Cadmium (Cd) is a chemical element with an atomic number of 48 in the periodic table of elements. Found in Earth’s crust at about 0.5 ppm (parts per million), it belongs to the group of rare metals. As a member of both the transition and toxic metals family of periodic table elements, this naturally occurring soft metal has two valence electrons that make cadmium relatively unreactive with the other chemical elements.
Chemical and Physical Properties of Cadmium
|Atomic Mass||112.411 g.mol-1|
|Color||Silvery-grey with a blue metallic hue|
|Physical state||A soft metal at room temperature|
|Density||8.65 g.cm-3 at 20°C|
|Melting point||321 °C|
|Boiling point||767 °C|
|Van der Waals radius||158 pm|
|Ionic radius||.95 (+2) Å|
|Atomic weight||112.411 g.mol-1|
|Most characteristic isotope||48Cd|
|Electronic shell||[Kr] 4d10 5s2|
|The energy of the first ionization||866 kJ.mol-1|
|The energy of the second ionization||1622 kJ.mol-1|
|Uses||Used in nickel-cadmium batteries, electroplating steel, manufacture of bearings, paint pigments.|
|Description||Rare, very soft, silver-white metal|
|Orbitals||[Kr] 4d10 5s2|
|Covalent Radius||1.48 Å|
|Atomic Radius||1.71 Å|
|Atomic Volume||13.1 cm³/mol|
|Name Origin||Greek: kadmeia (ancient name for calamine (zinc oxide))|
With the periodic table symbol Cd, atomic number 49, atomic mass of 112.41 g.mol -1, and electronic configuration [Kr] 4d10 5s2, cadmium is soft, ductile, malleable, burns to redness when exposed to heat, and has a deep yellow vapor.
It reaches its boiling point at 767 °C, while the melting point is achieved at 321 °C, which is lower than the transition metals arranged in the groups from 3 through 11 in the periodic table.
This member of the transition metals family of elements in the periodic table has an electronegativity of 1.69 according to Pauling, whereas the atomic radius according to van der Waals is 158 pm.
With oxidation states of +1 and +2, cadmium undergoes tarnishing as a result of its exposure to air (especially to oxygen – O2). It’s dissolved in acids, but not in alkalis.
When cadmium is dissolved by hydrochloric acid, nitric acid, or sulfuric acid, the resulting compounds are cadmium chloride (CdCl2), cadmium nitrate (Cd(NO3)2), and cadmium sulfate (CdSO4), respectively. In addition, this transition metal has strong anti-corrosive properties.
How Was Cadmium Discovered?
In the 17th century, zinc oxide was often used for the treatment of various skin conditions. In 1817, the German physician and a professor at the University of Göttingen, Friedrich Stromeyer (2 August 1776 – 18 August 1835), was asked by the government to inspect the Hildesheim pharmacies suspected of selling zinc carbonate instead of zinc oxide.
So, Stromeyer attempted an experiment by heating zinc carbonate taken from the pharmacies in an effort to prove the impurity of the substance sold by the pharmacies in this German town. He was expecting the white carbonate to turn into a white oxide, but instead, it ended up being yellow.
Since he could not trace any other elements that would give this color to the resulting oxide (such as lead or iron), this German chemist continued researching which led him to the place where the local pharmacies were obtaining the chemical substance, but he didn’t get any information that would clarify his scientific dilemma.
This led him to another experimental attempt when he reduced the elemental form of cadmium as the new, yellow-tinting substance of the analyzed zinc oxide rising from the yellow vapor from the soot and zinc oxide mixture. This turned out to be a completely new chemical element, and that’s how he discovered the elemental form of cadmium.
The same year, two other scientists, the German chemist Karl Samuel Leberecht (1765 – 1846) Hermann and the German physicist and apothecary Johann Christoph Heinrich Roloff (1783 – 1825), also succeeded in isolating cadmium in its elemental form, but Friedrich Stromeyer was accepted in the scientific world as the first discoverer of this member of the transition metals family of chemical elements.
How Did Cadmium Get Its Name?
“Kadmeia” is the Greek word used as a reference to zinc oxide. It was used to label the chemical element with an atomic number of 48 (Cd) due to the fact that cadmium is mainly obtained by heating zinc ores (Zn).
