Cadmium – facts and properties
- Symbol
- Cd
- Atomic number
- 48
- Atomic mass
- 112.414 u
- Period
- 5
- Group
- 12
- Category
- metal
- Electron configuration
- [Kr] 4d¹⁰ 5s²
- Electronegativity
- 1.69
- Atomic radius
- 161 pm
- Valence electrons
- 12
- State at standard conditions
- solid
- Melting point
- 321.07 °C
- Boiling point
- 767 °C
- Density
- 8.69 g/cm³
- Discovered
- 1817
Cadmium is a soft, bluish-white metal with atomic number 48. It is chemically similar to zinc and in nature occurs almost only together with zinc. Cadmium and its compounds are toxic and carcinogenic, which is why its use is strictly limited in the EU. It once went into device batteries, pigments and rust-protection coatings. Today it is mainly used in industrial batteries, solar modules and as a neutron absorber in nuclear reactors.
Position in the periodic table
Occurrence and extraction
Cadmium is scarce in the Earth's crust and forms hardly any ores of its own. It occurs in small amounts in zinc ores, above all in sphalerite. It is therefore obtained as a by-product of zinc production, and also from recycling nickel-cadmium batteries. Major producers are China, South Korea and Japan.
The element was discovered in 1817 by Friedrich Stromeyer in Göttingen. He examined zinc carbonate from a pharmacy that unexpectedly turned yellow on heating, and found a new metal in it. He took the name from the Latin cadmia, an old word for zinc ore.
Cadmium also reaches the environment through phosphate fertilisers, exhaust gases and mining. Plants take it up from the soil, so traces can occur in foods such as grain, potatoes and offal. Tobacco smoke contains cadmium as well.
Properties and reactions
Cadmium is in period 5 and group 12, below zinc and above mercury. Its electron configuration is [Kr] 4d¹⁰ 5s² and its electronegativity is 1.69. The metal has a density of 8.69 g/cm³, melts at 321.07 °C and boils at 767 °C. Both the melting point and the boiling point are low for a metal. Cadmium is soft and can be cut with a knife.
Compared with many other metals, cadmium is fairly stable. In air a thin oxide layer forms. In compounds it almost always has the oxidation number plus two. The cadmium ion forms colourless salts, whereas cadmium sulfide is yellow and mixed crystals of cadmium sulfide and cadmium selenide are orange to red.
Cadmium and its compounds are toxic and are classed as carcinogenic. Breathing in dust and fumes is especially dangerous. In the body cadmium builds up mainly in the kidneys and is excreted only very slowly. Long-term exposure damages the kidneys and the bones.
A well-known example is Itai-itai disease in Japan. Cadmium from mining reached the food chain through a river and rice fields and caused severe kidney damage and painful softening of the bones. Because of these dangers, cadmium and its compounds have no place in school experiments.
Uses in everyday life and technology
For a long time the most important use was the nickel-cadmium battery. It withstands many charging cycles and heavy loads, but because of its toxicity it was banned in the EU for portable batteries. Industrial batteries, for example for emergency power, railways and aviation, are still allowed. In everyday life lithium-ion batteries have largely replaced the NiCd battery. Old batteries belong in a take-back collection and never in household waste.
Cadmium sulfide and cadmium selenide served as brilliant pigments. Cadmium yellow and cadmium red are lightfast and opaque. They are found in paintings and artists' colours, but are largely banned in plastics. Cadmium coatings as rust protection on steel are also strongly restricted in the EU.
One important use is the thin-film solar cell made of cadmium telluride. Cadmium also absorbs neutrons very strongly, so it is found in control rods of nuclear reactors, which slow the chain reaction, for example in an alloy of silver, indium and cadmium. Strict legal rules apply to handling the metal, recycling and disposal.
Molecules containing this element
- Cadmium sulfide CdS
- Cadmium oxide CdO
- Cadmium chloride CdCl2
Quick question
What is the atomic number of cadmium (Cd)?
FAQ
Is cadmium toxic?
Yes. Cadmium and its compounds are toxic and are classed as carcinogenic. It builds up mainly in the kidneys and liver and is excreted only very slowly. That is why it is strictly limited in the EU, and it has no place in school experiments.
What was Itai-itai disease?
A cadmium poisoning in a region of Japan in the twentieth century. Cadmium from mining reached the river and rice fields. People who took in the polluted water and rice for years suffered kidney damage and very painful, brittle bones.
What was cadmium used for?
For rechargeable nickel-cadmium batteries, for yellow and red pigments, as rust protection on steel and in control rods of nuclear reactors. Many of these uses are banned or heavily restricted in the EU. Today it is mainly found in industrial batteries, in solar modules made of cadmium telluride and in control rods of nuclear reactors.