Cadmium

48 silver ← cadmium → indium Zn ↑ Cd ↓ Hg periodic table
General
Name, Symbol, Number	cadmium, Cd, 48
Chemical series	transition metals
Group, Period, Block	12, 5, d
Appearance	silvery gray metallic
Atomic mass	112.411(8) g/mol
Electron configuration	[Kr] 4d10 5s2
Electrons per shell	2, 8, 18, 18, 2
Physical properties
Phase	solid
Density (near r.t.)	8.65 g/cm³
Liquid density at m.p.	7.996 g/cm³
Melting point	594.22 K (321.07 °C, 609.93 °F)
Boiling point	1040 K (767 °C, 1413 °F)
Heat of fusion	6.21 kJ/mol
Heat of vaporization	99.87 kJ/mol
Heat capacity	(25 °C) 26.020 J/(mol·K)
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K 530 583 654 745 867 1040
Atomic properties
Crystal structure	hexagonal
Oxidation states	2 (mildly basic oxide)
Electronegativity	1.69 (Pauling scale)
Ionization energies	1st: 867.8 kJ/mol
	2nd: 1631.4 kJ/mol
	3rd: 3616 kJ/mol
Atomic radius	155 pm
Atomic radius (calc.)	161 pm
Covalent radius	148 pm
Van der Waals radius	158 pm
Miscellaneous
Magnetic ordering	no data
Electrical resistivity	(22 °C) 72.7 nΩ·m
Thermal conductivity	(300 K) 96.6 W/(m·K)
Thermal expansion	(25 °C) 30.8 µm/(m·K)
Speed of sound (thin rod)	(20 °C) 2310 m/s
Speed of sound (thin rod)	(r.t.) 50 m/s
Shear modulus	19 GPa
Bulk modulus	42 GPa
Poisson ratio	0.30
Mohs hardness	2.0
Brinell hardness	203 MPa
CAS registry number	7440-43-9
Notable isotopes
Main article: Isotopes of cadmium iso NA half-life DM DE (MeV) DP 106Cd 1.25% >9.5×1017 y εε2ν - 106Pd 108Cd 0.89% >6.7×1017 y εε2ν - 108Pd 109Cd syn 462.6 d ε 0.214 109Ag 110Cd 12.49% Cd is stable with 62 neutrons 111Cd 12.8% Cd is stable with 63 neutrons 112Cd 24.13% Cd is stable with 64 neutrons 113Cd 12.22% 7.7×1015 y β- 0.316 113In 113mCd syn 14.1 y β- 0.580 113In IT 0.264 113Cd 114Cd 28.73% >9.3×1017 y ββ2ν - 114Sn 116Cd 7.49% 2.9×1019 y ββ2ν - 116Sn

Cadmium (chemical symbol Cd, atomic number 48) is a relatively rare, soft, bluish-white metal. Its chemical properties are similar to those of zinc, and it is found in zinc ores. Unlike zinc, however, cadmium and solutions of its compounds are toxic and need to be handled with care.

This element and its compounds are used in electrical batteries, pigments, electroplating, plastics stabilizers, and many kinds of solder. In nuclear reactors, cadmium is used as a barrier to control nuclear fission reactions. Some compounds of cadmium function as semiconductors.

Occurrence and production

Cadmium metal

Cadmium-containing ores are rare. The only cadmium mineral of importance is Greenockite (cadmium sulfide, CdS), which is nearly always associated with sphalerite (zinc sulfide, ZnS). Consequently, cadmium is produced mainly as a byproduct from mining, smelting, and refining the sulfide ores of zinc. To a lesser degree, it is obtained from the ores of lead and copper.

Zinc sulfide ores are roasted in the presence of oxygen, thereby converting the sulfide to zinc oxide. To produce zinc metal, the oxide is either smelted with carbon or subjected to electrolysis in sulfuric acid. If the smelting process is used, cadmium is isolated from metallic zinc by vacuum distillation. If the electrolytic process is used, cadmium sulfate is precipitated out of the solution.

