Calcium – facts and properties
- Symbol
- Ca
- Atomic number
- 20
- Atomic mass
- 40.078 u
- Period
- 4
- Group
- 2
- Category
- alkaline earth metal
- Electron configuration
- [Ar] 4s²
- Electronegativity
- 1
- Atomic radius
- 194 pm
- Valence electrons
- 2
- State at standard conditions
- solid
- Melting point
- 842 °C
- Boiling point
- 1484 °C
- Density
- 1.54 g/cm³
- Discovered
- known since antiquity
Calcium is a silvery alkaline earth metal with atomic number 20. You will rarely see it as a metal, but its compounds are everywhere: limestone, marble and chalk are calcium carbonate, gypsum is calcium sulfate, and bones and teeth owe their hardness to a calcium phosphate. Calcium is the most abundant metal in the human body and a basic material of construction, since without lime there would be no mortar and no cement.
Position in the periodic table
Occurrence and extraction
Calcium is the fifth most abundant element in the Earth's crust. Because it is so reactive, it occurs only in compounds. Whole mountain ranges, such as the Dolomites, are built of limestone and dolomite rock, which formed largely from the shells and skeletons of marine organisms such as molluscs, corals and calcareous algae. Other important calcium minerals are gypsum, fluorite and apatite. Calcium ions are also dissolved in seawater and in tap water.
Lime and gypsum have been known since antiquity. The ancient Egyptians used gypsum mortar, the Romans built with lime mortar, and the name calcium comes from calx, the Latin word for lime. The metal itself, however, stayed hidden for a long time. Only in 1808 did Humphry Davy isolate calcium by electrolysis, first as an amalgam with mercury, from which he then distilled off the mercury. He obtained only small, impure amounts; Robert Bunsen and Augustus Matthiessen produced purer calcium several decades later. Today the metal is mostly made by reducing calcium oxide with aluminium under vacuum, or by electrolysis of molten calcium chloride.
Properties and reactions
Calcium has the electron configuration [Ar] 4s² and two valence electrons. It belongs to period 4 and group 2, the alkaline earth metals, below magnesium. Its electronegativity is 1. When the atom gives up both valence electrons it becomes a calcium ion with the electron arrangement of argon, so in compounds calcium has the oxidation state plus two.
The metal is fairly soft, with a density of 1.54 g/cm³. It melts at 842 °C and boils at 1484 °C. In air it becomes coated with a greyish white layer of oxide and nitride. Calcium reacts with water much more gently than sodium or potassium: it sinks and steadily gives off hydrogen while forming calcium hydroxide. Calcium compounds colour a flame brick red.
In everyday life the lime cycle matters more than the metal. Strongly heating limestone drives off carbon dioxide and leaves quicklime, which is calcium oxide. Quicklime reacts with water, releasing a lot of heat, to give slaked lime, calcium hydroxide. In mortar, slaked lime takes up carbon dioxide from the air and turns back into solid calcium carbonate. Water containing dissolved carbon dioxide, on the other hand, slowly dissolves limestone. This creates caves and produces the hard water that leaves limescale in kettles. Where the water loses carbon dioxide again, the dissolved lime is deposited once more and builds stalactites.
Uses in everyday life and technology
Calcium compounds are among the most widely used raw materials. Limestone and clay are burned together to make cement, the binder in concrete. Quicklime and slaked lime are used in mortar and plaster, in water treatment and in steelmaking, where they bind impurities into the slag. Gypsum is turned into plasterboard and plaster. Finely ground calcium carbonate is a filler in paper, plastics and toothpaste, and farmers spread lime to neutralise acidic soils.
The metal itself is used in smaller amounts, for example as a reducing agent in the production of other metals and to remove oxygen and sulfur from steel. Calcium carbide reacts with water to give ethyne, also called acetylene, a gas that once fuelled carbide lamps.
The body cannot do without calcium. An adult carries around one kilogram of it, almost all in bones and teeth. Calcium ions are also involved in muscle contraction, blood clotting and the transmission of nerve signals. Good sources are dairy products, green vegetables such as kale and broccoli, and calcium-rich mineral water.
Molecules containing this element
- Calcium oxide CaO
- Calcium chloride CaCl2
- Calcium hydroxide Ca(OH)2
Quick question
Which element has the electron configuration [Ar] 4s1?
FAQ
When was calcium discovered?
People have used calcium compounds such as lime and gypsum for thousands of years. The metal itself was first isolated by Humphry Davy in 1808, using electrolysis. That is why calcium is usually described as known since antiquity, even though the metal itself has been known for only a little over two centuries.
What is the difference between calcium and lime?
Calcium is the element, a reactive metal. Lime is a calcium compound; in the broad sense the word covers limestone, which is calcium carbonate, as well as quicklime and slaked lime. When food labels mention calcium, they mean calcium ions, not the metal.
Why is calcium important for bones?
Bones and teeth consist largely of hydroxyapatite, a hard calcium phosphate. The body constantly rebuilds bone and needs a regular supply of calcium from food to do so. If there is too little for a long time, it draws calcium from the bones, which can then lose strength.