Cadmium (symbol Cd) is the chemical element with atomic number Z = 48 It is a soft, bluish-white (silvery) metal in group 12 (the zinc group) of the periodic table, located in period 5 and the d-block At room temperature, cadmium is a solid metal (melting point 321 °C, boiling point 767 °C) It has a high density of about 8.65 g/cm³ The most common oxidation state of cadmium is +2 (its divalent cation Cd²⁺), although a rare +1 state (as the diatomic Cd₂²⁺ ion) can occur in specialized compounds Cadmium exhibits no unpaired electrons in its ground state Kr]4d¹⁰5s² configuration), so elemental cadmium is diamagnetic. The element is classified as a transition metal (often not treated as a “true” transition metal because its d shell is filled), and it is notably resistant to corrosion and oxidation for a metal

Like its group‐12 neighbors zinc and mercury, cadmium typically forms binary compounds (oxides, sulfides, halides, etc.) in which it is divalent. Its weak metallic bonds give it a low melting point by comparison to most metals, and it is fairly malleable and ductile Cadmium has no known essential biological role in humans or higher organisms and it is toxic even at low concentrations (for this reason it is tightly regulated worldwide).

Atomic Structure and Periodic Trends

Cadmium
Atomic number	48
Symbol	Cd
Group	12 (zinc group)
Boiling point	767 °C
Electron configuration	[Kr] 4d10 5s2
Density	8.65 g/cm^3
Discovery	F. Stromeyer, 1817
Cas number	7440-43-9
Oxidation states	+2, +1
Melting point	321.1 °C
Phase STP	Solid
Wikidata	Q1091

The neutral cadmium atom has 48 electrons arranged in shells as [Kr] 4d¹⁰ 5s² Its valence shell (the 5th shell) contains two 5s electrons and a filled 4d subshell. As a group-12 element, cadmium’s atomic radius is relatively large: its empirical atomic radius is about 155 pm (1.55×10⁻¹⁰ m) and its single-bond covalent radius is about 1.44 Å In the group-12 series down the column, atomic size increases from zinc to cadmium (and to mercury). Correspondingly, the first ionization energy (the energy to remove one electron) drops from Zn to Cd to Hg due to the larger radius and increased shielding. For cadmium, the first ionization energy is about 8.9937 eV (≈8.68×10^2 kJ/mol) lower than zinc (9.39 eV) but higher than mercury (10.43 eV). Electronegativity (Pauling scale) is modest: cadmium’s value is ~1.69 slightly higher than zinc (1.65) and lower than mercury (~2.0). In general, cadmium behaves more like a post-transition “post-metal” in many respects: it forms unpredictable covalent complexes and does not show multiple oxidation states beyond +1/+2.

Isotopes and Nuclear Properties

Naturally occurring cadmium consists of several isotopes. In terrestrial materials, there are eight recorded cadmium isotopes, of which five (¹⁰⁶Cd, ¹⁰⁸Cd, ¹¹⁰Cd, ¹¹²Cd, ¹¹⁴Cd) are essentially stable, and two (¹¹³Cd and ¹¹⁶Cd) are very long-lived radionuclides The abundances are roughly: ¹⁰⁶Cd 1.25%, ¹⁰⁸Cd 0.89%, ¹¹⁰Cd 12.5%, ¹¹¹Cd 12.8%, ¹¹²Cd 24.1%, ¹¹³Cd 12.2%, ¹¹⁴Cd 28.8%, ¹¹⁶Cd 7.51% (sums to 100%). Of these, ¹¹³Cd (spin 1/2) undergoes β-decay with a half-life ~7.7×10¹⁵ years, and ¹¹⁶Cd (spin 0) decays by very slow double beta decay (t₁/₂≈3.3×10¹⁹ years) the others are observationally stable (no decay observed under laboratory conditions). The more common stable isotopes (¹⁰⁶, ¹¹⁰–¹¹⁴Cd) mostly have nuclear spin 0, but ¹¹¹Cd and ¹¹³Cd (each ~12% abundant) have spin 1/2 and are NMR-active.

