All practice pages/Interatomic theory/Elements
Element 20, an alkaline earth metal
The metal in chalk, marble, bones and cement, and the most abundant metal in your body.
Period 4, group 2. The other alkaline earth metals are tinted, and every cell opens its own article.
Calcium has 20 electrons, one for every proton, so the atom carries no charge. Below are the same 20 electrons written two ways.
1s² 2s² 2p⁶ 3s² 3p⁶ 4s²
Short form: [Ar] 4s², where [Ar] stands for the 18 electrons argon already has.
The simple form counts electrons shell by shell. The orbital form says which subshell each one sits in: s holds 2, p holds 6, d holds 10 and f holds 14. Shell 2 here is 2s² 2p⁶, and 2 + 6 is 8.
Calcium sits in period 4 and has 4 shells. The period number counts the shells.
The outer shell holds 2 electrons. Those are the ones calcium can lose, gain or share, so they are the ones that decide what it bonds with. For a main-group element the group number gives that count: group 2 means 2.
Two valence electrons, which calcium gives up to form the Ca²⁺ ion in its compounds.
About 4% of the crust by mass, in limestone, chalk and marble (all calcium carbonate), gypsum and fluorite. Seashells and coral are calcium carbonate too, and about 1 kg of an adult's body is calcium, almost all of it in bones and teeth.
Humphry Davy isolated it in 1808 by electrolysis of a mixture of lime and mercury oxide. Lime itself had been made by heating limestone since ancient times. The name comes from the Latin calx, lime.
Cement and concrete depend on lime made from limestone, and plaster and drywall are gypsum. Calcium also refines steel, calcium chloride melts ice and settles dust on roads, and calcium carbonate is chalk, an antacid and a filler in paper, paint and toothpaste.