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Cadmium (pronounced /ˈkædmiəm/) is a chemical element with the symbol Cd and atomic number 48. The soft, bluish-white transition metal is chemically similar to the two other metals in group 12 , zinc and mercury. Similar to zinc it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low melting point for a transition metal. Cadmium is a relatively abundant element. Cadmium was discovered in 1817 by Friedrich Strohmeyer as an impurity in zinc carbonate.

Cadmium occurs as minor component in most zinc ores and therefore is a by-product of zinc production. Cadmium was for a long time used as pigment and for corrosion resistant plating on steel. Cadmium compounds were used to stabilize plastic. With the exception of its use in nickel-cadmium batteries, the use of cadmium is generally decreasing in all other applications. This decrease is due to the high toxicity and carcinogenicity of cadmium and the associated health and environmental concerns. Although cadmium is toxic, one enzyme, a carbonic anhydrase with a cadmium as reactive centre has been discovered.

Characteristics
Cadmium is a soft, malleable, ductile, toxic, bluish-white bivalent metal. It is similar in many respects to zinc but forms more complex compounds.

Chemical
The most common oxidation state of cadmium is +2, though rare examples of +1 can be found. Cadmium burns in air to form brown amorphous cadmium oxide (CdO). The crystalline form of the same compound is dark red and changes colour when heated, similar to zinc oxide. Hydrochloric acid, sulfuric acid and nitric acid dissolve cadmium by forming cadmium chloride (CdCl2) cadmium sulfate (CdSO4) or cadmium nitrate (Cd(NO3)2). The oxidation state +1 can be reached by dissolving cadmium in a mixture of cadmium chloride and aluminium chloride, forming the Cd22+ which is similar to the Hg22+ in mercury(I) chloride.

Cd + CdCl2 + 2 AlCl3 → Cd2[AlCl4]2

Isotopes
Main article: isotopes of cadmium
Naturally occurring cadmium is composed of 8 isotopes. For two of them, natural radioactivity was observed, and three others are predicted to be radioactive but their decay is not observed, due to extremely long half-life times. The two natural radioactive isotopes are 113Cd (beta decay, half-life is 7.7 × 1015 years) and 116Cd (two-neutrino double beta decay, half-life is 2.9 × 1019 years). The other three are 106Cd, 108Cd (double electron capture), and 114Cd (double beta decay); only lower limits on their half-life times have been set. At least three isotopes - 110Cd, 111Cd, and 112Cd - are stable. Among the isotopes absent in natural cadmium, the most long-lived are 109Cd with a half-life of 462.6 days, and 115Cd with a half-life of 53.46 hours. All of the remaining radioactive isotopes have half-lives that are less than 2.5 hours, and the majority of these have half-lives that are less than 5 minutes. This element also has 8 known meta states, with the most stable being 113mCd (t½ 14.1 years), 115mCd (t½ 44.6 days), and 117mCd (t½ 3.36 hours).

The known isotopes of cadmium range in atomic mass from 94.950 u (95Cd) to 131.946 u (132Cd). The primary decay mode before the second-most-abundant stable isotope, 112Cd, is electron capture, and the primary modes after are beta emission. The primary decay product before 112Cd is element 47 (silver), and the primary product after is element 49 (indium).

One isotope of cadmium, 113Cd, absorbs neutrons with very high probability if they have an energy below the cadmium cut-off and transmits them readily otherwise. The cadmium cut-off is about 0.5 eV Neutrons with energy below the cutoff are deemed slow neutrons, distinguishing them from intermediate and fast neutrons.

Applications
About three-quarters of all the cadmium used is in batteries, predominantly in rechargeable nickel-cadmium batteries. Nickel-cadmium cells have a nominal cell potential of 1.2 V. The cell consists of a positive nickel hydroxide electrode and a negative cadmium electrode plate separated by an alkaline electrolyte (potassium hydroxide). More recent nickel-metal hydride batteries reduce the use of Ni-Cd batteries. The European Union banned the use of cadmium in electronics in 2004 with several exceptions but reduced the allowed content of cadmium in electronics to 0.002 %.

 Other uses
Most of the remaining quarter is used mainly for cadmium pigments, coatings and plating, and as stabilizers for plastics. Other uses include:

In some of the lowest-melting alloys, for example Wood's metal
In bearing alloys, due to a low coefficient of friction and very good fatigue resistance
In electroplating (6% cadmium). Cadmium electroplating is widely used in aircraft industry due to the excellent corrosion resistance of cadmium-plated steel components. Cadmium provides cathodic protection to low-alloyed steels, since it is positioned lower in the galvanic series. The coating is usually passivated by chromate salts. A significant limitation of cadmium plating is hydrogen embrittlement of high-strength steels caused by the electroplating process. Therefore, steel parts heat-treated to tensile strength above 1300 MPa (200 ksi) should be coated by an alternative method (such as special low-embrittlement cadmium electroplating processes or physical vapor deposition).
In many kinds of solder
As a barrier to control neutrons in nuclear fissionThe pressurized water reactor designed by Westinghouse Electric Company used an alloy consisting of 80% Ag, 15% In, and 5% Cd.
Cadmium oxide in black and white television phosphors and in the blue and green phosphors for colour television picture tube
Cadmium sulfide (CdS) as a photoconductive surface coating for photocopier drums.
In paint pigments, cadmium forms various salts, with CdS being the most common. This sulfide is used as a yellow pigment. Cadmium selenide can be used as red pigment, commonly called cadmium red. To painters who work with the pigment, cadmium yellows, oranges, and reds are the most potent colours to use. In fact, during production, these colours are significantly toned down before they are ground with oils and binders, or blended into watercolours, gouaches, acrylics, and other paint and pigment formulations. These pigments are toxic, and it is recommended to use a barrier cream on the hands to prevent absorption through the skin when working with them.
In some semiconductors such as cadmium sulfide, cadmium selenide, and cadmium telluride, which can be used for light detection or solar cells. HgCdTe is sensitive to infrared.
In PVC as stabilizers.
In molecular biology, it is used to block voltage-dependent calcium channels from fluxing calcium ions.
See also Category:Cadmium compounds.