CADMIUM

Name: Cadmium

Symbol: Cd

Group: 12

Period: 5

Block: d

Category: Transition metals

Atomic number: 48

Atomic mass: 112.411 u

Electrons per shell: 2, 8, 18, 18, 2

Electronegativity: 1.69

Density: 8.65 g/cc

Melting point: 321°C

Boiling point: 767°C

Thermal conductivity: 96 W (m·K)

Electrical conductivity: 1.4 × 10^7 S/m

Magnetic order: Diamagnetic

Ordinary state: Solid

Oxidation states: -2, 1, 2

Mohs hardness: 2

Vickers hardness: None Data

Brinell Hardness: 203

Most stable isotopes: Cd-106 (1.25%), Cd-108 (0.89%), Cd-110 (12.47%), Cd-111 (12.80%), Cd-112 (24.11%), Cd-113 (12.23%), Cd-114 (28.75%), and Cd-116 (7.51%)

Discoverer: Karl S. L. Hermann and Friedrich Stromeyer, Germans (same year, independently; 1817)

Cadmium (Cd), chemical element with atomic number 48, is a soft, silvery transition metal, known for its toxicity and applications in alloys, batteries, and pigments. Its discovery in 1817 marks a curious milestone in the history of chemistry, as it was identified almost simultaneously by two German scientists, Karl Hermann and Friedrich Stromeyer, working independently. With an abundance of ~0.15 ppm in the Earth's crust, cadmium is relatively rare and is primarily found as an impurity in zinc (Zn) ores, reflecting its chemical proximity in Group 12 of the periodic table. The history of cadmium combines 19th-century scientific advances with the subsequent recognition of its environmental risks and industrial importance.

The discovery of cadmium occurred in 1817 when Karl Hermann, a German pharmacist, and Friedrich Stromeyer, a chemist and professor at the University of Göttingen, separately analyzed samples of zinc carbonate (calamine, ZnCO₃) contaminated with an unknown material. Hermann observed a yellow residue when heating calamine, while Stromeyer isolated the metal by reduction and noted its distinctive coloration when forming compounds. It was Stromeyer who named the element “cadmium,” derived from the Latin cadmia, in reference to calamine, and the Greek myth of Cadmus, the founder of Thebes. This name reflects cadmium's historical association with zinc minerals, as both metals often coexist in deposits like sphalerite (ZnS), where cadmium appears in traces (~0.1–0.5%).

In the 19th century, cadmium was little more than a scientific curiosity, but its use grew in the 20th century with the development of industrial applications. During the 1920s and 1930s, it became popular in pigments (such as cadmium yellow, CdS) for its brightness and stability, and in corrosion-resistant coatings for steel due to its corrosion resistance in saline environments. In the 1960s, nickel-cadmium (NiCd) batteries became a mainstay for portable devices, leveraging cadmium's high capacity as an electrode. However, cadmium's toxicity, fully recognized in the 1970s and 1980s, led to restrictions on its use, especially in Europe, due to its environmental impact and health risks (e.g., itai-itai disease in Japan, caused by cadmium contamination). Today, global production (~23,000 tons annually in 2025) is concentrated in Asia (China, South Korea), with applications limited to specialized sectors such as electronics and solar energy. The history of cadmium, from its parallel discovery to its role as a controversial material, illustrates the balance between technological innovation and environmental responsibility.

Cadmium (Cd), chemical element with atomic number 48, is a transition metal of Group 12 of the periodic table, known for its softness, toxicity, and applications in alloys, pigments, and batteries. With a density of 8.65 g/cm³ and an abundance of ~0.15 ppm in the Earth's crust, cadmium is a relatively rare metal found mainly as an impurity in zinc (Zn) ores, such as sphalerite (ZnS). Its grayish-blue appearance, duller than that of zinc, and its soft, almost silky texture, resemble copper (Cu) or precious metals like silver (Ag), although its toxicity classifies it as a "heavy metal" in terms of environmental and health impact, similar to lead (Pb) or mercury (Hg).

