Introduction to Cadmium
Cadmium (Cd) is a metallic chemical element with atomic number 48. It is a soft, silvery-white metal that can be easily cut with a knife. Cadmium is found in zinc ores and is often a byproduct of zinc mining, with significant historical production occurring in regions like Canada, China, and Australia. It belongs to Group 12 of the periodic table, alongside zinc and mercury.
Chemical Reactivity of Cadmium
Cadmium exhibits moderate chemical reactivity compared to highly reactive metals like sodium or potassium, but it is more reactive than noble metals such as gold or platinum. Its reactivity stems from its ability to lose two electrons from its outermost shell, forming a +2 oxidation state in most compounds.
Reactivity with Water
Cadmium does not react with water or steam at room temperature. Its metallic surface is generally stable when exposed to water, which distinguishes it from more reactive metals that rapidly corrode or produce hydrogen gas when in contact with water.
Reactivity with Air
When exposed to dry air, cadmium is relatively stable. However, in moist air, its surface slowly tarnishes, forming a thin layer of cadmium oxide (CdO) or cadmium carbonate (CdCO$_3$). This protective layer, similar to that formed on aluminum, can prevent further oxidation. When heated in air, cadmium readily burns to produce brown fumes of cadmium oxide.
Reactivity with Acids
Cadmium reacts with many acids, including hydrochloric acid (HCl) and sulfuric acid (H$_2$SO$_4$), to produce hydrogen gas and cadmium salts. For example, the reaction with hydrochloric acid is represented as:
Cd(s) + 2HCl(aq) → CdCl$_2$(aq) + H$_2$(g)
This demonstrates its position above hydrogen in the electrochemical series.
Hazards of Cadmium
Cadmium is recognized for several hazardous properties, which necessitate careful handling and disposal.
Toxicity
Cadmium and its compounds are highly toxic to humans and other living organisms. Exposure can occur through inhalation of dust and fumes, ingestion of contaminated food or water, or skin contact. Chronic exposure can lead to severe health problems, including kidney damage, bone softening (osteomalacia), and lung damage. Historically, industrial exposure was a concern in manufacturing facilities in various countries, leading to strict regulations regarding its use and environmental release. For example, the “Itai-itai disease” in Japan was linked to cadmium poisoning from mining activities.
Radioactivity
Naturally occurring cadmium is not radioactive. Its most abundant isotopes, such as Cadmium-110, Cadmium-111, and Cadmium-112, are stable. While some artificial radioactive isotopes of cadmium exist, they are not naturally found and are typically generated in research settings.
Flammability
In its bulk metallic form, cadmium is not considered flammable under normal conditions. However, fine powders of cadmium can ignite and burn in air, producing toxic cadmium oxide fumes. Therefore, handling cadmium in powdered form requires precautions to prevent dust explosions or fires.
Noteworthy Chemical Reaction: Nickel-Cadmium Batteries
A prominent example of cadmium’s chemical reactivity in a practical application is its use in nickel-cadmium (NiCd) rechargeable batteries. These batteries utilize reversible redox reactions involving cadmium electrodes.
During discharge, cadmium metal (Cd) acts as the anode, being oxidized to cadmium hydroxide (Cd(OH)$_2$). The reaction at the cadmium electrode is:
Cd(s) + 2OH$^-$(aq) → Cd(OH)$_2$(s) + 2e$^-$
Concurrently, nickel oxyhydroxide (NiOOH) at the cathode is reduced to nickel hydroxide (Ni(OH)$_2$). When the battery is recharged, these reactions are reversed, allowing the battery to store and release electrical energy repeatedly. NiCd batteries were historically widely used in portable electronic devices across the globe before being largely replaced by newer technologies due to cadmium’s toxicity and memory effect.