Calcium (Ca)
“The metal hidden in plain sight — your bones, eggshells, blackboard chalk and the concrete of the Padma Bridge all stand on the fifth most abundant element of the Earth’s crust.”
How its electrons are arranged
Computed from the aufbau principle, with known real-world exceptions applied automatically.
Characteristics
- Electron configuration [Ar] 4s² — the two valence electrons are shed so easily that virtually all calcium chemistry is the chemistry of the Ca²⁺ ion.
- Fifth most abundant element of the Earth’s crust: limestone, marble and chalk are all calcium carbonate (CaCO₃), piled up from the shells of ancient sea creatures over hundreds of millions of years.
- Reacts with water far more calmly than potassium — a steady stream of hydrogen bubbles rises as calcium becomes calcium hydroxide, whose solution is the "lime water" of laboratory tests.
- Its compounds paint a flame brick-red — the classic calcium flame test — and dissolved Ca²⁺ is exactly what makes water "hard".
A silvery-white, moderately soft metal that dulls to a grey coat within minutes in open air. In the laboratory it arrives as turnings or ribbon in sealed bottles, but in daily life you almost never meet the free metal — calcium prefers to travel as compounds such as chalk, marble and limestone.
Where you meet it
✦ Uses
- Cement and concrete — limestone roasted into clinker binds every structure from village pillars to the Padma Bridge, and lime mortar has plastered Bengali homes for centuries.
- Bone and teeth health — hydroxyapatite, a calcium phosphate, is the mineral of your skeleton, topped up by milk, small fish (mola, dhela) eaten with bones, eggshells and calcium tablets.
- Plaster of Paris and gypsum board — calcium sulfate hemihydrate (CaSO₄·½H₂O) sets into the cast that heals a broken arm and into the panels of modern ceilings.
- Slaked lime Ca(OH)₂ whitewashes village walls before festivals and corrects acidic farm soil, while bleaching powder born from lime disinfects water.
◍ Everyday examples
- Eggshells, snail shells and seashells are nearly pure CaCO₃ — drip vinegar on one and watch it fizz as carbon dioxide escapes.
- A stick of blackboard chalk is compressed calcium carbonate — and so is the antacid tablet that settles a sour stomach.
- Milk, yogurt, small fish eaten with their bones and leafy greens are the everyday calcium sources of a Bengali plate.
History & name
Lime — the white powder roasted out of limestone — plastered ancient homes and raised the pyramids, yet chemists could not crack what it was. In 1789 Antoine Lavoisier listed lime among his new "earths", suspecting each earth concealed a metal inside; the suspicion proved right. In 1808 Humphry Davy, fresh from his potassium triumph, passed current through a paste of lime and mercuric oxide, collected a calcium-mercury amalgam at the cathode, and boiled the mercury away to reveal shiny calcium metal at last. The element took its name from calx, the Latin word for lime.