| Discovery date | 1807 |
| Discovered by | Humphry Davy |
| Origin of the name | The name is derived from the English word 'potash'. |
| Allotropes |
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K
Potassium
19
39.098
| Group | 1 | Melting point | 63.5°C, 146.3°F, 336.7 K |
| Period | 4 | Boiling point | 759°C, 1398°F, 1032 K |
| Block | s | Density (g cm−3) | 0.89 |
| Atomic number | 19 | Relative atomic mass | 39.098 |
| State at 20°C | Solid | Key isotopes | 39K |
| Electron configuration | [Ar] 4s1 | CAS number | 7440-09-7 |
| ChemSpider ID | 4575326 | ChemSpider is a free chemical structure database | |
Image explanation
The image features the alchemical symbol for potash, from which the element was first isolated.
Appearance
A soft, silvery metal that tarnishes in air within minutes.
Uses
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.
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 (Å) | 2.75 | Covalent radius (Å) | 2.00 |
| Electron affinity (kJ mol−1) | 48.385 |
Electronegativity (Pauling scale) |
0.82 |
|
Ionisation energies (kJ mol−1) |
1st
418.81
2nd
3051.83
3rd
4419.607
4th
5876.92
5th
7975.48
6th
9590.6
7th
11342.82
8th
14943.65
|
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| 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 y | β- | |
|
|
β+ | ||||
| 41K | 40.962 | 6.7302 | - | - | |
|
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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) |
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| Pressure (Pa) |
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| Listen to Potassium Podcast |
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Transcript :
Chemistry in its element: potassium (Promo) You're listening to Chemistry in its element brought to you by Chemistry World, the magazine of the Royal Society of Chemistry. (End promo) 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. (Promo) Chemistry in its element is brought to you by the Royal Society of Chemistry and produced by thenakedscientists.com. There's more information and other episodes of Chemistry in its element on our website at chemistryworld.org/elements. (End promo)
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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.
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.
Derived in part from material provided by the British Geological Survey © NERC.
Elements 1-112, 114, 116 and 117 © John Emsley 2012. Elements 113, 115, 117 and 118 © Royal Society of Chemistry 2017.
Produced by The Naked Scientists.
Created by video journalist Brady Haran working with chemists at The University of Nottingham.
© Murray Robertson 1998-2017.
Data
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.
Podcasts
Produced by The Naked Scientists.
Periodic Table of Videos
Created by video journalist Brady Haran working with chemists at The University of Nottingham.
