CALCIUM


Physical Properties

Atomic Number: 20
Mass Number: 40.08
Electron Configuration: 1s²2s²2p⁶3s²3p⁶4s²
Crystal Structure: Face-Centered Cubic
Density: 1.55 g/cm³
Melting Point: 839^oC
First Ionization Energy: 590 kJ/mol
Second Ionization Energy: 1145 kJ/mol
Atomic Radius: 197 pm
Ionic Radius: 99 pm
Standard Reduction Potential: -2.87 V (Ca²⁺ + 2e^- = Ca)

Occurrence and Properties

Calcium is the third lightest member of the alkaline earth metals. Calcium is silvery in appearance, but when exposed to the air it quickly develops a white coating of magnesium oxide. Unlike magnesium, calcium is not particularly useful as a metal in itself. In nature calcium it is often found dissolved in water and is one ion responsible for the "hardness" of water. Limerock is composed primarily of calcium carbonate. Clam shells are also composed primarily of calcium carbonate. Calcium also occurs in the mineral fluorite, or calcium fluoride.

Reactions of Calcium

Calcium is generally more reactive than magnesium but not as reactive as sodium or potassium. Calcium will react quite vigorously with water to form calcium hydroxide and hydrogen gas. It will slowly oxidize to calcium oxide if left open to the air. Calcium will react with the halogens to form the corresponding halide. When heated, it will react with nitrogen to form calcium nitride and with hydrogen to form calcium nitride and with sulfur to form calcium sulfide.

Calcium Compounds

Calcium has no tendency to form covalent compounds and all calcium compounds are ionic in nature. In general, calcium compounds tend to be less soluble than the corresponding magnesium compounds. Calcium sulfate, carbonate, chromate, and oxalate are all insoluble compounds. Gypsum, used in the anufacture of wallboard, is calcium sulfate dihydrate. Plaster of paris is calcium sulfate hemihydrate. Calcium oxide is called quicklime, and calcium hydroxide is called slaked lime.

This photo shows the reaction between calcium metal and water. This is a displacement reaction in which hydrogen is displaced from the water; the bubbles you see are hydrogen gas. As the reaction proceeds, the solution will turn milky white in color due to the precipitation of calcium hydroxide.

Optical Calcite. Calcite is one form of calcium carbonate.

Fluorite. Fluorite is the mineral calcium fluoride. Pure fluorite is colorless, but the mineral can often be colored due impurities. The fluorite structure is a prototype ionic structure in which the calcium ions form a face-centered cubic array and the fluroride ions occupy the tetrahedral holes.