Cadmium (Cd) - Comprehensive Exam Study Guide

Introduction: Why Cadmium Matters

Cadmium (Cd) is a relatively rare, soft, silvery-white, and ductile metal. It is a d-block element, often found in zinc ores. Its significance arises from its unique chemical properties, leading to applications in various industries, but also from its high toxicity, posing significant environmental and health concerns. Understanding Cadmium is crucial for comprehending d-block chemistry and environmental science aspects.

CBSE/JEE Quick Revision Notes

• Symbol: Cd
• Atomic Number: 48
• Atomic Mass: 112.41 u
• Group: 12 (IIB)
• Period: 5
• Block: d-block
• Valency/Common Oxidation State: +2
• Nature: Soft, silvery-white metallic element
• Density: 8.65 g/cm³ (at 20 °C)
• Melting Point: 321.07 °C
• Boiling Point: 767 °C
• Occurrence: Primarily found as a minor component in zinc ores like sphalerite (ZnS).

Electron Configuration & Bonding Behavior

Cadmium’s electronic structure dictates its chemical properties.

• Ground State Electron Configuration: [Kr] 4d¹⁰ 5s²
• Bonding Behavior:
  • Cadmium typically exhibits a +2 oxidation state due to the loss of its two 5s electrons. The completely filled 4d¹⁰ subshell provides a stable configuration, making it difficult to remove electrons from the d-orbitals.
  • Unlike typical transition metals, Cadmium’s 4d¹⁰ configuration means its compounds are generally diamagnetic and colourless (unless the anion is coloured).
  • Cadmium forms predominantly covalent compounds, especially with halides and chalcogens, although its salts with highly electronegative elements can have significant ionic character.
  • It forms complex compounds with ligands like ammonia ([Cd(NH₃)₄]²⁺) and cyanide ([Cd(CN)₄]²⁻), utilizing its vacant 5p orbitals for coordination.

Crucial Chemical Reactions

Cadmium exhibits characteristic reactions typical of an active metal.

1. Reaction with Air/Oxygen: Upon heating in air, cadmium burns to form cadmium oxide. 2Cd(s) + O₂(g) → 2CdO(s) (Brown solid)

2. Reaction with Acids:

   • With Non-oxidizing Acids (e.g., HCl, dilute H₂SO₄): Cadmium reacts to produce hydrogen gas and cadmium salts. Cd(s) + 2HCl(aq) → CdCl₂(aq) + H₂(g) Cd(s) + H₂SO₄(aq, dilute) → CdSO₄(aq) + H₂(g)
   • With Oxidizing Acids (e.g., HNO₃): Reaction is more vigorous, forming oxides of nitrogen. 3Cd(s) + 8HNO₃(aq, dilute) → 3Cd(NO₃)₂(aq) + 2NO(g) + 4H₂O(l) Cd(s) + 4HNO₃(aq, conc.) → Cd(NO₃)₂(aq) + 2NO₂(g) + 2H₂O(l)

3. Reaction with Halogens: Cadmium reacts directly with halogens to form cadmium halides. Cd(s) + Cl₂(g) → CdCl₂(s) (Cadmium chloride) Cd(s) + Br₂(g) → CdBr₂(s) (Cadmium bromide)

4. Reaction with Sulfur: When heated with sulfur, cadmium sulfide is formed. Cd(s) + S(s) → CdS(s) (Yellow solid, used as pigment)

5. Formation of Hydroxide: Cadmium salts precipitate cadmium hydroxide with strong bases. Cd²⁺(aq) + 2OH⁻(aq) → Cd(OH)₂(s) (White precipitate, sparingly soluble in excess alkali)

Industrial and Biological Importance

Industrial Importance

1. Batteries: Cadmium is a key component in nickel-cadmium (Ni-Cd) rechargeable batteries. These batteries offer good performance at low temperatures and high discharge rates.
2. Pigments: Cadmium compounds, particularly cadmium sulfide (CdS) and cadmium sulfoselenide (CdSSe), are used as brilliant yellow, orange, and red pigments in paints, plastics, and ceramics due to their heat stability and vibrant colors.
3. Corrosion-Resistant Coatings: Cadmium plating provides excellent corrosion resistance to steel, especially in marine environments, by acting as a sacrificial coating.
4. Control Rods in Nuclear Reactors: Due to its high neutron absorption cross-section, cadmium is used in control rods in nuclear fission reactors to regulate the fission rate.
5. Stabilizers for PVC: Cadmium compounds are used as heat and light stabilizers in polyvinyl chloride (PVC) plastics.

Biological Importance and Toxicity

1. Essentiality: Cadmium has no known biological role as an essential element for humans or other organisms.
2. Toxicity: Cadmium and its compounds are highly toxic.
   • Accumulation: It accumulates in the body, primarily in the kidneys and liver, with a long biological half-life.
   • Health Effects: Chronic exposure can lead to kidney damage, bone demineralization (osteomalacia, osteoporosis, “Itai-itai disease”), lung damage, and is classified as a human carcinogen (lung, prostate, and kidney cancer).
   • Environmental Pollutant: Cadmium is a significant environmental pollutant, often released from mining, smelting, and manufacturing processes, entering the food chain through contaminated soil and water.