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] 4s²
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, 2
Number of electrons in each shell, from the nucleus outwards.
842 °C (1,115 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.
1,484 °C (1,757 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).
1.54 g/cm³
Mass of one cubic centimetre of the substance. For gases, at 0 °C and standard pressure.
1.00 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.
6.113 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.
0.025 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.
176 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.
+2
Charges the atom would have in its compounds if all its bonds were ionic; the main ones are listed.
Calcium (Ca) is a chemical element with atomic number 20. It sits in group 2 and period 4 of the periodic table, in the s-block. It is an alkaline earth metal. At room temperature it is a solid. Its standard atomic weight is 40.078.
Origin of the name
The name comes from the Latin calx, ‘lime’, the white powder made by heating limestone. The Romans already made lime, but chemists long took it for an element, until Antoine Lavoisier suspected in 1789 that it was the oxide of an unknown metal. In 1808 Humphry Davy proved him right: after Jöns Jacob Berzelius and Magnus Pontin had made a calcium amalgam, Davy passed electricity through moist lime mixed with mercury oxide, distilled off the mercury and named the metal he was left with calcium.
Where is calcium found?
Calcium reacts too readily ever to occur free, yet it is the fifth most abundant element in the Earth’s crust and the third most common metal after aluminium and iron. Limestone, chalk and marble are all calcium carbonate, much of it built from the shells and skeletons of ancient sea creatures; corals and sea shells are made of it today. Gypsum, fluorite and apatite are calcium minerals too.
What is calcium used for?
The metal itself has few uses: it cleans oxygen and sulfur out of molten steel and helps extract uranium and thorium. Its compounds are everywhere. Limestone heated in a kiln gives quicklime, and quicklime with water gives slaked lime, a cheap base used throughout industry; lime and limestone go into cement and mortar, and gypsum into plaster and plasterboard.
Interesting facts
People have built with lime for thousands of years.
Water containing carbon dioxide slowly dissolves limestone, then leaves it behind again drop by drop: that is how caves grow their stalactites and stalagmites.
Calcium is the most abundant metal in the human body, and almost all of it is in bones and teeth.