Calcium
Atomic Data
| Atomic Number | 20 |
| Symbol | Ca |
| Atomic Weight | 40.078 u |
| Density (STP) | 1.55 g/cm³ |
| Melting Point | 841.85 °C (1115 K) |
| Boiling Point | 1483.85 °C (1757 K) |
| Electronegativity | 1.0 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p6 4s2 |
| Oxidation States | +2 |
| Phase at STP | Solid |
| Category | Alkaline-Earth Metal |
| Period / Group | 4 / 2 |
| CAS Number | 7440-70-2 |
Electron Configuration
[Ar] 4s2
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 6 | 18 |
| N | 4 | 4s | 2 | 20 |
| Total | 20 | 20 | ||
Isotopes of Calcium
Calcium has six naturally occurring stable isotopes. The most abundant is ⁴⁰Ca, comprising 96.941% of all naturally occurring Calcium.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Calcium-40 | ⁴⁰Ca | 20 | 20 | 96.941 | Stable |
| Calcium-42 | ⁴²Ca | 20 | 22 | 0.647 | Stable |
| Calcium-43 | ⁴³Ca | 20 | 23 | 0.135 | Stable |
| Calcium-44 | ⁴⁴Ca | 20 | 24 | 2.086 | Stable |
| Calcium-46 | ⁴⁶Ca | 20 | 26 | 0.004 | Stable |
| Calcium-48 | ⁴⁸Ca | 20 | 28 | 0.187 | Stable |
Abundance & Occurrence
Calcium is present in Earth's crust at approximately 41500 ppm by mass and at approximately 64 ppm by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Water and moisture reactivity: Calcium metal reacts with water to produce calcium hydroxide and hydrogen gas; the reaction is slower than sodium or potassium but accelerates with heat — keep calcium metal dry and away from moisture.
- Skin and eye irritation: Calcium metal causes skin and eye irritation; calcium hydroxide (lime) produced on contact with moist tissue is a caustic alkali that can cause chemical burns with prolonged exposure.
- Calcium oxide (quicklime): Quicklime reacts violently and exothermically with water; contact with skin or eyes can cause severe burns — slake only in controlled conditions with appropriate protective equipment.
- Dust inhalation: Calcium oxide and calcium hydroxide dusts irritate the respiratory tract, eyes, and mucous membranes; work in well-ventilated conditions with dust-rated respiratory protection.
Calcium in the Real World
Real-World Uses
- Building materials — Calcium compounds dominate construction: limestone (CaCO₃) is quarried for aggregate, calcined to produce quicklime, and slaked to make calcium hydroxide for mortar; Portland cement is primarily calcium silicate and aluminate phases.
- Dietary supplement — Calcium carbonate and calcium citrate supplements are widely taken to maintain bone density and prevent osteoporosis, particularly in post-menopausal women and older adults.
- Food additive — Calcium chloride firms pickles, olives, and canned tomatoes; calcium sulfate sets tofu and is a yeast nutrient in bread; calcium propionate inhibits mould in packaged baked goods.
- Steel refining — Calcium is injected as calcium silicide or calcium carbide wire into molten steel to modify sulfide and alumina inclusions, improving the steel's formability, surface quality, and machinability.
- Water treatment — Lime (calcium hydroxide) is added to drinking water and industrial effluents to adjust pH, precipitate heavy metals, and soften water by converting soluble magnesium and bicarbonate ions to insoluble carbonates.
- Paper and plastics filler — Precipitated and ground calcium carbonate is added as a low-cost filler and brightening agent to paper, PVC, paints, and rubber, improving whiteness, opacity, and printability.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is calcium used for?
Calcium compounds are central to construction: limestone (CaCO₃) and Portland cement (calcium silicates) are the basis of concrete and mortar. Calcium is an essential dietary nutrient for bones and teeth (as hydroxyapatite). Calcium chloride is used to de-ice roads and accelerate concrete setting. Calcium carbonate is a filler in paper, paint, and plastics. In steel refining, calcium is added to modify non-metallic inclusions and improve machinability.
Why do we need calcium?
Calcium is the most abundant mineral in the human body, with about 99% stored in bones and teeth as calcium phosphate (hydroxyapatite). The remaining 1% in blood and cells is critical for muscle contraction (calcium ions trigger myosin-actin interaction), nerve signal transmission, blood clotting (calcium activates several clotting factors), enzyme activation, and cell signalling. Insufficient dietary calcium during childhood and adolescence leads to lower peak bone mass and increased lifetime risk of osteoporosis and fractures.
Is calcium a metal?
Yes, calcium is a soft, silver-grey alkaline-earth metal in Group 2 of the periodic table. It is the fifth most abundant element in Earth's crust and the third most abundant metal (after aluminium and iron). However, calcium metal is rarely seen in everyday life because it reacts with water and air; it is only used in pure metallic form as a deoxidiser and desulfuriser in steel production and for special chemical reducing applications.
How does calcium make bones hard?
Bones are composite materials: the organic component (mainly collagen fibres) provides tensile strength and flexibility, while the mineral component (mainly calcium phosphate in the form of hydroxyapatite crystals) provides compressive strength and rigidity. Hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂] nanocrystals nucleate and grow along the collagen fibres during bone mineralisation, interlocking with the organic matrix to create a composite that is stronger and more fracture-resistant than either component alone.