Cadmium has no isotopes of practical radiometric dating for geology (the long half-lives are too great for useful clocks). However, certain isotopes are important in radiation sources and instruments. For example, synthetic ^109Cd (half-life ≈463 days) decays by electron capture to stable ^109Ag and emits Ag K-line X-rays (~22 keV) – it is used in X-ray fluorescence and calibration devices. Notably, ^113Cd has an enormous thermal neutron absorption cross-section: neutrons with energy below the ~0.5 eV cadmium cutoff are almost certainly captured by ^113Cd, while higher-energy neutrons pass through This property makes cadmium metal or compounds useful in nuclear reactor control rods and neutron shielding. Some nuclear power reactors (e.g. certain PWR control assemblies) use alloys containing cadmium (often mixed with silver and indium) to absorb neutrons and regulate fission reactions The metastable nuclide ¹¹⁵mCd (44.6-day half-life) and others have limited technical uses in research.

Allotropes and Typical Compounds

Cadmium has no allotropic forms under normal conditions – it exists only as the metallic element (HCP crystal) and compounds. In compounds, cadmium is usually in the +2 oxidation state, with a full d¹⁰ configuration. Typical cadmium compounds include oxide (CdO), sulfide (CdS), selenide (CdSe), telluride (CdTe), chloride (CdCl₂), bromide (CdBr₂), iodide (CdI₂), carbonate (CdCO₃), hydroxide (Cd(OH)₂), nitrate (Cd(NO₃)₂), sulfate (CdSO₄), and various organometallics. Cadmium(II) oxide is a dense red-brown solid, insoluble in water, that dissolves in strong acids to form soluble Cd²⁺ salts. It is mildly amphoteric, dissolving in strong bases to form [Cd(OH)₄]²⁻. Cadmium sulfide (CdS) is a bright yellow solid used as “cadmium yellow” pigment, and is also a photoconductive semiconductor (photocells, solar PV) CdSe (orange-red) and CdTe (green sapphire-colored) are also semiconductors used in optoelectronic applications; for example, cadmium telluride (CdTe) is the absorber layer in commercial thin-film solar cells Cadmium halides like CdCl₂ and CdI₂ are white, hygroscopic solids; anhydrous CdCl₂ has a layered lattice. One unusual species is the cadmium(I) chloride, which in suitable molten chloride mixtures yields the dimeric Cd₂²⁺ cation (analogous to the Hg₂²⁺ species) Cadmium also forms organometallic compounds (e.g. dimethylcadmium) and coordination complexes with amines or thiols, but these are mainly of academic interest due to toxicity.

Hydrogen forms a cadmium hydride. Cadmium hydride (CdH₂) is chemically known but highly unstable; it can be made under pressure but decomposes on mild heating, releasing hydrogen and metallic Cd Because Cd–H bonds are weak, CdH₂ is typically of interest only to researchers studying metal hydrides, not practical chemical feedstock

Physical Properties

Cadmium is a relatively soft metal (malleable and ductile) with a bluish-white metallic luster. It crystallizes in the hexagonal close-packed (hcp) structure At 20 °C, its density is about 8.65 g/cm³ (much heavier than Zn or Al). Its melting point is 594.2 K (≃321.1 °C) and boiling point 1040 K (≃767 °C) which are low compared to most transition metals (cadmium sublimes easily). The heat of fusion is about 6.19 kJ/mol and heat of vaporization ~99.6 kJ/mol

Cadmium is a good electrical and thermal conductor (thermal conductivity ≈97 W·m⁻¹·K⁻¹ at room temperature It is paramagnetic when molten, but in solid form with all electrons paired it behaves as a diamagnetic metal (negligible permanent magnetism). Spectroscopically, cadmium has several notable atomic emission lines. In fact, the international standard length (the ångström) was once defined (in 1907) by the wavelength of a red cadmium spectral line (6438.46963 Å) (This definition was later superseded, but it highlights cadmium’s sharp emission lines used in atomic spectroscopy.)

Chemical Reactivity and Trends

Cadmium metal is moderately reactive. It does not dissolve in water but will burn vigorously in air at high temperature, forming cadmium oxide (CdO). A superficial oxide layer may form in moist air, but it is not as quickly corrosive as many metals; indeed, cadmium is unusually corrosion-resistant for a reactive metal and was widely used for protective plating on steel Upon heating, cadmium ignites and burns to brown CdO; the oxide is a basic (slightly amphoteric) refractory solid that in powder form is brownish and crystalline form is reddish Cadmium dissolves readily in strong oxidizing acids: hydrochloric, sulfuric or nitric acid convert Cd to soluble CdCl₂, CdSO₄ or Cd(NO₃)₂, liberating hydrogen or NOₓ gases It also reacts with halogens (e.g. Cd + Cl₂ → CdCl₂) and sulfur (Cd + S → CdS).