Small amounts of cadmium, about ten percent of consumption, are produced from secondary sources—mainly from dust generated when recycling iron and steel scrap. Production in the United States began in 1907, but it was not until after World War I that cadmium came into wide use.

History

Cadmium (Latin cadmia, Greek kadmeia meaning "calamine") was discovered in Germany in 1817 by Friedrich Strohmeyer. Strohmeyer found the new element as an impurity in zinc carbonate (calamine), after he noticed that some impure samples of calamine changed color when heated, but pure calamine did not. The element was named after the Latin word for calamine, having been found in this zinc compound. For about 100 years, Germany remained the only important producer of the metal.

Although cadmium and its compounds are highly toxic, the British Pharmaceutical Codex from 1907 states that cadmium iodide was used as a medicine to treat "enlarged joints, scrofulous glands, and chilblains."

In 1927, the International Conference on Weights and Measures redefined the meter in terms of a red cadmium spectral line (one meter = 1,553,164.13 wavelengths). This definition has since been changed (see krypton).

Notable characteristics

Cadmium is a soft, malleable, ductile, bluish-white metal that can be easily cut with a knife. In the periodic table, it lies in group 12 (former group 2B), between zinc and mercury. In addition, it is located in period five, immediately following silver.

Cadmium is similar in many respects to zinc but reacts to form more complex compounds. The most common oxidation state of cadmium is +2, though rare examples of +1 can be found. The most common salt of cadmium is cadmium sulfide, which has a yellow color.

As an element in group 12, cadmium has traditionally been classified as a "transition metal." This view, however, has been changed, based on the current definition of transition elements stated by the International Union of Pure and Applied Chemistry (IUPAC). According to that definition, a transition element is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Cadmium does not fit this definition because its atoms and ions contain electrons that completely fill the d orbitals.

Isotopes

Image of the violet light from a helium-cadmium metal vapor laser.

Naturally occurring cadmium is composed of eight isotopes. At least three of them—¹¹⁰Cd, ¹¹¹Cd, and ¹¹²Cd—are known to be stable. Two others have been observed to be radioactive, but with extremely long half-lives: ¹¹³Cd, with a half-life of 7.7 X 10¹⁵ years; and ¹¹⁶Cd, with a half-life of 2.9 X 10¹⁹ years. The other three—¹⁰⁶Cd, ¹⁰⁸Cd, and ¹¹⁴Cd—are predicted to be radioactive, but their decays have not been observed, as they too are expected to have extremely long half-lives.

Among the radioactive cadmium isotopes absent in nature, 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 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 five minutes. This element also has eight known meta states, with the most stable being ^113mCd (half-life = 14.1 years), ^115mCd (half-life = 44.6 days), and ^117mCd (half-life = 3.36 hours).

Compounds

• Cadmium chloride (CdCl₂): This white crystalline compound is highly soluble in water and slightly soluble in ethanol. Although it is considered to be ionic, it has considerable covalent character to its bonding. Its crystal structure is a two-dimensional layer lattice that is often used as a reference for describing other crystal structures. It is used for the preparation of cadmium sulfide and for electroplating. In the laboratory, anhydrous CdCl₂ can be used for the preparation of organocadmium compounds of the type R₂Cd (where R is an aryl or primary alkyl group).

• Cadmium selenide or cadmium(II) selenide (CdSe): This is a solid, semiconducting material, but it has yet to find many applications in manufacturing. It is transparent to infrared (IR) light and has seen limited use in windows for instruments utilizing IR light. It is also being developed for use in opto-electronic devices, laser diodes, nanosensing, and biomedical imaging. Much current research on cadmium selenide has focused on the controlled synthesis and properties of CdSe nanoparticles, and determination of their potential uses. It should be noted that selenides are toxic in large amounts.

