Cadmium (Cd) is a soft, malleable, metallic element (atomic number 48) found in Group 12 of the periodic table. Chemically similar to zinc, it is primarily obtained as a byproduct during the mining and processing of zinc ores. This discussion details the physical characteristics of cadmium, focusing on its surface quality and how its appearance changes depending on its form.
The Silvery Luster of Pure Cadmium
When freshly cut or polished, pure cadmium exhibits a bright, highly reflective silvery-white metallic luster. This elemental state allows the metal to take a high polish, giving it a characteristic shiny appearance. It is highly ductile and malleable, meaning it can be drawn into thin wires or rolled into sheets without fracturing.
Despite its initial bright appearance, cadmium’s surface luster is not permanent when exposed to the atmosphere. The metal reacts with oxygen, especially in moist conditions, quickly forming a thin layer of cadmium oxide. This oxidation process dulls the surface, transforming the bright metal into a grey or bluish-grey coating that reduces reflectivity. This dense, non-porous oxide layer protects the underlying metal from further corrosion.
Common Forms and Appearance in Applications
The visual presentation of cadmium varies widely once it is incorporated into commercial products or compounds. In electroplating, where a thin layer is deposited onto materials like steel for corrosion resistance, the finish is typically bright and silvery-white, sometimes with a slight bluish cast. This deposited layer is smooth and uniform, offering a clean, attractive finish on precision parts.
The appearance is often intentionally altered through post-plating treatments, resulting in golden, iridescent yellow, black, or olive drab finishes. Outside of metallic plating, cadmium compounds are known for their vibrant, non-metallic colors when used as pigments. For example, cadmium sulfide creates the intense yellow hue known as cadmium yellow, and cadmium selenide is used for bright orange and red powders. These pigments, used in artist paints and plastics, are opaque, non-reflective solids that are chemically stable and resistant to fading.
Cadmium’s Significance in Health and Toxicology
Beyond its physical appearance, the element’s main relevance involves its biological effects. Cadmium is classified as a heavy metal, and its presence in the environment is a concern due to its toxicity and tendency to accumulate in living organisms. Human exposure primarily occurs through the inhalation of dust or fumes, such as from cigarette smoke, or through dietary intake from contaminated crops and seafood.
Once absorbed, the body struggles to excrete cadmium, resulting in a biological half-life that can span 10 to 30 years. This prolonged retention leads to bioaccumulation, as the element is initially stored in the liver before being redistributed throughout the body. The kidneys are the organ most sensitive to contamination, where the metal accumulates in the renal cortex. Long-term exposure can result in damage to the renal tubular cells and impaired kidney function.