In chemical behavior, cadmium resembles zinc but is less electropositive; it is positioned below zinc and above hydrogen in the reactivity/electrochemical series. Thus cadmium displaces hydrogen from dilute acids (producing Cd²⁺ and H₂ gas). It does not react with alkaline water. In strong alkali, cadmium hydroxide (Cd(OH)₂), which is otherwise insoluble, can dissolve to form complex ions like [Cd(OH)₄]²⁻. Cadmium has a relatively large atomic size and a full d^10 configuration, so it has a low ability to polarize anions (it is not a “hard” Lewis acid). It forms coordination compounds with amines and halides, but these are generally less stable than those of zinc. One notable chemistry trend: unlike most first-row metals, cadmium has very little tendency to form higher oxidation states or π bonds; it almost never exceeds +2. It does, however, exhibit a rare +1 state in certain compounds, where two Cd atoms share a bond (for example, Cd₂I₂ units in [Cd₂I₄]²⁻)

Because of its filled d shell, cadmium’s chemistry is “softer” (more covalent) than zinc’s. For example, cadmium sulfide and selenide (CdS, CdSe) are semiconductors, and cadmium forms thiolate and phosphine complexes in homogeneous catalysis. Historically, cadmium salts were used to stabilize polyvinyl chloride (PVC) plastics, but this use declined due to toxicity. In summary, cadmium is chemically less reactive than zinc (it is “nobel-like” enough to plate and resist corrosion) but more reactive than mercury. Like zinc, cadmium is a borderline metal that readily dissolves in acids, forming the highly toxic Cd²⁺ ion, and it forms mostly ionic compounds with a significant covalent character.

Occurrence and Production

Cadmium is relatively scarce in nature. Its average concentration in Earth’s crust is only on the order of 0.1–0.5 parts-per-million No large cadmium ore deposits are known; the element occurs almost exclusively as a byproduct of mining and refining other metals. The principal source of cadmium is zinc sulfide (sphalerite) ore: cadmium substitutes into sphalerite crystals up to ~1.4% by weight The only mineral consisting mainly of cadmium is greenockite (CdS), but this is almost always found alongside zinc ores Small amounts of cadmium can also be recovered from lead and copper ores that contain zinc. Overall, about 90% of cadmium is produced during the roasting, smelting and refining of zinc ores; only a small fraction comes from direct mining. Recycling of steel and zinc scrap provides the remainder (about 10% from secondary sources such as dust from scrap smelters

Industrial production of cadmium typically follows the zinc-metal production pathway. Zinc sulfide concentrate is roasted with oxygen to form ZnO (and SO₂), then reduced to metallic zinc by smelting or electrolytic methods. During smelting, cadmium, which is more volatile than zinc, distills off from the molten zinc when heated under vacuum or low pressure. For electrolytic zinc production, cadmium remains in the leachate solution and is recovered by precipitation (usually as cadmium hydroxide or sulfate) and subsequent isolation For example, one common method is to precipitate cadmium from the spent zinc sulfate electrolyte by adding a sulfide (CdS precipitates) or adjusting pH, then smelt the precipitate to metal. The metal is refined by vacuum distillation or electrorefining.

Principal cadmium-producing countries (via zinc refining) include China, South Korea, Japan, and various European and North American producers. Historically, France and Germany were early leaders (Germany produced nearly all world supply in the 19th century), but today China dominates output. Typical global production is on the order of 20 000–30 000 metric tons of cadmium per year, mostly as a byproduct of the larger zinc industry

Applications and Technology

Cadmium’s uses arise from its corrosion resistance, thermoelectric properties, and unique semiconductor chemistry, but all are constrained by its toxicity. Historically, cadmium metal was widely used for electroplating steel and other metals to prevent corrosion (a technique called cadmium plating). Such coatings form a passive oxide that protects the underlying metal; cadmium-plated hardware (fasteners, bearings, aircraft parts) was common in military and aerospace applications Its use for consumer hardware has declined due to health regulations.

