Calcium - Ca General Information Discovery Calcium was first isolated by Sir Humphry Davy in 1808 in London, although lime, or calcium oxide, was prepared by the Romans in the first century. Appearance Calcium is a silvery-white, relatively soft metal. Source Calcium is the fifth most abundant metal in the Earth's crust, greater than 3% by mass. It is not found free in nature, but occurs abundantly as limestone (calcium carbonate), gypsum (calcium sulfate), fluorite (calcium fluoride) and apatite (calcium chloro- or fluoro-phosphate). Calcium is prepared commercially by the electrolysis of fused calcium chloride, to which calcium fluoride is added to lower the melting point. Uses Calcium and its compounds are widely used. Quicklime (calcium oxide), which is made by heating limestone and can be changed into slaked lime by the addition of water, is a substance often used by the chemical industry. It has the advantage of being cheap and readily available. When mixed with sand it takes up carbon dioxide from the air and hardens as mortar and plaster. Calcium from limestone is an important constituent of Portland Cement. Calcium is also used as a reducing agent in preparing other metals such as thorium and uranium, and as an alloying agent for aluminium, beryllium, copper, lead and magnesium alloys. Biological Role Calcium is an essential constituent of cells, teeth and bones. The normal amount found in an adult is over one kilogram, located mostly in the teeth and bones. General Information Calcium readily forms a coating of oxide and nitride in air, it reacts with water and burns with a yellow-red flame, forming mostly the nitride. Calcium carbonate is soluble in water containing carbon dioxide, and this causes hardness in water. This calcium carbonate is also the constituent of stalactites and stalagmites in caves where water drips slowly and evaporates in situ.

	Physical Information
	Atomic Number		20
	Relative Atomic Mass (12C=12.000)		40.078
	Melting Point/K		1112
	Boiling Point/K		1757
	Density/kg m-3		1550 (293K)
	Ground State Electron Configuration		[Ar]4s2
	Electron Affinity(M-M-)/kJ mol-1		+186

	Key Isotopes
	nuclide	40Ca	42Ca	43Ca	44Ca	45Ca
	atomic mass	39.963	41.959	42.959	43.955	44.956
	natural abundance	96.94%	0.647%	0.135%	2.086%	0%
	half-life	stable	stable	stable	stable	165 days
	nuclide	46Ca	47Ca	48Ca
	atomic mass	45.954	46.954	47.952
	natural abundance	0.004%	0%	0.187%
	half-life	stable	4.53 days	stable

Other Information
Enthalpy of Fusion/kJ mol-1		9.33
Enthalpy of Vaporisation/kJ mol-1		150.6
Oxidation States
Ca+2

	Ionisation Energies/kJ mol-1
	M - M+	589.7
	M+ - M2+	1145
	M2+ - M3+	4910
	M3+ - M4+	6474
	M4+ - M5+	8144
	M5+ - M6+	10496
	M6+ - M7+	12320
	M7+ - M8+	14207
	M8+ - M9+	18191
	M9+ - M10+	20385