Cadmium (Cd): An Introduction
Cadmium, designated by the chemical symbol Cd, is a soft, silvery-white, ductile metal with atomic number 48. It belongs to Group 12 of the periodic table, alongside zinc and mercury. In its elemental form, cadmium exhibits several unique chemical properties that are important to understand.
Reactivity with Water
Cadmium generally displays low reactivity with water at ordinary temperatures.
- At room temperature: Cadmium metal does not react with cold or hot water.
- With steam: When heated to high temperatures (typically red heat) and exposed to steam, cadmium reacts to form cadmium oxide and hydrogen gas. This reaction can be represented by the following chemical equation:
Cd(s) + H₂O(g) → CdO(s) + H₂(g)
Reactivity with Air
The interaction of cadmium with air depends on the conditions of exposure.
- In dry air: Cadmium remains stable in dry air at room temperature, showing minimal tarnishing.
- In moist air: Over time, in the presence of moist air, a thin, protective layer of cadmium oxide (CdO) may form on the surface, which can hinder further oxidation.
- Upon heating: When cadmium is heated strongly in air, it readily oxidizes, producing brown fumes of cadmium oxide. This reaction is exothermic:
2Cd(s) + O₂(g) → 2CdO(s)
Toxicity
Cadmium is highly toxic to humans and other living organisms. Exposure, even at low concentrations, can lead to severe health issues.
- Accumulation: Cadmium accumulates in the body, primarily in the kidneys and liver, over long periods. Its biological half-life is very long, meaning it is excreted very slowly.
- Health effects: Chronic exposure can cause kidney damage, bone demineralization (leading to conditions like osteomalacia), and respiratory problems. It is also classified as a human carcinogen.
- Environmental concern in India: Cadmium can enter the environment through industrial effluents from industries like electroplating, pigment manufacturing, and nickel-cadmium battery production. Contamination of agricultural soils and water bodies, particularly near industrial zones, can pose significant health risks when it enters the food chain.
Radioactivity
In its naturally occurring isotopes, cadmium is not considered a radioactive element. The most abundant isotopes of cadmium are stable. While some artificial, extremely rare isotopes of cadmium can exhibit radioactivity, these are not relevant to the general understanding of the element.
Flammability
Bulk cadmium metal is not readily flammable under normal conditions. However, in finely divided powder form, or when heated to very high temperatures, cadmium can ignite and burn in air. When it burns, it produces toxic fumes of cadmium oxide.
A Chemical Reaction Involving Cadmium
Cadmium participates in various chemical reactions. A notable example is its reaction with hydrogen sulfide gas to form cadmium sulfide (CdS). Cadmium sulfide is a bright yellow pigment historically used in paints, although its use has declined due to toxicity concerns.
Cd²⁺(aq) + H₂S(g) → CdS(s) + 2H⁺(aq)
In this reaction, cadmium ions (typically from a soluble cadmium salt in solution) react with hydrogen sulfide to precipitate insoluble cadmium sulfide, which is recognized by its distinct yellow colour.