A major industrial application for decades was nickel-cadmium (NiCd) rechargeable batteries, in which cadmium provides a stable negative electrode. These batteries have high cycle life and performance in extreme temperatures, so they were used in power tools, aviation, and emergency lighting. However, due to environmental concerns, NiCd batteries have largely been replaced by nickel-metal-hydride (NiMH) and lithium-ion batteries in most consumer devices Cadmium is also used in alloys (e.g. low-melting solders), and in some specialized electronics (e.g. Peltier thermoelectric modules, since cadmium has useful Seebeck coefficients).

Cadmium compounds have notable niche roles. Cadmium sulfide and selenide are used as pigments (“cadmium yellow”, “cadmium red”) and for their semiconducting photoelectric properties Cadmium telluride (CdTe) is the basis of a commercially important thin-film solar cell technology, as CdTe has a near-ideal 1.45 eV bandgap for photovoltaics Large CdTe solar modules are produced by companies like First Solar. Cadmium sulfide is also used in photoresistors and optical sensors (it conducts in light).

In nuclear technology, cadmium rods and control assemblies are used to absorb neutrons and control fission in some reactors, leveraging cadmium’s high neutron capture (especially from ^113Cd) Cadmium oxide and sulfate have been used as catalysts and for electrochemical plating baths (e.g. for saturating electrodeposition solutions). Certain cadmium salts (e.g. CdS quantum dots, CdTe quantum wells) are being explored in optoelectronics and display technologies (quantum-dot LED displays), though their use is limited by health regulations. Overall, cadmium’s technological roles have been greatly curtailed by legislation (e.g. its inclusion in the EU Restriction of Hazardous Substances (RoHS) list). Still, it remains important in applications where its specific properties cannot easily be replaced.

Biology, Environmental Fate, and Safety

Cadmium and its compounds are highly toxic to living organisms, accumulating in soils, water, and biological tissues There is no known essential or beneficial role for cadmium in humans or higher animals In contrast, cadmium is a potent poison: chronic exposure damages kidneys and bones, and it is a human carcinogen (lung and prostate cancer) by inhalation. The most common routes of human exposure are ingestion of cadmium-contaminated food (plant uptake from soils) and inhalation of industrial dusts or fumes. Once in the body, cadmium accumulates with a biological half-life of 10–30 years (especially in the kidneys), leading to long-term harm. Well-known examples include itai-itai disease in Japan (1970s), where rice crops irrigated with cadmium-polluted water caused severe bone and kidney damage in local people

Cadmium is regulated as a priority pollutant. For instance, the U.S. EPA limit for cadmium in drinking water is 5 µg/L. Occupational exposure limits are very low (OSHA’s Permissible Exposure Limit is 5 µg/m³ for an 8-hour TWA; ACGIH TLV is even lower). Because of its toxicity, uses of cadmium are restricted by law (for example, cadmium-containing paint pigments or plating are banned in many consumer products). In the environment, cadmium is very mobile in acidic conditions; it can bioaccumulate in the food chain (especially in oysters, mussels, and rice). Detoxifying cadmium pollution typically involves raising soil pH and adding zinc or phosphate to precipitate cadmium, but its persistence means it remains a health concern where it has been used or released.

For laboratory and industry safety, cadmium metal and compounds must be handled with strict controls. Inhalation of cadmium fumes or dust is particularly hazardous (causes “metal fume fever” acutely and lung injury chronically). Soluble cadmium salts are especially toxic, so they are avoided when possible. Modern workplaces use closed systems and respirators for any cadmium, and biological monitoring (urine cadmium) is standard. At high levels, cadmium exposure causes kidney damage (proteinuria), bone demineralization, and severe lung damage. Because of its toxicity, cadmium use is gradually being phased out: for example, nickel-cadmium batteries are replaced by alternatives, and cadmium pigments and stabilizers are mostly banned.

