Structure
Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.
Measured values
Every bar shows where cadmium sits among all 118 elements for that property.
Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.
Size, to scale
Radius 151 pm — that is 0.151 nm, so about 3311 million of them side by side would span a millimetre.
Thermal range
Cadmium is liquid over a 446 K window, from 594 K to 1040 K.
The story
A soft, toxic metal that gave the world rechargeable NiCd cells and the brilliant yellow of old paint.
Cadmium was discovered in 1817 by German chemist Friedrich Stromeyer while investigating a peculiar yellow compound that was contaminating medicinal zinc preparations. This accidental discovery would reveal one of the periodic table's most toxic elements.
Friedrich Stromeyer, a professor at the University of Göttingen, was tasked with investigating reports of yellow discoloration in zinc oxide used for medicinal purposes. Pharmacists had complained that some batches of zinc oxide turned yellow when heated, making them unsuitable for medical use. Stromeyer suspected the presence of an unknown substance.
Stromeyer methodically analyzed the contaminated zinc oxide samples. He dissolved them in acid and performed precipitation reactions to separate different components. The yellow color persisted through various chemical treatments, indicating it wasn't from known impurities like iron or arsenic. His careful analytical work revealed this was something entirely new.
Through a series of chemical separations, Stromeyer successfully isolated a small amount of metallic cadmium by reducing the yellow compound with charcoal. The new metal was silvery-white, malleable, and had unusual properties. He named it "cadmium" after the Greek word "kadmeia" (calamine), referring to the zinc ore from which it was extracted.
Remarkably, two other chemists made similar discoveries around the same time. Karl Samuel Leberecht Hermann in Germany and Jöns Jacob Berzelius in Sweden both isolated cadmium independently in 1818, confirming Stromeyer's work. This parallel discovery highlighted the systematic approach of early 19th-century chemistry.
Within decades of its discovery, cadmium found use as a pigment. Cadmium yellow (cadmium sulfide) and cadmium red (cadmium selenide) became prized by artists for their brilliant, lightfast colors. These pigments were more vibrant and stable than previous yellow and orange colorants.
It wasn't until the mid-20th century that cadmium's severe toxicity became widely recognized. The tragic "Itai-itai disease" in Japan during the 1950s-60s, caused by cadmium pollution from mining operations, highlighted the element's dangerous health effects. This led to strict regulations and the phase-out of many cadmium applications.
Today, cadmium serves as a model for studying heavy metal toxicity and environmental contamination. Research into cadmium's biological effects has contributed significantly to our understanding of cellular stress, DNA damage, and environmental health. Despite its toxicity, cadmium remains important in specialized applications like nuclear control rods and advanced solar cells.
Stromeyer's discovery of cadmium exemplifies the careful analytical chemistry of the early 19th century. His systematic approach to isolating and characterizing unknown substances became a model for elemental discovery. The subsequent recognition of cadmium's toxicity also helped establish the field of environmental toxicology and the importance of studying elemental health effects.
Applications
Due to cadmium's toxicity, many traditional uses have been phased out or heavily regulated. The EU has banned cadmium in most consumer products, and many countries have strict limits on cadmium emissions and disposal.
NiCd batteries were once common in rechargeable applications due to their durability and ability to deliver high currents. They powered cordless phones, emergency lighting, and power tools. However, environmental concerns have led to their replacement with lithium-ion and NiMH batteries in most applications.
Cadmium sulfide produces brilliant yellow pigments, while cadmium selenide creates vibrant reds and oranges. These pigments were prized by artists for their intensity and lightfastness. Famous painters like Monet and Van Gogh used cadmium yellows. Today, safer alternatives are preferred for most applications.
Cadmium plating provides excellent corrosion resistance, especially in marine environments. It was widely used on aircraft hardware, bolts, and electronic components. Military and aerospace applications still use cadmium plating where alternatives cannot match its performance, but under strict controls.
Cadmium's exceptional ability to absorb neutrons makes it valuable in nuclear reactors as control rods and neutron shields. Cadmium-113 has one of the highest neutron absorption cross-sections of any stable isotope, making it essential for nuclear safety systems.
Cadmium telluride (CdTe) thin-film solar cells are highly efficient and cost-effective. Despite environmental concerns about cadmium, these solar panels have excellent energy payback ratios and are recyclable. They represent one of the few growing applications for cadmium.
Low-melting cadmium alloys are used in specialized applications like automatic fire sprinkler systems and safety plugs in pressure vessels. These alloys melt at predetermined temperatures, providing automatic safety releases in emergency situations.
Cadmium compounds serve as research tools in biochemistry and toxicology studies. Cadmium chloride is used to study cellular stress responses and environmental toxicology. These applications require strict safety protocols and waste management procedures.
Due to cadmium's toxicity, most everyday uses have been phased out or heavily regulated. Modern consumers rarely encounter cadmium directly, as safer alternatives have replaced most traditional applications.
Older rechargeable batteries in power tools, emergency lighting, and cordless phones contained cadmium. These are being replaced with lithium-ion and nickel-metal hydride batteries. Proper recycling of old NiCd batteries is crucial to prevent environmental contamination.
Professional artists may still use cadmium-based paints for their exceptional color properties, but many manufacturers now offer cadmium-free alternatives. Art students are often taught to use safer substitutes that mimic cadmium's vibrant yellows, oranges, and reds.
Some thin-film solar panels contain cadmium telluride, though consumers don't directly interact with this material. These panels are sealed and must be recycled properly at end-of-life to prevent cadmium release into the environment.
Vintage ceramics and glazes sometimes contain cadmium pigments, which can leach into food and drinks. Regulations now prohibit cadmium in food-contact surfaces. Antique or imported dishware should be tested before use with food.
