Cadmium
Cadmium is a chemical element with symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Like zinc, it prefers oxidation state +2 in most of its compounds and like mercury it shows a low melting point compared to transition metals.
Identity
- Atomic mass
- 112.414 u
- Category
- Transition metals
- Group
- 12
- Period
- 5
- Block
- d
- Oxidation States
- +2
- Phase (25 °C)
- Solid
- Appearance
- silvery bluish-gray metallic
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10
- Short configuration
- [Kr] 4d10 5s2
- Electrons per shell
- 2, 8, 18, 18, 2
- Electronegativity (Pauling)
- 1.69
- Electron affinity
- -68 kJ/mol
- 1st ionization energy
- 868 kJ/mol
- Ionization energies
- 868 · 1631 · 3616 kJ/mol
Physical properties
- Melting point
- 321.1 °C (594 K)
- Boiling point
- 766.9 °C (1040 K)
- Density
- 8.65 g/cm³
- Molar heat capacity
- 26.0 J/(mol·K)
Chemical behavior of cadmium
Cadmium has a full d-shell configuration ([Kr] 4d10 5s2). It loses only its two outer 5s electrons to give the diamagnetic Cd2+ ion. A soft ductile group 12 metal akin to zinc but less reactive, it forms very stable complexes and slowly films over in moist air.
Reactivity
Cd + 2 HCl → CdCl₂ + H₂
Dissolves slowly in dilute non-oxidizing acids such as hydrochloric acid, releasing hydrogen gas. Nitric acid attacks much faster.
2 Cd + O₂ → 2 CdO
Strongly heated in air it burns with dense brown fumes of cadmium oxide, a brown powder used in electrodes and enamels.
Cd²⁺ + H₂S → CdS + 2 H⁺
Quantitatively precipitates canary-yellow cadmium sulfide with hydrogen sulfide in acid, once the basis of vivid pigments.
Cd + 2 NiO(OH) + 2 H₂O ⇌ Cd(OH)₂ + 2 Ni(OH)₂
In nickel-cadmium rechargeables it is the negative electrode, oxidizing to cadmium hydroxide on discharge and reversing on charge.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Cadmium sulfide | CdS | Cadmium yellow; historic vivid stable pigment for paints and plastics |
| Cadmium telluride | CdTe | Thin-film semiconductor for efficient photovoltaic panels |
| Cadmium oxide | CdO | Brown powder; battery electrodes, catalysts, ceramic glazes |
| Cadmium chloride | CdCl₂ | Dyeing, electroplating and coordination-chemistry reagent |
Electron shell distribution
View in the atom builderIsotopes
Natural cadmium mixes eight isotopes: six stable plus two primordial long-lived radionuclides (Cd-113, Cd-116). Cadmium-113 has one of the largest thermal-neutron capture cross-sections, hence control rods.
Occurrence and production
Scarce crustal element (0.1-0.15 ppm) with almost no workable ores of its own. It hides in Sphalerite (0.1-0.5 %) and is won solely as a by-product of Smelting.
History and origin of the name
Friedrich Stromeyer found it in 1817 in impure Salzgitter calamine turning suspiciously yellow on heating; Hermann and Karsten saw it the same year in Silesia. Named cadmium after cadmia (zinc ore).
Applications and relevance of cadmium
Uses of the element
Coatings once protected steel; Ni-Cd rechargeables powered tools and aircraft. Both shrank under RoHS/REACH limits. Remaining niches are control rods and specialized uses under strict control.
Uses of its compounds
Sulfide and selenide pigments color artists' paints and plastics where still allowed; telluride films make efficient solar panels; oxide, chloride and stearates serve electrodes and stabilizers under tight regulation.
Health, safety and environment
Biological role
Cadmium is highly toxic with no human function. It lodges in liver and kidney bound to metallothionein, with a biological half-life of decades. Chronic intake damages kidney tubules and bone.
Safety
Metal, dust, fume and all compounds need stringent control: ventilation, respirators, no dry sweeping, biological monitoring. Never weld cadmium-plated steel without extraction.
Environment
Persistent and bioaccumulative; mining, smelting and battery waste are the main releases. Regulated as hazardous waste with take-back and recycling circuits.
Frequently asked questions about Cadmium
Why is cadmium so strictly regulated?
It accumulates in kidneys with decades-long half-life, harming tubules and bone at chronic low doses, plus lung risks from fume. Rules cap it in electronics, toys, paints and packaging.
What was Itai-itai disease in Japan?
From the 1910s Toyama zinc-lead mines flushed cadmium into the Jinzu River; rice irrigated with it poisoned villagers with kidney failure and brittle bones. Cries of pain named it 'it hurts, it hurts'.
How can cadmium be in solar panels?
Cadmium telluride films absorb sunlight efficiently in microns on glass, cutting semiconductor use versus silicon. Manufacture, recycling and end-of-life must contain cadmium.
Why does cadmium linger so long in the body?
After blood uptake it binds metallothionein proteins in liver and kidney cells that retain it. Excretion is very slow, with biological half-life reaching decades.
How does cadmium absorb neutrons in reactors?
Cd-113 grabs thermal neutrons with a huge cross-section. Inserting cadmium rods soaks up neutrons and slows fission; withdrawing restarts it. Burn-up limits rod life.
What is special about cadmium pigments?
Intense yellows, oranges and reds with covering power and lightfastness prized by artists and plastics makers. Toxic dust needs precautions; avoid inhaling dry pigment.
What was the 'memory effect' of Ni-Cd batteries?
Repeated partial recharges grew large cadmium crystals, shrinking active area so packs 'remembered' the shallow level and quit early. Modern Ni-MH and Li-ion largely replaced them.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.