History and Etymology

Cadmium was discovered in 1817 by two German chemists simultaneously: Friedrich Stromeyer in Göttingen and Karl Samuel Leberecht Hermann in Göttingen and later Hanover. Both were analyzing zinc compounds (calamine ores and industrial zinc oxide) and found an unknown impurity. Stromeyer recognized yellow deposits that could not be pure zinc sulfide; he isolated the new metal by reducing its sulfide and named it cadmium after the Latin word cadmia – the historical name for zinc carbonate ore (“calamine”) The Latin cadmia derives from Greek κάδμεία (kadmeía, “calamine”), itself from the mythological Prince Cadmus (Cadmus, Κάδμος) who was said to have discovered the ore.

The pure metal and its compounds were studied through the 19th century. Cadmium pigments (especially cadmium yellow, CdS) were introduced around the 1840s once enough cadmium could be produced For about a century in the early 1900s, cadmium had a niche use: the international standard unit of length (“the angström”) was defined by the red cadmium spectral line (6438.46963 Å) In the 1930s–40s, large-scale production began (mainly in Germany) for plating steel. Use soared after World War II: at mid-century, plating and pigments each consumed the majority of cadmium production In the 1960s–80s, the rise of nickel-cadmium batteries made NiCd the dominant application (peaking at ~80% of U.S. demand by 2006) However, from the 1970s onward environmental and health awareness led to restrictions: by 2006 plating and pigments each accounted for under 10% of consumption and NiCd battery use is now declining with replacement by NiMH and Li-ion technologies.

The element’s name, discovered origin, and key uses reflect its history: its Latin/Greek root cadmia highlights the calamine (zinc ore) from which it was first isolated Its discovery in 1817 shortly followed the discoveries of other group-12 metals (zinc in antiquity, mercury much earlier). Cadmium’s unique properties only became technologically significant in the 20th century, and by the 21st its role is largely defined by balancing utility (in batteries, electronics, CdTe solar cells) against stringent safety regulations.

Data Table

Property	Cadmium (Cd)	Notes / Value (& Reference)
Atomic Number	48
Standard Atomic Weight	112.414	(weighted mean; 6 stable lso 2 radioactive)
Electron Configuration	[Kr] 4d¹⁰ 5s²	Valence: 2 electrons in 5s shell
Oxidation States	+2 (common); +1 (rare)	Cd²⁺ compounds are dominant. +1 found in Cd₂²⁺ species
Electronegativity (Pauling)	1.69	Moderate; group 12 trend (Zn~1.65, Hg~2.0)
First Ionization Energy	8.9937 eV (≈868 kJ/mol)	Lower than Zn but higher than Hg
Atomic Radius (empirical)	~155 pm	Size for element–element bond
Covalent Radius (single bond)	144 pm	Cadmium covalent radius
Density (20 °C)	8.65 g/cm³	High density metal
Melting Point	594.2 K (321.1 °C)	Critics: relatively low for a metal
Boiling Point	1040 K (767 °C)	Sublimes; see also vacuum distillation behavior
Crystal Structure	hcp (hexagonal close-packed)	Cadmium metal atomic packing
Thermal Conductivity	97 W·m⁻¹·K⁻¹	Comparable to copper (≈400), higher than many metals
Electrical Conductivity	≈2.5×10^7 S/m (solid)	Good conductor (similar order as Cu, but somewhat less)
Gas Density (g/L, 273 K)	0.005 (Cd vapor at BP)	(Not usually tabulated; Cd vapor pressure low under ambient)
Electron Affinity	– (no stable Cd⁻)	No bound anion (undefined)
Natural Abundance	0.1–0.5 ppm (crust)	Rare in crust
Stable Isotopes	¹⁰⁶Cd, ¹⁰⁸Cd, ¹¹⁰Cd, ¹¹¹Cd, ¹¹²Cd, ¹¹⁴Cd	(five observationally stable)
Notable Radioisotopes	¹¹³Cd (t₁/₂≈7.7×10¹⁵ y); ¹¹⁶Cd (t₁/₂≈2.9×10¹⁹ y)	Very long half-lives
Discovery	1817	By Friedrich Stromeyer and K. Hermann (as Zn impurity)
Name Origin	Latin cadmia (calamine), from Greek Kadmeía (Ζinc ore)	Named after “Cadmus” myth

References: Authoritative sources including the Encyclopædia Britannica and periodic table databases have been used to compile these values Units are SI (kelvin, kilograms, joule, etc., as shown), with temperatures also given in °C. The atomic weight is the standard atomic weight (relative atomic mass) of cadmium.