Calcium (Ca)

About 99% of the calcium in a human body is in bone and teeth, but the other 1% does most of the interesting work: a calcium ion crossing a membrane is the trigger for every muscle contraction, including each heartbeat. Limestone, chalk, marble and seashells are all calcium carbonate, laid down mostly by organisms, and heating it to about 900 °C to drive off carbon dioxide gives quicklime — a reaction humans have been running for at least 9,000 years. The metal itself is silvery and soft and almost nobody has seen it, because it tarnishes within seconds in air.

Properties

Atomic number

20

Symbol

Ca

Standard atomic weight

40.078 u

Category

Alkaline earth metal

Group

2

Period

4

Block

s-block

Electron configuration

[Ar] 4s²

Electrons per shell

2, 8, 8, 2

Melting point

1,115 K — 841.9 °C — 1547.3 °F

Boiling point

1,757 K — 1483.8 °C — 2702.9 °F

Density

1.54 g/cm³

Electronegativity

1 (Pauling)

Oxidation states

+2

Discovered

1808 by Humphry Davy

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s²

Noble gas shorthand

[Ar] 4s²

Shells

K: 2 · L: 8 · M: 8 · N: 2

What it is used for

• Cement and concrete, via limestone and quicklime
• Steelmaking, where lime removes phosphorus and sulfur
• Plaster, mortar and agricultural lime
• Calcium carbonate as an antacid and a dietary supplement
• A reducing agent for extracting uranium, thorium and zirconium

Neighbours in the table

Group 2

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Calcium sits in group 2 with beryllium, magnesium, strontium, barium, radium.