Where Can You Find Cadmium?
This chemical element is rarely found in Earth’s crust but is nonetheless widely distributed in nature. Cadmium occurs as a by-product of zinc ores, among which zinc blende (or sphalerite) is the main cadmium smelting ore. Greenockite (CdS) is the only crystal form of cadmium consisting of cadmium sulfide.
It also occurs in some lead ores and in a small number of minerals, combined with oxygen, chlorine, or sulfur. China, Kazakhstan, Japan, Canada, and South Korea comprise the group of the world’s leading countries in cadmium production. In the United States, cadmium is mostly obtained as a by-product of zinc, lead, or copper. Mineral deposits of this chemical element are found in Washington, Utah, Missouri, Colorado, and Illinois.
Some foods, such as oil-seeds, shellfish, fish, and leafy vegetables, also contain low-levels of cadmium.
Cadmium in Everyday Life
Despite being a rare metal, cadmium is widely used in everyday life for producing rechargeable batteries (more specifically, the rechargeable Ni-Cd batteries), solar cells, UV lamps, TV tubes (both black and white, and in color), bearings, in cadmium stabilizers, cadmium telluride solar panels, etc.
Furthermore, due to its strong anti-corrosive properties, cadmium is used as an electroplating agent in the making of airplanes and all other vehicles and machines that can be damaged by corrosion. By electroplating cadmium onto alloys containing iron, steel, brass, or copper, this substance serves as protection from corrosion.
The control rods made of cadmium often find their use in the work of nuclear reactors due to their ability to absorb neutrons. Cadmium salts are applied in the production of pigments, where cadmium selenide (also known as cadmium red) produces a red pigment, while cadmium sulfide produces a yellow pigment.
How Dangerous Is Cadmium?
This chemical element belongs to the category of toxic metals. Due to its high toxicity and adverse health effects, cadmium is less and less applied in everyday life. The general population is exposed to cadmium mainly through cigarette smoke, food containing cadmium, soldering, welding, etc.
This toxic metal has been labeled by the European Restriction of Hazardous Substances Directive, the United States Department of Health and Human Services, Human Services National Institute for Occupational Safety and Health, and the Agency For Toxic Substances and Disease registry as a human carcinogen.
Namely, cadmium exposure can have adverse health effects on multiple systems of the body, such as the gastrointestinal system, the reproductive system, the immune system, the nervous system, the respiratory system, the urinary system, and the kidneys.
Basically, the long-term exposure to high levels of this substance classified as a human carcinogen may result in kidney diseases leading to permanent kidney damage, lung cancer, etc.
The inhalation of cadmium vapor or dust is especially toxic. Cadmium poisoning can occur after consuming contaminated food, drinking water, or inhaling air contaminated with high levels of this heavy metal.
By accumulating in the kidneys, this toxic substance impairs our biological mechanism that filters the waste products of the body and regulates both the production of red blood cells and hormones.
What Are the Risk Factors of Cadmium Poisoning?
Exposure to even low levels of cadmium concentration can lead to adverse effects on human health, which can develop into cancer and have a fatal outcome. The risk factors that trigger these adverse health effects include:
- Smoking (even non-smokers that are exposed to cigarette smoke risk cadmium poisoning or even lung cancer after prolonged exposure);
- Working or living near a production factory of cadmium batteries or pigments.
What Are the Symptoms of Cadmium Poisoning?
The symptoms of cadmium poisoning include:
- Gastrointestinal problems (such as vomiting and nausea);
- Loss of consciousness;
- Kidney problems accompanied by pain;
- Fragile and brittle bones;
- Fatigue and weakness of the body;
- Muscle pains;
- Lung problems;
- Difficulty breathing;
- Pain in the bones;
- Lung cancer.
Acute Tubular Necrosis (Renal Tubular Damage)
Acute tubular necrosis (ATN) or renal tubular damage is caused by a hazardous substance, such as cadmium. When cadmium is absorbed in the bloodstream, it cuts off the delivery of oxygenated blood and nutrition to the kidneys.
