Calcium

Calcium is the chemical element with atomic number 20 and the symbol Ca. It is a soft, silvery-white alkaline earth metal in group 2 of the periodic table, and it is the fifth most abundant metal in the Earth’s crust. Calcium is essential to bones, teeth and shells, and its compounds, such as limestone, are among the most widely used materials in construction.

History of calcium

Lime, or calcium oxide, is the useful product made by heating limestone, and for centuries it was used to make plaster and mortar. The name of the element comes from the Latin calx, meaning lime. Antoine Lavoisier listed lime among the earths, substances that seemed impossible to break down further, but he suspected that it was the oxide of an unknown element.

In 1808 Humphry Davy tried to split moist lime by electrolysis, as he had done with sodium and potassium, but it did not work. He then electrolyzed a mixture of lime and mercury oxide, which produced an amalgam, an alloy of calcium and mercury. That alone did not prove that he had a new element. The Swedish chemist Jöns Jacob Berzelius had carried out a similar experiment and also obtained the amalgam.

Davy repeated the work with more lime in the mixture, made more of the amalgam, and distilled off the mercury. What remained was calcium. The Royal Society of Chemistry credits Davy with the discovery in 1808.

Properties of calcium

Calcium has a standard atomic weight of about 40.08 and an electron configuration of [Ar] 4s2. The two outer electrons are lost easily, so calcium almost always has an oxidation state of +2 in its compounds. PubChem lists an electronegativity of 1.0 and a first ionization energy of 6.113 electronvolts.

The metal is softer than most others and is silvery white when freshly cut, but it tarnishes rapidly in air. It reacts with water to release hydrogen, although less violently than the metals below it in group 2. Calcium melts at about 842 degrees Celsius, boils at about 1,484 degrees Celsius and has a density of 1.54 grams per cubic centimeter.

When heated in a flame, calcium compounds give a brick-red color, which chemists use for identification and fireworks designers use for red effects. Its chemistry is similar to that of magnesium, which lies directly above it in the periodic table.

Occurrence and production of calcium

Calcium makes up about 4.1 percent of the Earth’s crust by mass. It is never found uncombined. Instead it occurs as limestone, which is calcium carbonate, gypsum, which is calcium sulfate, fluorite, which is calcium fluoride, and apatite, a calcium phosphate mineral that also holds fluorine or chlorine.

Hard water contains dissolved calcium bicarbonate. When this water filters through the ground and reaches a cave, calcium carbonate precipitates out of it, forming stalactites and stalagmites over thousands of years. Marine animals use calcium in a similar way to build shells, and over geologic time their remains have formed huge limestone deposits.

Calcium metal is prepared commercially by heating lime with aluminum in a vacuum. Because the metal has limited demand compared with its compounds, most quarrying and mining targets limestone and gypsum instead.

Uses of calcium

Calcium metal is used as a reducing agent, a substance that removes oxygen from other compounds, in preparing metals such as thorium and uranium. It is also added as an alloying agent to aluminum, beryllium, copper, lead and magnesium alloys. An alloy is a metallic mixture designed to have better properties than its separate parts.

Calcium compounds are used far more widely. Limestone is cut and used directly as a building stone and is also the raw material for cement. When limestone is heated in a kiln, it gives off carbon dioxide and leaves behind quicklime, which is calcium oxide. Quicklime reacts vigorously with water to give slaked lime, or calcium hydroxide.

Slaked lime is used to make cement, as a soil conditioner, in water treatment to reduce acidity, and in the chemical industry. It is also used in steelmaking to remove impurities from molten iron. Mixed with sand, slaked lime absorbs carbon dioxide from the air and hardens into lime plaster.

Gypsum is used by builders as a plaster and, in the form of plaster of Paris, by medical staff for setting bones. Calcium carbonate is added to antacids, chalk and food products as a calcium supplement or additive.

Isotopes of calcium

Calcium has six naturally occurring isotopes. Calcium-40 is by far the most common, at about 96.9 percent. The others are calcium-42, calcium-43, calcium-44, calcium-46 and calcium-48, each present in much smaller amounts, with calcium-44 the next most abundant at about 2.1 percent.

Calcium-48 is technically radioactive, but its half-life is so long, far greater than the age of the universe, that it behaves as stable for nearly all purposes. Calcium-40 is the final product of the decay of potassium-40, which is described in the entry on isotope and half-life. Calcium-41, with a half-life of about 99,000 years, is a rare radioactive isotope that has been studied for dating bones and rocks.

Calcium in life and the environment

Calcium is essential to all living things, and particularly to the growth of healthy teeth and bones. Calcium phosphate is the main mineral in bone, and the average person holds about 1 kilogram of calcium. Calcium ions also take part in muscle contraction, nerve signaling and blood clotting.

The Royal Society of Chemistry lists milk and dairy products, leafy green vegetables, fish, nuts and seeds as sources of calcium. This entry is general background rather than dietary or medical advice, and recommended intakes differ between countries and age groups. Calcium also supports healthy soils, plants and corals.

Quarrying limestone and burning it to make cement and lime release carbon dioxide, so these industries are significant contributors to greenhouse gas emissions. Dissolved calcium also affects the hardness of drinking water, which can leave scale in pipes and kettles.

Editorially reviewed by Specialty Digest Editorial TeamLast reviewed November 4, 2026Researched and drafted with AI assistanceReport an issue