Calcium (Ca) — Atomic Number 20

Calcium is a chemical element with the symbol Ca and atomic number 20. It is a silvery-grey alkaline earth metal — the fifth most abundant element in Earth’s crust. Its compounds (limestone, marble, chalk, gypsum) have been used in construction for millennia. The name comes from Latin calx (“lime”).

At a Glance

Discovery

Calcium compounds were used since antiquity. The pure metal was first isolated in 1808 by Sir Humphry Davy via electrolysis of lime (CaO) and mercuric oxide.

Chemical Properties

Calcium loses two 4s² electrons to form Ca²⁺ (configuration 2,8,8, like argon).

Key Reactions

1. With water: $\text{Ca}+2{\text{H}}_2\text{O}\to {\text{Ca(OH)}}_2+{\text{H}}_2$ — steady fizzing, milky suspension
2. With oxygen: $2\text{Ca}+{\text{O}}_2\to 2\text{CaO}$ (quicklime)
3. With acid: $\text{Ca}+2\text{HCl}\to {\text{CaCl}}_2+{\text{H}}_2$

Characteristic Test

Flame test: Brick-red — distinctive crimson-orange

The Limestone Cycle (IGCSE Classic)

1. Thermal decomposition: ${\text{CaCO}}_3\stackrel{\Delta }{\to }\text{CaO}+{\text{CO}}_2$
2. Slaking: $\text{CaO}+{\text{H}}_2\text{O}\to {\text{Ca(OH)}}_2$ (exothermic — “hisses”)
3. Limewater test: ${\text{Ca(OH)}}_2+{\text{CO}}_2\to {\text{CaCO}}_3+{\text{H}}_2\text{O}$ (turns milky)

Compounds & Uses

Biological Role

Essential for bones and teeth (as calcium phosphate), muscle contraction, nerve transmission, and blood clotting. The human body contains ~1.5 kg of calcium.

IGCSE Essentials

1. Flame test: brick-red
2. Limestone cycle — thermal decomposition → slaking → carbonation
3. Limewater test for CO₂ — turns milky
4. Forms Ca²⁺ (loses 2 electrons → 2,8,8)
5. Group 2 reactivity: Be < Mg < Ca < Sr < Ba

Sources

• Calcium — Wikipedia
• Calcium — Ptable.com
• Calcium — RSC Periodic Table