The particles of this toxic substance also damage the kidney’s tissue, ultimately resulting in acute kidney failure by triggering necrosis of the kidney’s structures – the renal tubules. Without sufficient supply of oxygen, the neuron cells of the renal tubules break down and the renal tubules suffer irreparable damage and loss of their function.
What Are the Symptoms of Acute Tubular Necrosis?
The following are the most common symptoms that signal acute tubular necrosis, or renal tubular damage:
- Swelling of some parts of the body;
- Inability to urinate;
- A taste of metal in the mouth;
- Pain in the stomach;
- Seizures and confusion.
The Health Risks of Cadmium in Cigarette Smoke
This toxic heavy metal is also found in tobacco, which reaffirms the adverse health effects of both cadmium and smoking. Cigarette smoke may contain approximately one microgram of cadmium, but it has nonetheless extremely adverse effects upon the health.
Since cigarette smoke enters the bodily systems via the lungs, half of the inhaled cadmium particles also enter the bloodstream that further damage other tissues and organs in the body.
Environmental Effects of Cadmium
Naturally present in low levels, cadmium levels in our environment are increased by human activities. The environmental pollution caused by cadmium is usually a result of the combustion of fossil fuels creating cadmium emissions in the air, forest fires, volcano fumes, cadmium producing factories, disposal of nickel-cadmium batteries, and disposal of electronic and electrical waste, thus affecting both the health of the general population living in the contaminated area, as well as the environmental health. Improper waste disposal and tobacco smoke containing cadmium also contribute to air pollution.
The toxic cadmium levels that exceed the cadmium concentration in waters, air, soil, or food, are regulated by the federal agencies dealing with environmental health and environmental protection, such as EPA, the Occupational Safety and Health Administration (OSHA), and the Food and Drug Administration (FDA). For more information, please refer to the ATSDR.
The Cadmium Cycle in the Environment
According to a study by the World Health Organisation (WHO) on the cadmium cycle in the environment, cadmium has no degradable property. Hence, it continues to circulate and contaminate the soil, air, and waters after it’s been released into the environment.
Volcanoes, sea spray, and forest fires are the largest sources of cadmium-based air contamination. The weathering of rocks also contributes to the quantities of this heavy metal released into the environment. Due to its water-soluble property, cadmium is mainly accumulated in the soil from which the plants absorb it easily.
This is the reason why the largest exposure to the toxicity of this chemical element in humans comes from food, i.e the agricultural products.
Way to Reduce Cadmium Levels in the Environment
The occupational and environmental exposure to toxic levels of cadmium can be reduced by:
- Recycling of electronic and electrical waste (especially the nickel-cadmium batteries);
- Minimization of cadmium emissions during the mining of this transition metal;
- Avoiding the application of cadmium in the products we use every day, such as plastic items, toys, batteries, jewelry;
- Avoiding smoking;
- Campaigning against smoking in order to raise awareness of the health and environmental hazards of cadmium and to protect the health of individuals, public health, as well as environmental health.
Isotopes of Cadmium
Out of the eight naturally occurring cadmium isotopes, two isotopes of this chemical element are radioisotopes (113Cd with a half-life of 7.7×1015 years, and 116Cd with a half-life of 2.9×1019). 110Cd, 111Cd, and 112Cd are considered to be stable cadmium isotopes, while the 109Cd has the longest half-life of 462.6 days.