Physically, cadmium is extremely soft (Mohs hardness of 2), allowing it to be cut with a household knife with sufficient force. It is malleable and ductile, which facilitates its shaping into sheets or wires, although less so than precious metals like gold (Au). Its low melting point (321.07 °C) and boiling point (767 °C) make it ideal for forming low-melting-point alloys, such as Wood's metal (Cd, Pb, Sn, Bi), used in solders and low-temperature applications. Cadmium's electrical conductivity (1.4 × 10⁷ S/m) is good, though inferior to copper, and its thermal conductivity (~96 W/(m·K)) makes it suitable for coatings. Cadmium has eight stable isotopes (Cd-106, Cd-108, Cd-110, Cd-111, Cd-112, Cd-113, Cd-114, Cd-116), two of them metastable, a high number that reflects its proximity to tin (Sn), the element with the most stable isotopes (11, due to its “magic number” of protons, Z=50).

Chemically, cadmium is more reactive than noble metals like silver, behaving in many respects like a p-block metal rather than a typical transition metal. It reacts with acids (like HCl, forming CdCl₂) and bases, although it forms a passivating layer in some environments that retards corrosion. Its compatibility with p-block metals, such as bismuth (Bi) or tin, and with silver, allows it to form stable alloys, while its incompatibility with many transition metals (like iron, Fe) limits its mixing at high temperatures. Cadmium's toxicity, which can cause kidney and bone damage (as in itai-itai disease), restricts its use in modern applications, especially in nickel-cadmium (NiCd) batteries and pigments (CdS), where it has been replaced by safer alternatives. Its combination of softness, reactivity, and ability to form alloys makes it valuable in specialized applications, but its handling requires caution due to its environmental and health impact.

Cadmium (Cd), chemical element with atomic number 48, is a Group 12 transition metal known for its toxicity and softness (Mohs hardness ~2), with a density of 8.65 g/cm³ and an abundance of ~0.15 ppm in the Earth's crust. Although its toxicity limits its use, cadmium exhibits moderate corrosion resistance, comparable to that of zinc (Zn), with which it shares chemical properties due to its proximity in the periodic table. This resistance, though not comparable to that of noble metals like gold (Au) or platinum (Pt), allows for specific applications, such as protective coatings, despite the associated environmental and health risks.

Cadmium is stable in dry and humid air at room temperature, forming a thin passivating layer of cadmium oxide (CdO) that protects the underlying metal from further oxidation by oxygen (O₂). This layer, although less toxic than metallic cadmium, remains hazardous and can be released into the environment if mishandled. In fresh and salt water, cadmium resists corrosion for moderate periods, but in salt water (with sodium chloride, NaCl), the formation of cadmium chloride (CdCl₂) accelerates degradation, especially in the presence of electrolytes. The passivating layer is easily removed by abrasion or chemical dissolution, allowing progressive corrosion that penetrates the part, compromising its structural integrity over time.

Cadmium is vulnerable to acids, both reducing (like hydrochloric acid, HCl) and oxidizing (like nitric acid, HNO₃), which rapidly dissolve the metal, forming compounds such as CdCl₂ or cadmium nitrate (Cd(NO₃)₂). Strong bases, such as sodium hydroxide (NaOH), also attack cadmium, generating hydroxocomplexes. Despite its reactivity, cadmium's moderate resistance to non-acidic environments makes it useful in protective coatings for steel, similar to those of zinc. In galvanizing processes, cadmium is applied by electrodeposition, offering sacrificial protection in corrosive environments, such as marine ones, although its use has decreased due to environmental regulations concerning its toxicity (e.g., restrictions in the European Union under the RoHS directive). The combination of limited but functional resistance, along with its ability to form protective layers, makes cadmium a viable material in specific industrial applications, provided its risks are managed with caution.

Cadmium is one of those metals, like Yttrium for example, that are more useful as compounds than in pure form or as an alloying agent, since although it is part of some alloys, its use has been systematically attempted to be eradicated, and for good reason, due to its toxic nature.