Some specialized industrial equipment may contain cadmium-plated components for corrosion resistance. Workers in these industries receive special training and monitoring. Consumer exposure is minimal due to industrial containment practices.
Cadmium can accumulate in soil and plants, particularly leafy vegetables and grains. Modern agricultural practices monitor cadmium levels to ensure food safety. Smoking tobacco is a significant source of cadmium exposure for smokers.
Regulatory agencies like the EPA and FDA strictly limit cadmium in consumer products. Most everyday exposure comes from food and tobacco rather than direct contact with cadmium-containing products. Proper recycling and disposal prevent environmental accumulation.
Where it comes from
0.15 mg/kg of Earth's crust · more abundant than 43% of elements
Cadmium is one of the rarest stable elements in Earth's crust, occurring at an average concentration of only 0.1-0.2 parts per million. This toxic heavy metal rarely forms its own minerals, instead occurring as a minor component in zinc, lead, and copper ores.
Cadmium's chemical similarity to zinc means it almost always occurs alongside zinc minerals. The most important cadmium-bearing mineral is greenockite (CdS), which forms as a coating on zinc ores. Cadmium typically comprises 0.1-0.5% of zinc concentrates, making zinc refining the primary source of cadmium production.
Major cadmium-producing regions include China (60% of world production), South Korea, Canada, and Kazakhstan. The Red Dog mine in Alaska produces significant cadmium as a byproduct of zinc mining. Australia's Mount Isa and Broken Hill deposits also contain substantial cadmium reserves.
Pure cadmium minerals are extremely rare. Besides greenockite (CdS), other cadmium minerals include otavite (CdCO₃) and monteponite (CdO). These minerals are primarily of scientific interest rather than economic importance, as cadmium is almost exclusively recovered as a byproduct of other metal production.
Human activities have significantly increased environmental cadmium levels. Coal burning, metal smelting, and industrial processes release cadmium into air, water, and soil. Municipal waste incineration and the use of phosphate fertilizers also contribute to environmental cadmium contamination.
Cadmium bioaccumulates in living organisms, particularly in kidneys and liver. Plants can absorb cadmium from contaminated soils, with leafy vegetables and grains showing higher concentrations. Shellfish and some fish species concentrate cadmium from water, making seafood a potential exposure source.
Seawater contains approximately 0.1 parts per billion of cadmium, while fresh waters typically have lower concentrations. Cadmium pollution from mining and industrial activities can significantly elevate concentrations in rivers and lakes near contaminated sites.
Cadmium undergoes long-range atmospheric transport, allowing pollution from industrial areas to affect remote regions. Volcanic activity also releases natural cadmium into the atmosphere. The element's persistence and toxicity make it a global environmental concern requiring international cooperation for management.
Handling
Cadmium is classified as a Group 1 carcinogen by the International Agency for Research on Cancer (IARC). It poses serious health risks through inhalation, ingestion, and skin contact. Exposure should be minimized and strictly controlled.
Acute inhalation: Cadmium fumes can cause severe lung irritation, pulmonary edema, and potentially fatal pneumonitis. Symptoms may be delayed 4-24 hours after exposure. Chronic exposure: Long-term inhalation causes irreversible lung damage, emphysema, and increased lung cancer risk.
Primary target organ: Cadmium accumulates in kidneys with a biological half-life of 10-30 years. Chronic exposure causes kidney dysfunction, proteinuria, and eventual kidney failure. Even low-level exposure over time can cause irreversible kidney damage.
Calcium interference: Cadmium disrupts calcium metabolism, leading to bone demineralization, osteoporosis, and increased fracture risk. The notorious "Itai-itai disease" in Japan was characterized by severe bone pain and fractures from cadmium contamination.
Multiple cancer types: Cadmium exposure is linked to lung, prostate, kidney, and bladder cancers. The mechanism involves DNA damage, oxidative stress, and disruption of cellular repair mechanisms. No safe exposure threshold has been established.
Strict controls required: OSHA limits workplace exposure to 0.005 mg/m³ for an 8-hour workday. Workers must use respiratory protection, protective clothing, and undergo regular health monitoring. Eating, drinking, and smoking are prohibited in cadmium work areas.
Contamination prevention: Proper disposal of cadmium-containing batteries and electronics is essential. Agricultural monitoring prevents cadmium accumulation in food crops. Remediation of contaminated sites requires specialized techniques and long-term monitoring.
Immediate actions: Remove from exposure, provide fresh air, and seek immediate medical attention for inhalation. Wash thoroughly after skin contact. Chelation therapy may be considered for acute poisoning, though its effectiveness is limited. Long-term medical monitoring is essential for any significant exposure.
Quick answers
Cadmium (symbol Cd) is element 48 on the periodic table, a transition metal in period 5, group 12. A soft, toxic metal that gave the world rechargeable NiCd cells and the brilliant yellow of old paint. At room temperature it is a solid.
[Kr] 4d¹⁰ 5s², giving 5 occupied shells holding 2, 8, 18, 18, 2 electrons respectively.Cadmium melts at 594.2 K (321.1 °C) and boils at 1040 K (766.9 °C).
The standard atomic weight of Cadmium is 112.414 u. That is a weighted average across its naturally occurring isotopes, which is why it is rarely a whole number.
Cadmium has a density of 8.65 g/cm³. Water is 1.0 g/cm³, so a block of cadmium is about 8.7× heavier.
Cadmium has a Pauling electronegativity of 1.69. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). Values in this middle band tend to form covalent rather than strongly ionic bonds.
Cadmium was discovered in 1817 by Stromeyer & Hermann. It is named after latin cadmia, "calamine".
Cadmium makes up about 0.15 mg/kg of the Earth's crust — genuinely rare, which is why it is expensive.