Average mass of an atom in unified atomic mass units (1 u is 1/12 of the mass of a carbon-12 atom). Accounts for the natural mix of the element’s isotopes.
Number of the vertical column, 1 to 18 in IUPAC numbering. Elements in a group share the same outer-shell layout, so their properties are alike.
Number of the horizontal row. Usually equal to the number of electron shells in the atom; palladium is the exception, with four shells in period 5.
The subshell (s, p, d or f) that receives the last electrons in the usual filling order; it divides the table into four blocks. Real atoms sometimes differ: lanthanum starts the f-block but has no f electrons.
[Ar] 4s¹
How electrons are arranged in subshells. The bracket stands for the previous noble gas; the superscript is the number of electrons in a subshell.
2, 8, 8, 1
Number of electrons in each shell, from the nucleus outwards.
63.4 °C (336.53 K)
Temperature at which the solid turns liquid at standard pressure. Helium, carbon and arsenic do not melt at normal pressure: helium needs about 25 atm, carbon and arsenic turn straight into vapour.
759 °C (1,032 K)
Temperature at which the liquid boils at standard pressure. For carbon and arsenic, the temperature at which the solid turns straight into vapour (sublimation).
0.89 g/cm³
Mass of one cubic centimetre of the substance. For gases, at 0 °C and standard pressure.
0.82 Pauling
How strongly an atom pulls electrons towards itself in a chemical bond. On the Pauling scale it runs from 0.79 for caesium to 3.98 for fluorine.
4.341 eV
Energy needed to remove the most loosely bound electron from an atom. The lower it is, the more easily the element gives up an electron, which is typical of metals.
0.501 eV
Energy released when a neutral atom gains an electron. Highest for the halogens. Noble gases, nitrogen and some metals form no stable negative ion.
203 pm
Half the length of a single covalent bond between two atoms of the element. It shows the size of an atom inside a molecule or crystal: 76 pm for carbon, 132 pm for iron.
+1
Charges the atom would have in its compounds if all its bonds were ionic; the main ones are listed.
Potassium (K) is a chemical element with atomic number 19. It sits in group 1 and period 4 of the periodic table, in the s-block. It is an alkali metal. At room temperature it is a solid. Its standard atomic weight is 39.098.
Origin of the name
Humphry Davy, who in 1807 passed electricity through molten caustic potash and got the first metal ever isolated by electrolysis, named it potassium after potash, the salt once made by soaking wood ash in water and boiling it down in pots. The symbol K comes from kalium, a name coined in German in 1809; it goes back to the Arabic al-qalyah, ‘plant ashes’, which also gave the word alkali.
Where is potassium found?
Potassium is too reactive ever to occur free, though it is the seventh most abundant element in the Earth’s crust. It is mined from salts such as sylvite and carnallite, left behind when ancient seas and lakes dried up; Canada, with deposits deep under Saskatchewan, is the largest producer. Sea water holds much less potassium than sodium.
What is potassium used for?
Most potassium goes into fertilizers: with nitrogen and phosphorus it is one of the three main nutrients plants need, the K in NPK. Potassium nitrate (saltpetre) is in matches, fireworks and gunpowder, potassium hydroxide makes soft soaps, and potassium superoxide releases oxygen in breathing equipment on submarines and spacecraft.
Interesting facts
The metal is soft enough to cut with a knife. Dropped into water it releases hydrogen so violently that the gas catches fire, so potassium is kept under oil.
Potassium and its salts colour a flame lilac.
A tiny share of natural potassium is radioactive potassium-40, which slowly turns into argon; measuring the two lets geologists find the age of rocks.
Potassium ions moving in and out of nerve cells carry every nerve impulse in your body.