The primary use of Cadmium (almost 9 out of 10 times) is in the manufacture of rechargeable Nickel-Cadmium batteries where both metals are bonded by an electrolytic reaction in a potassium hydroxide solution ("Caustic Soda"). Health authorities (and a humble server) recommend not throwing batteries (of any type) into general waste bins but rather to specialized collection sites where they will be given separate treatment.

The second most important use of Cadmium (which, as you may have guessed, has been gradually decreasing) is in the manufacture of paints. These paints had a much higher quality than alternatives, but aesthetics are sacrificed for the health of the population.

The third use of Cadmium, already mentioned in this book in the section on thermoplastics, is as a stabilizer in them: indeed, it was used as a dopant in common and less common plastics (depending on use requirements) because it improved mechanical properties, made them more resistant (or directly, immune) to degradation by solar radiation (UV), and also increased heat tolerance. Cadmium and Lead, also formerly used, have been replaced and continue to be replaced by combinations of Calcium-Zinc, among others.

Finally, Cadmium has traditionally been a component of "pot metal"/"white metal" type alloys, p-block metal alloys based on Tin, Lead, or a mixture of both in applications such as large bearings, pressure-molded parts, et cetera. Cadmium is a great metal, with the misfortune of being poisonous, which restricts its range of possible uses.

Cadmium (Cd), chemical element with atomic number 48, is a heavy transition metal known for its high toxicity, posing significant risks to human health and the environment. With a density of 8.65 g/cm³ and an abundance of ~0.15 ppm in the Earth's crust, it is primarily found as an impurity in zinc (Zn) ores, such as sphalerite (ZnS). Although it does not cause immediate death except in extreme cases of massive ingestion, as could happen with a small child, chronic exposure to cadmium has devastating effects, particularly on the kidneys, bones, and nervous system, due to its ability to accumulate in the body with a biological half-life of 10 to 30 years. Its use in industrial applications, such as nickel-cadmium (NiCd) batteries, pigments (CdS), and coatings, demands strict precautionary measures to minimize risks.

The toxicity of cadmium lies in its ability to chemically mimic essential elements, such as calcium (Ca) or zinc, thanks to its atomic radius (~151 pm) and oxidation states (+2). This phenomenon, known as ionic mimicry, allows cadmium to interfere with metabolic and enzymatic processes, causing kidney damage, osteoporosis, and, in historical cases like itai-itai disease in Japan, severe bone pain. Classified as a human carcinogen by the International Agency for Research on Cancer (IARC), cadmium is linked to lung and prostate cancers. Exposure can occur through inhalation of vapors or dust in industrial environments, ingestion through contaminated food or water, especially in crops like rice, or prolonged dermal contact.

To handle cadmium safely, rigorous measures are crucial. In industrial environments, such as galvanizing plants or battery recycling facilities, the use of personal protective equipment is required, including gloves, masks, and respirators, to prevent the inhalation of particles or contact with cadmium compounds, such as cadmium chloride (CdCl₂). Ventilation and filtration systems are essential to reduce the release of cadmium dust or vapors into the environment. Waste management, such as NiCd batteries or electrodeposition process waste, must be carried out in specialized facilities to prevent soil and water contamination, as soluble cadmium can leach into ecosystems. Regulations, such as the European Union's RoHS directive, restrict its use in electronic products to less than 0.01% by weight, promoting safer alternatives. Furthermore, monitoring crops in contaminated soils is fundamental to prevent the accumulation of cadmium in the food chain.

Due to these risks, the use of cadmium has decreased in favor of less toxic options, such as lithium-ion (Li-ion) batteries or non-cadmium-based pigments. However, it remains relevant in specialized applications, such as cadmium telluride (CdTe) solar cells, where its controlled handling is critical. The combination of its toxicity and industrial utility requires a careful balance, ensuring that adequate precautions protect human health and the environment while leveraging its properties in specific contexts.