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 isotopes, all considered stable for practical purposes, with ⁴⁰Ca dominating at 96.941% natural abundance — one of the highest single-isotope fractions of any multi-isotope element. The rarest natural isotope, ⁴⁶Ca, accounts for only 0.004% of natural 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 the fifth most abundant element in Earth's crust at about 41,500 ppm by mass, occurring mainly as carbonate rocks such as limestone, chalk, and marble, and in silicate minerals like plagioclase feldspar. In the universe it is much rarer at approximately 64 ppm, produced primarily by nuclear fusion reactions in massive stars.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Safety & Handling
- Reactivity with water: Calcium metal reacts with water to produce calcium hydroxide and flammable hydrogen gas. The reaction generates enough heat to potentially ignite the hydrogen; calcium metal must therefore be stored under mineral oil or in a dry, sealed container away from any moisture.
- Fire hazard: Burning calcium metal produces an intense white flame and calcium oxide fume. Water must not be used to fight a calcium metal fire; dry sand or a Class D fire extinguisher is appropriate. Fine calcium powder is particularly prone to ignition in air.
- Skin and eye burns: Both calcium metal and freshly formed calcium hydroxide are strongly alkaline and can cause chemical burns on contact with skin or eyes. Protective gloves, safety goggles, and a lab coat should be worn whenever handling the metal or concentrated calcium solutions.
- Dust inhalation: Calcium oxide and calcium hydroxide dusts irritate the respiratory tract, eyes, and mucous membranes. Adequate ventilation and a dust mask or respirator are required when handling powdered calcium compounds in bulk quantities.
- Calcium compounds — general safety: Most calcium compounds encountered in everyday life — calcium carbonate in chalk, calcium chloride as a food additive — are of low toxicity and are generally regarded as safe. Excessive supplemental calcium intake over long periods may contribute to kidney stone formation in susceptible individuals, but standard dietary levels do not represent an acute health hazard.
Calcium in the Real World
Real-World Uses
- Cement and concrete — Calcium silicate and calcium aluminate phases formed during cement production bind aggregate particles into concrete, making calcium one of the most industrially consumed elements on Earth with billions of tonnes of cement produced annually.
- Steel deoxidizer — Calcium metal and calcium-silicon alloys are injected into molten steel to remove dissolved oxygen and sulfur, improving cleanliness, strength, and machinability of the final product and consuming a significant share of global calcium metal production.
- Dietary supplements — Calcium is an essential macronutrient required for building and maintaining bones and teeth, enabling muscle contraction, and transmitting nerve impulses; calcium carbonate and calcium citrate are the most common forms used in supplements and food fortification.
- Water treatment — Calcium hydroxide (slaked lime) is added to drinking water and wastewater to raise pH, precipitate heavy metals, and soften water by removing excess magnesium and bicarbonate ions, making it one of the lowest-cost and most widely used water treatment chemicals.
- Cheese and dairy coagulant — Calcium chloride is added to pasteurised milk during cheesemaking to restore the calcium balance lost during heat treatment, improving curd formation and yield; it is also used as a firming agent in canned vegetables and as a food-grade de-icing agent.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is calcium used for?
Calcium has a wide range of industrial and biological uses. In construction it is the backbone of cement and concrete through calcium silicate compounds. In steelmaking it acts as a deoxidizer to remove oxygen and sulfur impurities. Biologically, calcium is the primary mineral in bones and teeth, and it is widely sold as a dietary supplement. It is also used in water treatment to adjust hardness and in dairy processing to help coagulate cheese.
Is calcium a metal?
Yes, calcium is a metal — specifically an alkaline-earth metal in Group 2 of the periodic table. As a pure element it is a soft, silvery-white solid that is lighter than most structural metals. Pure calcium metal is rarely encountered outside laboratories because it reacts readily with air and water, so it is far more commonly found as a compound such as limestone (CaCO₃) or gypsum (CaSO₄).
How abundant is calcium in Earth's crust?
Calcium is the fifth most abundant element in Earth's crust at approximately 41,500 parts per million (ppm) by mass, making up about 4.15% of the crust. It occurs predominantly as carbonate minerals such as limestone and chalk, and as silicate minerals in igneous rocks. In the universe calcium is far less common, at about 64 ppm by mass.
Who discovered calcium and when?
Calcium as a pure metal was first isolated by the British chemist Humphry Davy in 1808. Davy used electrolysis of a mixture of lime (calcium oxide) and mercuric oxide to produce a calcium amalgam, from which he then removed the mercury to obtain the metal. The name comes from the Latin calx, meaning lime, which was already well known to ancient civilisations thousands of years earlier.