Calcium (Ca)
Alkaline earth metal
Calcium is the most abundant metal in the human body — your skeleton is essentially a calcium phosphate framework. In the ground it forms limestone, marble and chalk; in industry it becomes cement and concrete.
Position on the periodic table
Atomic properties
| Atomic number | 20 |
|---|---|
| Atomic mass | 40.0784 amu |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 4s2 |
| Noble gas shorthand | [Ar] 4s2 |
| Electrons per shell | 2, 8, 8, 2 |
| Valence electrons (outer shell) | 2 |
| Common oxidation states | +2 |
| Electronegativity (Pauling) | 1 |
| Covalent radius (approx.) | 176 pm |
| First ionization energy | 589.8 kJ/mol |
| Electron affinity | 2.4 kJ/mol |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Density | 1.55 g/cm³ |
| Melting point | 841.85 °C (1115 K) |
| Boiling point | 1483.8 °C (1757 K) |
| Appearance | Not available |
| Radioactive | No |
| Origin | Occurs naturally |
Electron configuration of calcium
Calcium's ground-state electron configuration is 1s2 2s2 2p6 3s2 3p6 4s2, usually shortened to [Ar] 4s2. Its electrons occupy 4 shells (2, 8, 8, 2), placing it in period 4 of the s-block. The 2 outer-shell electrons drive its bonding behaviour. Explore it interactively in the electron configuration calculator.
Uses of calcium
- Cement, mortar and concrete (from limestone)
- Steelmaking flux
- Plaster and drywall (calcium sulfate)
- Dietary supplements and antacids
Biological role: Essential — builds bones and teeth, triggers muscle contraction and helps blood clot.
Occurrence: About 4% of the Earth's crust, mostly as limestone, gypsum and fluorite.
Common compounds of calcium
CaCO3
Calcium carbonate (limestone)
CaO
Quicklime
Ca(OH)2
Slaked lime
CaSO4
Gypsum
History and discovery
Discovered: 1808 — Humphry Davy. Name origin: From Latin calx, meaning lime (the building material).
Humphry Davy isolated calcium in 1808, though lime had been used in building for thousands of years.
Safety notes
Calcium metal reacts with water; everyday calcium compounds like limestone and chalk are harmless.
Educational context only — always follow your school's laboratory rules and never handle chemicals without proper supervision. See our disclaimer.
Practice questions
Quick practice: Calcium
1. What is the chemical symbol of Calcium?
2. What is the atomic number of Calcium?
3. Which category does Calcium belong to?
4. What is the state of Calcium at room temperature?
5. Which period of the periodic table is Calcium in?
Calcium FAQs
What is the atomic number of calcium?
Calcium's atomic number is 20 — every calcium atom has 20 protons in its nucleus.
What is the symbol for calcium?
Ca. From Latin calx, meaning lime (the building material).
Is calcium a metal, nonmetal, or metalloid?
Calcium is classified as a alkaline earth metal.
What state is calcium at room temperature?
At about 25 °C, calcium is a solid. It melts at 841.85 °C.
How many valence electrons does calcium have?
Calcium has 2 electrons in its outer shell (shell pattern: 2, 8, 8, 2).
What is calcium used for?
Key uses include: cement, mortar and concrete (from limestone); steelmaking flux; plaster and drywall (calcium sulfate).
Related elements
Keep working with this element
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