PTL:Cadmium

From ChemEd Collaborative

Cadmium

Cadmium is a bluish-white metal that can be cut with a knife. It is in Group IIB, along with zinc (Zn) and mercury (Hg), and is in the 5th period. It almost always exists as the Cd(II) ion in its compounds.

1 Discovered
2 Name
3 Reactions
4 Uses
5 Emission Spectra

Discovered

Cadmium was discovered in 1817 by Strohmeyer from an impurity in ZnCO₃. It is usually found in zinc ores.

Name

Indeed, the name of the element comes from the Latin word cadmia, the ancient name for calamine, zinc carbonate.

Reactions

The metal reacts with air when heated to give brown CdO,

2 Cd(s) + O₂(g) 2 CdO(s)

and it reacts with acids to give Cd(II) salts.

2 Cd(s) + 2 HCl(aq) CdCl₂(aq) + H₂(g)

Uses

Cadmium is used in rechargeable batteries and as a yellow pigment in paints

Cadmium is used as a pigment to make the red glass in traffic lights

The metal, which is obtained as a by-product of zinc production, is widely used to electroplate objects to protect them from corrosion. It is also used in Ni-Cd [rechargeable batteries] where the anode reaction is

Cd(s) + 2 OH^-(aq) Cd(OH)₂(s) + 2 e^-

Cd metal is also an important component of low-melting alloys used in bearings, solders, and nuclear-reactor control rods.

Cadmium pigments based on yellow cadmium(II) sulfide (CdS) are among the most [brilliant inorganic pigments]. If mixed with ZnS, the material is a greenish yellow, while mixing HgS or CdSe gives a yellow to bordeaux red color. Cadmium selenosulfide (a mixture of CdS and CdSe) is deep red and is used to make the red [glass] in traffic lights.

Since CdS and similar compounds are stable at high temperatures and have a high brightness, they are incorporated in plastics that have high processing temperatures (such as polyethylene, polystyrene, and polycarbonates). They are used in brilliant paints, and their ability to absorb in the UV protects the organic binder of the paint from degradation.

Cadmium metal is classified as a toxic metal, most cases of poisoning occurring through inhalation or ingestion. Cadmium sulfide and similar compounds, however, are extremely insoluble (so the "lethal dose" is very high). For this reasons plastics pigmented with cadmium compounds can be used safely for household products.