - the only element named after a cooking utensil. It was named in 1807 by Humphry Davy after the compound from which he isolated the metal, potash, or potassium hydroxide.

An extract from the 1730s by the Dutch chemist Herman Boerhaave describes how potash got its name:

"Potas or Pot-ashes is brought yearly by the Merchant's Ships in great abundance from Coerland (now part of Latvia and Lithuania), Russia, and Poland. It is prepared there from the Wood of green Fir, Pine, Oak, and the like, of which they make large piles in proper Trenches, and burn them till they are reduced to Ashes... These ashes are then dissolved in boiling Water, and when the Liquor at top, which contains the Salt, is depurated, i.e. freed from impurities, by standing quiet, it is poured off clear. This, then, is immediately put into large copper Pots, and is there boiled for the space of three days, by which means they procure the Salt they call Potas, (which signifies Pot-Ashes) on account of its being thus made in Pots.

Even earlier in the 16th Century, Conrad Gesner tells us that "Of the hearbe called Kali, doe certayne prepare a Salt"

He describes this plant, Kali whose Latin name is Salsola kali but is more commonly known as Saltwort:

"Kali is of two Cubites of heygth, hauing no prickles or thornes, & is sometymes very red, saltye in taste, with a certayne vngratefull smell, found & gathered in saltie places: out of which, the Salt of Alkali maye be purchased"

His method of production of this Salt of Alkali is pretty similar to that described by Boerhaave with both processes actually yielding an impure mixture of what we would now call potassium and sodium carbonate; the wood ash method yielding more potassium carbonate, potash, the salty herbs giving more sodium carbonate, soda. However, it is from the herb kali, that we owe the word that describes both - al-kali or alkali; the 'al' prefix simply being Arabic definite article 'the'.

The crude potash can be made more caustic or 'pure' by treating a solution of it with lime water, calcium hydroxide. The potassium carbonate and calcium hydroxide solutions react with a bit of chemical partner-swapping: insoluble calcium carbonate or chalk precipitates out, leaving a solution of potassium hydroxide. It was from this pure hydroxide that Davy first isolated the metal potassium. To do this he used the relatively new force of electricity.

After unsuccessfully trying to electrolyse aqueous solutions of potash, during which he only succeeded in breaking apart the water, he reasoned that he needed to do away with the water and try to electrolyse molten potassium hydroxide. This he did on the sixth of October, 1807 using the large Voltaic pile he had built at the Royal Institute in London. His younger cousin, Edmund Davy, was assisting Humphry at the time and he relates how when Humphry first saw "the minute globules of potassium burst through the crust of potash, and take fire as they entered the atmosphere, he could not contain his joy".

Davy had every right to be delighted with this amazing new metal: it looked just like other bright, shiny metals but its density was less than that of water. This meant the metal would float on water --at least, it would do if it didn't explode as soon as it came into contact with the water. Potassium is so reactive , it will even react and burn a hole through ice. This was the first alkali metal to be isolated, but Davy went on to isolate sodium, calcium, magnesium and barium.

Whilst Davy named his new metal potassium after the potash, Berzelius, the Swedish chemist who invented the international system of chemical symbols now used by chemists the world over, preferred the name kalium for the metal, better reflecting its true origins, he thought. Hence it is due a small salty herb that we now end up with the symbol K for the element pot-ash-ium, potassium.

Chris Smith

Cambridge chemist Peter Wothers. Next time beautiful but deadly is the name of the game.

Bea Perks

Arsenic gets its name from a Persian word for the yellow pigment now known as orpiment. For keen lexicographers apparently the Persian word in question Zarnikh was subsequently borrowed by the Greeks for their word arsenikon which means masculine or potent. On the pigment front, Napoleon's wallpaper just before his death is reported to have incorporated a so called Scheele's green which exuded an arsenic vapour when it got damp.

Chris Smith

So potent or not, licking the wallpaper in Napoleon's apartments is definitely off the menu. That's Bea Perks who will be with us next time to tell us the deadly tale of arsenic, I hope you can join us. I'm Chris Smith, thank you for listening and goodbye.


Chemistry in its element is brought to you by the Royal Society of Chemistry and produced by There's more information and other episodes of Chemistry in its element on our website at

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Visual Elements images and videos
© Murray Robertson 1998-2017.



W. M. Haynes, ed., CRC Handbook of Chemistry and Physics, CRC Press/Taylor and Francis, Boca Raton, FL, 95th Edition, Internet Version 2015, accessed December 2014.
Tables of Physical & Chemical Constants, Kaye & Laby Online, 16th edition, 1995. Version 1.0 (2005), accessed December 2014.
J. S. Coursey, D. J. Schwab, J. J. Tsai, and R. A. Dragoset, Atomic Weights and Isotopic Compositions (version 4.1), 2015, National Institute of Standards and Technology, Gaithersburg, MD, accessed November 2016.
T. L. Cottrell, The Strengths of Chemical Bonds, Butterworth, London, 1954.


Uses and properties

John Emsley, Nature’s Building Blocks: An A-Z Guide to the Elements, Oxford University Press, New York, 2nd Edition, 2011.
Thomas Jefferson National Accelerator Facility - Office of Science Education, It’s Elemental - The Periodic Table of Elements, accessed December 2014.
Periodic Table of Videos, accessed December 2014.


Supply risk data

Derived in part from material provided by the British Geological Survey © NERC.


History text

Elements 1-112, 114, 116 and 117 © John Emsley 2012. Elements 113, 115, 117 and 118 © Royal Society of Chemistry 2017.



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