Difference between revisions of "Cadmium" - New World Encyclopedia

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[[Image:HeCd laser.jpg|thumb|left|150px|Image of the violet light from a helium-cadmium metal vapor laser.]]
 
[[Image:HeCd laser.jpg|thumb|left|150px|Image of the violet light from a helium-cadmium metal vapor laser.]]
  
Naturally occurring cadmium is composed of eight [[isotope]]s. For two of them, natural [[radioactivity]] was observed, and other three are predicted to be [[radioactive]] but their decays were never observed, due to extremely long [[half-life]] times. The two natural radioactive isotopes are <sup>113</sup>Cd ([[beta decay]], [[half-life]] is 7.7 X 10<sup>15</sup> years) and <sup>116</sup>Cd (two-neutrino [[double beta decay]], [[half-life]] is 2.9 X 10<sup>19</sup> years). Other three ones are <sup>106</sup>Cd, <sup>108</sup>Cd ([[double electron capture]]), and <sup>114</sup>Cd ([[double beta decay]]); only lower limits on their [[half-life]] times have been set. At least three isotopes - <sup>110</sup>Cd, <sup>111</sup>Cd, and <sup>112</sup>Cd - are absolutely stable. Among the isotopes absent in the natural cadmium, the most long-lived are <sup>109</sup>Cd with a half-life of 462.6 days, and <sup>115</sup>Cd with a half-life of 53.46 hours. All of the remaining [[radioactive]] isotopes have half-lifes that are less than 2.5 hours and the majority of these have half-lifes that are less than 5 minutes. This element also has 8 known [[meta state]]s with the most stable being <sup>113m</sup>Cd (t<sub>½</sub> 14.1 years), <sup>115m</sup>Cd (t<sub>½</sub> 44.6 days) and <sup>117m</sup>Cd (t<sub>½</sub> 3.36 hours).
+
Naturally occurring cadmium is composed of eight [[isotope]]s. At least three of them&mdash;<sup>110</sup>Cd, <sup>111</sup>Cd, and <sup>112</sup>Cd&mdash;are known to be stable. Two others have been observed to be [[radioactive]], but with extremely long half-lives: <sup>113</sup>Cd, with a [[half-life]]* of 7.7 X 10<sup>15</sup> years; and <sup>116</sup>Cd, with a [[half-life]] of 2.9 X 10<sup>19</sup> years. The other three&mdash;<sup>106</sup>Cd, <sup>108</sup>Cd, and <sup>114</sup>Cd&mdash;are predicted to be [[radioactive]]*, but their decays have not been observed, as they too are expected to have extremely long half-lives.
  
The known isotopes of cadmium range in [[atomic weight]] from 96.935 [[atomic mass unit|u]] (<sup>97</sup>Cd) to 129.934 amu (<sup>138</sup>Cd). The primary [[decay mode]] before the second most abundant stable isotope, <sup>112</sup>Cd, is [[electron capture]] and the primary modes after are [[beta emission]] and [[electron capture]]. The primary [[decay product]] before <sup>112</sup>Cd is element 47 ([[silver]]) and the primary product after is element 49 ([[indium]]).
+
Among the radioactive cadmium isotopes absent in nature, the most long-lived are <sup>109</sup>Cd, with a half-life of 462.6 days, and <sup>115</sup>Cd, 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 5 minutes. This element also has 8 known [[meta state]]*s, with the most stable being <sup>113m</sup>Cd (t<sub>½</sub> 14.1 years), <sup>115m</sup>Cd (t<sub>½</sub> 44.6 days), and <sup>117m</sup>Cd (t<sub>½</sub> 3.36 hours).
  
 
== Applications ==
 
== Applications ==

Revision as of 17:42, 14 November 2006

48 silvercadmiumindium
Zn

Cd

Hg
Cd-TableImage.png
periodic table
General
Name, Symbol, Number cadmium, Cd, 48
Chemical series transition metals
Group, Period, Block 12, 5, d
Appearance silvery gray metallic
Cd,48.jpg
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 {{{isotopesof}}}]]
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 that occurs with zinc ores.

  • transition metal
  • cadmium is known to cause cancer.
  • Cadmium is used largely in batteries.

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.[1]

Small amounts of cadmium, about 10% 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 (1 m = 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 5, 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.

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"[2]. 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 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).

Applications

About three-fourths of cadmium is used in batteries (especially Ni-Cd batteries) and most of the remaining one-fourth is used mainly for pigments, coatings and plating, and as stabilizers for plastics. Other uses;

  • Used in some of the lowest melting alloys.
  • Due to a low coefficient of friction and very good fatigue resistance, it is used in bearing alloys.
  • 6% of cadmium finds use in electroplating.
  • Many kinds of solder contain this metal.
  • As a barrier to control nuclear fission.
  • Compounds containing cadmium are used in black and white television phosphors and also in the blue and green phosphors for color television picture tubes.
  • Cadmium forms various salts, with cadmium sulfide 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, casesin, acrylics and other paint and pigment formulations. These pigments are toxic and it is recommended to use a barrier cream on your hands to prevent absorption through the skin when working with them. There is no such thing as cadmium blue, green or violet.
  • Used 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.
  • Some cadmium compounds are employed in PVC as stabilizers.
  • Used in the first neutrino detector.
  • Used to block voltage-dependent calcium channels from fluxing calcium ions in molecular biology.

See also Cadmium compounds.

Precautions

Cadmium is toxic

Cadmium has no constructive purpose in the human body. This element and solutions of its compounds are toxic even in low concentrations, and will bioaccumulate in organisms and ecosystems. One possible reason for its toxicity is that it interferes with the action of zinc-containing enzymes. Zinc is an important element in biological systems, but cadmium, although similar to zinc chemically in many ways, apparently does not substitute or "stand in" for it well at all. Cadmium may also interfere with biological processes containing magnesium and calcium in a similar fashion. 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 the kidneys.

Compounds containing cadmium are also carcinogenic [3], and can induce many types of cancer [4].

Cadmium poisoning is the cause of the itai-itai disease, which literally means "ouch ouch" in Japanese. In addition to kidney damage, patients suffered from osteoporosis and osteomalacia.

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

Footnotes

External links

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