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Calcium (Ca), element 20
Calcium (Ca) is the chemical element with atomic number 20: an alkaline earth metal in group 2, period 4. At 25 °C it is a solid, with an atomic mass of 40.08 u.
Physical and chemical data
| Atomic number | 20 |
|---|---|
| Symbol | Ca |
| Atomic mass | 40.08 u |
| Density | 1.54 g/cm³ |
| Melting point | 841.85 °C (1,115 K) |
| Boiling point | 1,483.85 °C (1,757 K) |
| Electronegativity (Pauling) | 1 |
| Electron configuration | [Ar]4s2 |
| Atomic radius (van der Waals) | 231 pm |
| Oxidation states | +2 |
| Physical state at 25 °C | solid |
| Family | alkaline earth metal |
| Group and period | 2 / 4 |
| Discoverer and year | Humphry Davy (1808) |
The discovery record belongs to Humphry Davy, in 1808.
Neighbours on the table
Horizontally, one proton less lands on Potassium (Z=19); one more lands on Scandium (Z=21). Vertically, group 2 gathers relatives with similar behaviour: Beryllium, Magnesium, Strontium, Barium.
The story behind the name
It comes from the Latin calx, the lime made by burning limestone, a material the Romans knew so well the word survives in calculus (the little counting stones of the abacus). Calcium compounds have accompanied civilisation since the pyramids, but the metal itself only appeared in 1808, when Humphry Davy pulled it out of lime by electrolysis. In the human body it is the most abundant mineral: about 1 kg in an adult, nearly all of it stored in bones and teeth, and the calcium stiffening your skeleton was forged inside exploded stars.
Physical and chemical data: PubChem Periodic Table (NCBI, compiling NIST and IUPAC), public domain, snapshot of 2026-07-10. Temperatures converted from kelvin; historical notes cross checked against classic chemistry references.
Last updated: · Methodology and sources