• Cadmium sulfide or cadmium sulphide (CdS): This compound, which forms yellowish, hexagonal crystals, is found in nature as the mineral greenockite. It has a specific gravity of 4.7 and Mohs hardness of 3.8. Synthetic cadmium pigments based on cadmium sulfide are valued for their good thermal stability in many polymers, particularly plastics. By adding selenium to its chemical formula, it is possible to obtain pigments ranging in color from greenish yellow to red-violet. In addition, cadmium sulfide is a direct bandgap semiconductor that is useful for photosensitive and photovoltaic devices. When mixed with zinc sulfide, cadmium sulfide acts as a phosphor with a long afterglow.

Applications

Cadmium, its alloys, and its compounds have a variety of applications. About three-fourths of commercially available cadmium is used in batteries (especially nickel-cadmium batteries), and most of the remaining one-fourth is used for pigments, coatings, electroplating, and stabilizers for plastics.

Additional uses of cadmium are as follows:

• It is used in some of the lowest melting alloys.
• Given its low coefficient of friction and high fatigue resistance, it is used in alloys for bearings.
• It is a component of many kinds of solder.
• It is used in nuclear reactors as a barrier to control nuclear fission.
• Compounds containing cadmium are used in black and white television phosphors, and in the blue and green phosphors for color television picture tubes.
• Cadmium sulfide is used as a yellow pigment, and cadmium selenide is used as a red pigment commonly called cadmium red. Cadmium yellows, oranges, and reds are among the most potent colors available. During production, these colors are significantly toned down before being ground with oils and binders, or blended into watercolors, gouaches, casesin, acrylics, and other paint and pigment formulations. These pigments, however, are toxic, and one needs to prevent absorption through the skin by, for instance, applying a barrier cream on one's hands.
• Some semiconductors are made of cadmium sulfide, cadmium selenide, or cadmium telluride, which can be used for light detection or solar cells. Mercury cadmium telluride (HgCdTe) is sensitive to infrared.

Precautions

Cadmium and its compounds are toxic.

Cadmium has no known function in the human body. This element and solutions of its compounds are toxic even at low concentrations, and they bioaccumulate in organisms and ecosystems. One possible reason for the toxicity of cadmium is that it interferes with the action of zinc-containing enzymes. Zinc is an important element in biological systems, but cadmium, although chemically similar to zinc, is not a healthy substitute for zinc. Likewise, cadmium may also interfere with biological processes requiring magnesium and calcium. Pathways of human contact include soil contamination from industrial releases or landfill and associated leachate processes.

Inhaling cadmium-laden dust quickly leads to respiratory tract infection and kidney problems, which can be fatal (often from renal failure). Ingestion of any significant amount of cadmium causes immediate poisoning and damage to the liver and kidneys. Compounds containing cadmium are also carcinogenic and can induce many types of cancer.

Cadmium poisoning is the cause of itai-itai disease, which literally means "ouch ouch" in Japanese. In addition to kidney damage, patients suffer from osteoporosis and osteomalacia (the equivalent of rickets among adults).

While working with cadmium, it is important to do so under a fume hood to protect against dangerous fumes. Cadmium-containing silver solder should be handled with care. Serious toxicity problems have resulted from long-term exposure to cadmium plating baths.

References

ISBN links support NWE through referral fees

• Greenwood, N. N., and A. Earnshaw. Chemistry of the Elements, 2nd ed. Oxford, UK: Butterworth-Heinemann, 1997.
• March, J. Advanced Organic Chemistry, 4th ed. New York: Wiley, 1992.
• Nicholls, D. Complexes and First-Row Transition Elements. London: Macmillan Press, 1973.
• Wells, A. F. 'Structural Inorganic Chemistry, 5th ed. Oxford, UK: Oxford University Press, 1984.
• Handbook of Chemistry and Physics, 71st ed. Ann Arbor, Michigan: CRC Press, 1990.
• The Merck Index, 7th ed. Rahway, New Jersey: Merck & Co, 1960.

External links

All links retrieved November 25, 2023.

• WebElements.com – Cadmium

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