|Z||N||Isotopic mass (Da)
[n 2][n 3]
[n 6][n 7]
[n 8][n 9]
|Natural abundance (mole fraction)|
|Excitation energy[n 9]||Normal proportion||Range of variation|
|97Cd||48||49||96.93494(43)#||2.8(6) s||β+ (>99.9%)||97Ag||9/2+#|
|β+, p (<.1%)||96Pd|
|98Cd||48||50||97.92740(8)||9.2(3) s||β+ (99.975%)||98Ag||0+|
|β+, p (.025%)||97Ag|
|98mCd||2427.5(6) keV||190(20) ns||8+#|
|99Cd||48||51||98.92501(22)#||16(3) s||β+ (99.78%)||99Ag||(5/2+)|
|β+, p (.21%)||98Pd|
|β+, α (10−4%)||94Rh|
|106Cd||48||58||105.906459(6)||Observationally Stable[n 10]||0+||0.0125(6)|
|108Cd||48||60||107.904184(6)||Observationally Stable[n 11]||0+||0.0089(3)|
|109m1Cd||59.6(4) keV||12(2) µs||1/2+|
|109m2Cd||463.0(5) keV||10.9(5) µs||11/2|
|111mCd||396.214(21) keV||48.50(9) min||IT||111Cd||11/2−|
|113Cd[n 12][n 13]||48||65||112.9044017(29)||8.04(5)×1015 y||β−||113In||1/2+||0.1222(12)|
|113mCd[n 12]||263.54(3) keV||14.1(5) y||β− (99.86%)||113In||11/2−|
|114Cd[n 12]||48||66||113.9033585(29)||Observationally Stable[n 14]||0+||0.2873(42)|
|115Cd[n 12]||48||67||114.9054310(29)||53.46(5) h||β−||115mIn||1/2+|
|115mCd||181.0(5) keV||44.56(24) d||β−||115mIn||(11/2)−|
|116Cd[n 12][n 13]||48||68||115.904756(3)||2.8(2)×1019 y||β−β−||116Sn||0+||0.0749(18)|
|117mCd||136.4(2) keV||3.36(5) h||β−||117mIn||(11/2)−|
|119mCd||146.54(11) keV||2.20(2) min||β−||119mIn||(11/2−)#|
|121mCd||214.86(15) keV||8.3(8) s||β−||121mIn||(11/2−)|
|123mCd||316.52(23) keV||1.82(3) s||β−||123In||(11/2−)|
|125mCd||50(70) keV||570(90) ms||β−||125In||11/2−#|
|129Cd||48||81||128.93215(32)#||242(8) ms||β− (>99.9%)||129In||3/2+#|
|129mCd||0(200)# keV||104(6) ms||11/2−#|
|130Cd||48||82||129.9339(3)||162(7) ms||β− (96%)||130In||0+|
|β−, n (4%)||129In|
List of Cadmium Compounds
Despite being relatively chemically unreactive, cadmium forms a number of nitrides, sulfides, selenides, tellurides, hydrides, chlorides, ionides, and oxides.
- Cadmium acetate
- Cadmium arsenide
- Cadmium bromide
- Cadmium chloride
- Cadmium chromate
- Cadmium cyanide
- Cadmium fluoride
- Cadmium hydride
- Cadmium hydroxide
- Cadmium iodide
- Cadmium nitrate
- Cadmium oxide
- Cadmium pigments
- Cadmium selenide
- Cadmium stearate
- Cadmium sulfate
- Cadmium sulfide
- Cadmium telluride
- Cadmium tetrafluoroborate
- Cadmium tungstate
- Cadmium zinc telluride
- Cadmium(I) tetrachloroaluminate
- Caesium cadmium bromide
- Caesium cadmium chloride
- Mercury cadmium telluride
- Organocadmium compound
- Cadmium phosphide
- Zinc cadmium phosphide arsenide
- Zinc cadmium sulfide
5 Interesting Facts and Explanations
- Tarnishing is the loss of the metallic shine of a metallic compound after being exposed to air.
- The mass cadmium poisoning that occurred in Japan in 1912 was labeled as the “Itai-itai disease”, meaning ‘severe pains’ in Japanese. As a result of this man-made disease triggered by environmental pollution, the general population of the Toyama Prefecture suffered severe pains in the joints and spine due to the release of this heavy metal into the environment by the mining companies.
- Heavy metal contamination of the environment from the landfills of cadmium-nickel batteries and products containing cadmium occurred in the Xinxiang municipality of China. The environmental protection agencies refer to this large-scale pollution as one of the furthest spread pollutions, since it also polluted the crops which, in turn, contributed to a widespread cadmium hazard due to the food chain being affected.
- Breathing in cigarette smoke can be as toxic as exposure to cadmium in any other form.
- The non-smokers that are exposed to cigarette smoke are labeled as ‘passive (secondhand) smokers’ because they breathe in the same toxic tobacco smoke as the smokers.