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Element Fact
The liquid helium-4 possesses the most unusual physical properties that defy common sense. Below 2.2 K it becomes a superfluid that can flow, against gravity, up the wall of a container.

Cadmium, Cd

cadmium sample

Atomic number: 48
Atomic mass: 112.411(8)
Natural abundance: 0.16 ppm
Isotopes:
106Cd (105.906458) - 1.25%
108Cd (107.904183) - 0.89%
110Cd (109.903006) - 12.49%
111Cd (110.904182) - 12.80%
112Cd (111.902757) - 24.13%
113Cd (112.904401) - 12.22%
114Cd (113.903358) - 28.73%
116Cd (115.904755) - 7.49%

History and Production

From Latin cadmia or Greek kadmeia, ancient name for calamine (ZnCO3). It was discovered by F. Stromeyer in 1817, from a sample of tainted calamine with yellow, instead of the usual white. The coloration was identified to be due to the presence of a new element and not iron. The name was given after the zinc ore in which it had been found. It is extracted from zinc ores which usually retain about 0.2-0.4% of cadmium. It is obtained from zinc finish product by vacuum distillation. The metal is used in electroplating, in Ni-Cd batteries, solders and some low-melting alloys. Some compounds such as cadmium sulfide is used as a yellow pigment.

Physical Data

It is a bluish-white metal that can be easily cut with a knife and tarnishes quickly in moist air. Its occurance in nature is usually associated with zinc ores such as sphalerite (ZnS). Cadmium and its compounds are toxic and should handle with care.

Interatomic distance: 297.8 pm
Melting point: 321.07C
Boiling point: 767C
Thermal conductivity/Wm-1K-1: 96.8 (27C)
Density/kgm-3: 8650 (20C), 7996 (m.p.)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 111.8 kJ/mol
Gibbs free energy of formation: 51.8 kJ/mol
Entropy: 167.7 J/mol K
Heat capacity: 20.8 J/mol K

Electronic data

Electronic configuration: [Kr] 4d10 5s2
Term symbol: 1S0
Electron affinity: not stable   Electronegativity (Pauline): 1.69
Ionization energy (first, second, third): 867.77, 1631.41, 3616.27 kJ/mol

Chemical properties

Cadmium is resist to attack in dry air at room temperature. In moist air it forms a protective coating of oxide and carbonate. On strong heating in air, it burns to produce brown cadmium oxide.

Test for cadmium compounds
(1) Hydrogen sulfide gives a yellow precipitate insoluble in ammonium sulfide, but soluble in dilute nitric acid.
(2) With a hydroxide solution, cadmium compound gives a white precipitate soluble in ammonia.
(3) Potassium cyanide gives a white precipitate of cadmium cyanide, soluble in excess.
(4) Solid cadmium compounds when fused with sodium carbonate on a charcoal block give a brown incrustation of cadmium oxide.


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