CALCIUM

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

Calcium is the third lightest member of the alkaline earth metals. Calcium is more reactive than magnesium. It is silvery in appearance, but when exposed to the air it quickly develops a white coating of calcium oxide. The calcium in the photo above is slightly tarnished due to exposure to siar. Calcium does not occur in nature in elemental form but it is widely distributed in compound form. Some common calcium ores include calcite (calcim carbonate), dolomite (calcium magnesium carbonate), gypsum (calcium sulfate dihydrat) and fluorite (calcium fluorite)

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.

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.

Ca + 2H₂O = Ca(OH)₂ + H₂

Calcium slowly oxidizes upon exposure to air, forming calcium oxide.

2Ca + O₂ = 2CaO

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 Metal Reacting with Water 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.

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.

Several calcium compounds are common household chemicals. Gypsum, used in the manufacture of wallboard, is calcium sulfate dihydrate. Plaster of paris is calcium sulfate hemihydrate (hemihydrate = half-hydrate). Calcium oxide is called lime or quicklime. It reacts with water to produce calcium hydroxide, and is often used to raise the pH of acidic soils.

CaO + H₂O = 2Ca(OH)₂

Calcium hydroxide is known as slaked lime, and solutions of calcium hydroxide are known as limewater. When carbon dioxide is bubbled through a solution of limewater, a precipitate of calcium carbonate is produced. In this case a precipitate is produced becasue calciumc carbonate is insoluble. Other hydroxides may be used; on space missions the air is "scrubbed" by passing it through a solution of lthium hydroixde (lithium is used because it is ligter).

CO₂ + Ca(OH)₂ = CaCO₃ + H₂O

Limerock (or limestone) and coral are both composed prmarily of calcium carbonate. There are two forms of calcium carbonate, the calcite form and the aragonite form. Calcium carbonate deposited on land is primarily the calcite form, while calcium carbonate deposited in the sea, such as coral, is primarily the aragonite form.

Although considered to be insoluble, small amounts of calcium carbonate will dissolve in water and water obtained in areas where the bedrock is primarily limerock is said to be "hard" due to the presence of dissolved calcium and magnesium. This solubiliy decreases with increasing temperature; in boilers this is a problem because heating causes the dissolved calcium and magnesium carbonates to be deposited as "bouiler scale."

All carbonates, including calcium and magnesium carbonate, react with acids to expel carbon dioxide gas.

CaCO₃ + 2HCl = CaCl₂ + H₂O + CO₂