Ca · Element 20 · Alkaline earth metal
Structural element of bone, shell and cement: calcium is how life learned to build hard things.
Structure
Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.
Measured values
Every bar shows where calcium sits among all 118 elements for that property.
Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.
Same element, different neutrons
All calcium atoms have the same number of protons, 20, and differ only in their number of neutrons: those variants are its isotopes. The most common, Ca-40, has 20 neutrons and makes up 96.941% of natural calcium.
| ISOTOPE | Neutrons | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|---|
| Ca-40 most common | 20 | 96.941% | Stable | — |
| Ca-41 | 21 | — | 99400 yrs | ε |
| Ca-42 | 22 | 0.647% | Stable | — |
| Ca-43 | 23 | 0.135% | Stable | — |
| Ca-44 | 24 | 2.086% | Stable | — |
| Ca-45 | 25 | — | 162.61 days | β⁻ |
| Ca-46 | 26 | 0.004% | Stable | — |
| Ca-48 | 28 | 0.187% | ~2.9 × 10¹⁹ yrs | 2β⁻, 2β⁻, β⁻ |
Here the classroom shortcut, rounded atomic mass (40.078) minus atomic number, gives the same answer: 20.
Decay modes
~ marks a half-life whose published uncertainty is larger than the value itself.
Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.
Size, to scale
Radius 197 pm — that is 0.197 nm, so about 2.54 million of them side by side would span a millimetre.
Thermal range
Calcium is liquid over a 642 K window, from 1115 K to 1757 K.
The story
Calcium (Ca) is element 20, a soft, silvery alkaline earth metal in group 2. It is the fifth most abundant element in Earth's crust and the most abundant metal in the human body, where it builds bones and teeth. Calcium metal is never found free in nature because it reacts with water and air. It melts at 842 °C (1,115 K) and has a low density, 1.54 g/cm³.
Calcium's 20 electrons sit in four shells, arranged 2, 8, 8, 2, with the configuration [Ar] 4s². It readily loses its two outer electrons to form the Ca²⁺ ion found in limestone, gypsum and bone.
Natural calcium is a mix of six isotopes. Calcium-40 makes up about 97%. Calcium-48, only about 0.19%, has 20 protons and 28 neutrons, a doubly magic nucleus that physicists accelerate into heavy targets to make superheavy elements such as oganesson.
Calcium compounds have been used for thousands of years: the ancient Egyptians, Greeks and Romans made lime mortar and plaster, and the element's name comes from the Latin calx, meaning lime. Antoine Lavoisier listed lime among the elements in 1789, suspecting that it was the oxide of an unknown metal.
In 1808 Humphry Davy in London isolated calcium by electrolysis of a moist mixture of lime and mercuric oxide, which gave a calcium amalgam; distilling off the mercury left the metal. In Stockholm, Jöns Jacob Berzelius and Magnus Martin af Pontin prepared the amalgam the same year.
Davy obtained only small, impure amounts. Robert Bunsen and Augustus Matthiessen made larger quantities in 1855 by electrolysis of molten calcium chloride. Industrial production began in the early 20th century.
Applications
Most calcium is used as compounds, not as metal. Limestone (calcium carbonate) is the raw material of cement and lime. Heating limestone gives quicklime (calcium oxide), used in steelmaking, water treatment, flue gas cleaning and paper making. Gypsum (calcium sulfate) is made into plaster and plasterboard.
Where it comes from
41500 mg/kg of Earth's crust · more abundant than 97% of elements
Calcium makes up about 4% of Earth's crust by mass. It is dissolved in seawater and in hard water, and living things concentrate it into shells, coral and bone.
Limestone is quarried almost everywhere. Calcium metal is made in small amounts, mainly by reducing lime with aluminum under vacuum, much of it in China.
Handling
Calcium metal reacts with water to release flammable hydrogen gas and form calcium hydroxide, which is caustic. The reaction is slower than with sodium but can ignite the hydrogen. Store the metal in a dry, sealed container, and fight fires with a Class D extinguisher or dry sand, never water.
Quicklime (calcium oxide) and slaked lime (calcium hydroxide) are corrosive to the eyes and skin, and quicklime gets very hot when it is wetted. Wear eye protection and gloves. Wet cement and concrete are also strongly alkaline and can cause chemical burns on prolonged skin contact.
Calcium is essential, but too much from supplements can cause constipation, kidney stones and high blood calcium. The NIH sets an upper limit of 2,500 mg a day for adults aged 19 to 50 and 2,000 mg a day from age 51.
Quick answers
Calcium (symbol Ca) is element 20 on the periodic table, an alkaline earth metal in period 4, group 2. Structural element of bone, shell and cement: calcium is how life learned to build hard things. At room temperature it is a solid.
[Ar] 4s², giving 4 occupied shells with the distribution 2, 8, 8, 2.Calcium melts at 1115 K (841.9 °C) and boils at 1757 K (1483.9 °C).
The standard atomic weight of calcium is 40.078 u. That is a weighted average across its naturally occurring isotopes, which is why it is rarely a whole number.
The most common calcium atom, Ca-40, has 20 neutrons: its mass number, 40, minus its atomic number, 20. It makes up 96.941% of natural calcium. The classroom shortcut, rounded atomic mass (40.078) minus atomic number, gives the same 20.
Calcium has a density of 1.54 g/cm³. Water is 1.0 g/cm³, so a block of calcium is about 1.5× heavier.
Calcium has a Pauling electronegativity of 1. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). A value this low means it readily gives its outer electrons away, forming positive ions.
Calcium was discovered in 1808 by Humphry Davy. Origin of the name: Latin calx, "lime".
Calcium makes up about 4.15% of the Earth's crust — one of the most abundant elements there is.