Calcium
Calcium is the fifth most abundant element in the Earth's crust and the most abundant metal in the human body. It builds your bones and teeth, helps your blood clot, makes your muscles contract and keeps your heart beating in rhythm. It is also the mineral behind limestone, marble, chalk, coral reefs and the White Cliffs of Dover.
- Atomic Number2020 protons, 20 electrons
- Atomic Mass40.08 uAbout 40× heavier than hydrogen
- State at Room TempSolidsoft, silvery metal
- Density1.54 g/cm³About 1.5× denser than water
- Melting / Boiling841.9°C / 1483.8°CMelts at a moderate 842°C
- DiscoveredAncientHumphry Davy, 1808
How does calcium compare to the other alkaline earth metals?
Calcium is the most abundant alkaline earth metal in the Earth's crust and in the human body.
Calcium at 40 u is heavier than magnesium (24 u) but lighter than strontium (88 u). The alkaline earth metals roughly double in mass each step down Group 2, reflecting the increasing number of electrons filling larger and larger electron shells.
What is calcium?
Calcium is an alkaline earth metal in Group 2 of the periodic table. It has 20 protons and two electrons in its outer shell, which it gives away readily to form Ca²⁺ ions. Pure calcium is a silvery-white metal, softer than steel but harder than sodium or potassium. It reacts slowly with oxygen in air and more vigorously with water, producing hydrogen gas and calcium hydroxide (lime water).
Calcium gets its name from the Latin word calx, meaning lime (calcium oxide). Lime has been made by heating limestone since ancient times and used in construction, agriculture and as a flux in metal smelting. Humphry Davy chose the name when he first isolated the metal in 1808, connecting it to this long-familiar and widely used substance.
Where you find calcium
In space
Calcium is produced inside stars and is widespread across the universe. Calcium spectral lines are visible in the light from stars. The surfaces of the Moon and Mars contain calcium-rich minerals, and calcium compounds have been found in meteorites. On Earth, calcium in the form of limestone and other minerals has shaped landscapes on every continent.
On Earth
Calcium makes up approx. 4.1% of the Earth's crust by mass, found mainly as calcium carbonate and calcium silicate minerals.
- Limestone, chalk and marble. All three are forms of calcium carbonate (CaCO₃). Limestone is the most widespread rock in many parts of the world; chalk is a fine-grained variety; marble is limestone transformed by heat and pressure. The White Cliffs of Dover are chalk.
- Gypsum. Calcium sulfate (CaSO₄·2H₂O) is a soft mineral mined on a large scale for making plaster, drywall and cement additives.
- Coral and shells. Coral reefs, oyster shells, clamshells and sea urchin spines are all made of calcium carbonate, extracted from seawater by marine creatures.
How we use calcium
- Building materials. Limestone is one of the most quarried rocks in the world, used for building, road aggregate and heated to make quicklime (calcium oxide) for cement, mortar and plaster. Concrete depends on calcium chemistry.
- Steel making. Calcium oxide (lime) is added to blast furnaces to remove impurities such as silica from molten iron, forming a slag that floats to the surface and can be removed.
- Agriculture. Ground limestone and calcium oxide are spread on acid soils to neutralise them and make them suitable for crops. Most plants grow poorly in very acidic conditions.
- Bone and dental health. Calcium supplements and calcium-rich foods (dairy products, leafy vegetables, fish with bones) help maintain strong bones and teeth throughout life.
How it was discovered
Calcium was isolated in 1808 by Humphry Davy at the Royal Institution in London. He passed an electric current through a mixture of lime (calcium oxide) and mercury oxide, then distilled off the mercury to leave behind an impure calcium amalgam. It was not a perfectly pure sample, but it was enough to confirm that lime contained a previously unknown metal. Davy named it calcium from the Latin calx. Completely pure calcium metal was not obtained until much later, after better purification techniques were developed.
Deeper dive: calcium in biology: bones, muscles and blood clotting
Calcium is the most abundant mineral in the human body, with 99% of it locked in bones and teeth as hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂), a combination of calcium and phosphate that gives bone its hardness while collagen fibres give it flexibility. The remaining 1% of body calcium, circulating in the blood and inside cells, is crucial for a huge range of processes.
Muscle contraction requires calcium ions: when a nerve signal reaches a muscle cell, calcium ions are released from internal storage. They bind to proteins in the muscle fibre, causing it to shorten. When the signal stops, calcium is pumped back into storage and the muscle relaxes. Your heart is just a specialised muscle that repeats this process approx. 70 times a minute throughout your life.
Blood clotting (coagulation) involves a cascade of proteins that need calcium ions at almost every step. Without calcium, blood would not clot properly, which is why sodium citrate or EDTA (chemicals that bind calcium tightly) are added to blood samples for testing, to prevent clotting so the sample can be analysed. Vitamin D is essential for absorbing calcium from food, which is why vitamin D deficiency leads to weak bones (rickets in children, osteoporosis in adults).
Calcium builds the physical structures of life, from your skeleton to coral reefs to chalk cliffs. Moving to 21 protons on the periodic table brings us to scandium, the first of the transition metals.