Cadmium (Cd)

Cadmium has the electron configuration [Kr] 4d¹⁰ 5s². The completely filled 4d and 5s subshells confer extra stability to the neutral atom, making it less chemically reactive than many transition metals.

Cadmium primarily exhibits the +2 oxidation state (Cd²⁺) in its compounds. Common examples include:

• CdO – cadmium oxide
• CdCl₂ – cadmium chloride
• CdS – cadmium sulfide (yellow pigment)

Cadmium(1+) is rare and unstable under normal conditions.

Cadmium is highly toxic because it mimics zinc and calcium ions, interfering with essential enzymatic processes. Chronic exposure affects the kidneys, liver, and bones, and inhalation of cadmium dust or fumes can cause severe lung and kidney damage. It is classified as a Group 1 human carcinogen by the IARC.

Despite its toxicity, cadmium has several important applications:

• Rechargeable batteries – especially Ni–Cd cells (now declining).
• Electroplating – corrosion-resistant coatings for steel and aircraft components.
• Pigments – CdS (yellow) and CdSe (red) for plastics and glass.
• Alloys and solders – for low melting point and improved wear resistance.

All three elements (Zn, Cd, Hg) have filled d-subshells and similar +2 oxidation states. Trends include:

• Atomic radius increases down the group (Zn < Cd < Hg).
• Cd is less electropositive than Zn and forms more covalent bonds.
• Hg differs significantly due to relativistic effects and weak metallic bonding.

In a Ni–Cd cell, cadmium serves as the anode material and undergoes reversible oxidation and reduction:

Meanwhile, nickel oxyhydroxide is reduced at the cathode. Despite their efficiency, Ni–Cd batteries are being replaced due to environmental concerns.

Cadmium occurs as an impurity in zinc sulfide ores (sphalerite, \(\mathrm{ZnS}\)). During roasting and smelting:

The CdO formed is later reduced by carbon or recovered through distillation in zinc refining processes.

Cadmium pigments include cadmium yellow (CdS), cadmium red (CdSe), and related compounds known for high color stability and brilliance. However, due to toxicity and environmental persistence, their use has been restricted in paints, plastics, and consumer goods in many countries.

Cadmium coatings provide excellent corrosion resistance, lubricity, and solderability. It protects steel parts in harsh environments (e.g., marine and aviation). However, due to environmental regulations, cadmium plating is being replaced by zinc–nickel or tin–zinc alternatives.

Precautions include:

• Use fume hoods or local exhaust ventilation.
• Wear gloves, masks, and protective clothing.
• Avoid heating cadmium or Cd-containing alloys due to toxic vapor release.
• Proper disposal and recycling to prevent soil and water contamination.

Regulations strictly limit cadmium exposure in workplaces and consumer products.