Cadmium

Cadmium is a soft, blue-white metal that is highly toxic and was a major industrial pollutant of the 20th century. Yet it is also the material that made colourful photography possible, pigments that produced some of the most vibrant yellows and reds in modern art, and for decades the key technology in rechargeable batteries.

• Atomic Number4848 protons, 48 electrons
• Atomic Mass112.41 u48× heavier than hydrogen
• State at Room TempSolidSolid
• Density8.69 g/cm³
• Melting / Boiling321.1°C / 766.9°C
• Discovered1817

What is Cadmium?

Cadmium is a post-transition metal in Group 12 of the periodic table, sitting below zinc and above mercury. With 48 protons and primarily a +2 oxidation state, cadmium is soft, malleable and easily cut. It is toxic to humans and animals even in small amounts, accumulating in the kidneys and causing serious damage over years of exposure. Despite its toxicity, its compounds have useful optical and electrical properties.

Cadmium gets its name from cadmia, the ancient Greek and Latin name for zinc oxide (calamine), because cadmium was found as an impurity in zinc carbonate ores. Friedrich Stromeyer discovered it in 1817 in Hanover, Germany, while investigating why some zinc carbonate samples from pharmacies did not turn white when heated, as expected. He identified the yellow colour as a new element.

Fact Cadmium telluride (CdTe) solar panels are now the second most popular solar panel technology in the world, after silicon. CdTe panels are cheaper to manufacture than silicon panels and can be made into large, flexible sheets. In 2023, they had approx. 5% of the global solar market despite the toxicity of cadmium, a fact that has driven research into cadmium containment and recycling.

Where you find Cadmium

In space

Cadmium is produced in stellar nucleosynthesis and is found in the Sun.

On Earth

Cadmium is rare in the Earth's crust, approx. 0.15 parts per million, and is almost always found associated with zinc minerals.

• Greenockite (CdS). The only significant cadmium mineral, but rarely mined directly. Almost all commercial cadmium comes as a by-product of zinc smelting.
• Zinc ores. Cadmium substitutes for zinc in sphalerite (ZnS) at concentrations of 0.1-0.5%. Recovering it from zinc smelter flue dusts is the main source of cadmium.

How we use Cadmium

• Nickel-cadmium (NiCd) batteries.. Rechargeable NiCd batteries were the dominant portable battery technology from the 1960s to the 1990s, used in power tools, cameras and electronics. Largely replaced by NiMH and lithium-ion batteries.
• Cadmium pigments.. Cadmium sulfide (yellow) and cadmium selenide (red) are brilliant, heat-stable pigments used by artists and in industrial coatings. Claude Monet and Henri Matisse both used cadmium yellow extensively.
• Cadmium telluride solar cells.. CdTe thin-film solar cells are the second most common solar cell technology after silicon, offering good efficiency at lower manufacturing cost.
• Nuclear reactor control.. Cadmium absorbs neutrons very effectively and was used in early nuclear reactor control rods.

Did you know? The Itai-itai disease ("it hurts-it hurts" in Japanese) that emerged in the Toyama prefecture of Japan in the 1950s was one of the first recognised cases of environmental cadmium poisoning. Locals ate rice grown in fields irrigated by a river contaminated with cadmium from a zinc mine. The cadmium accumulated in their kidneys and bones, causing severe pain, skeletal damage and kidney failure.

How it was discovered

Friedrich Stromeyer discovered cadmium in 1817 in Hanover while investigating a discrepancy in the colour of zinc carbonate samples. He isolated the pure metal by roasting and reducing the yellow cadmium oxide with carbon. Independently and almost simultaneously, Karl Samuel Leberecht Hermann identified it in another zinc ore. Stromeyer is credited as the primary discoverer.

Deeper dive: cadmium chemistry and applications

Cadmium is remarkably toxic because it mimics zinc and calcium in biological systems. In the kidneys: the main target organ for cadmium toxicity, cadmium replaces zinc in zinc-dependent enzymes, disrupting their function. Cadmium also triggers the production of metallothionein, a protein that binds cadmium and other metals, sequestering it but also concentrating it in kidney cells. Chronic low-level exposure over decades causes progressive kidney tubular damage and eventually bone demineralisation (because cadmium disrupts calcium metabolism). Cadmium has been classified as a Group 1 carcinogen (definitely causes cancer) by the WHO.

CdTe solar cells are made by depositing thin layers of cadmium sulfide and cadmium telluride onto glass substrates. The cell structure is a heterojunction between the two semiconductor materials that efficiently converts sunlight to electricity. First Solar: the largest CdTe solar manufacturer, has argued convincingly that the cadmium locked in glass solar panels poses no environmental risk during normal operation, and that proper end-of-life recycling recovers it safely.

Moving to 49 protons on the periodic table brings us to Indium.