Chemical Element: Lead (Pb) - Study Guide

Introduction: Why Lead Matters

Lead (Pb), from the Latin plumbum, is a heavy metal primarily known for its density, malleability, and resistance to corrosion. While many of its historical applications (like in paints and plumbing) have been phased out due to its toxicity, lead remains crucial in modern technology, particularly in lead-acid batteries and radiation shielding. Understanding its chemical properties is essential for comprehending its industrial uses and environmental impact.

CBSE/JEE Quick Revision Notes

• Atomic Number (Z): 82
• Atomic Mass: 207.2 g/mol
• Group: 14 (Carbon family)
• Period: 6
• Block: p-block
• Common Oxidation States: +2, +4
• Nature: Soft, malleable, ductile, silvery-blue metal that tarnishes upon exposure to air, forming a dull grey layer of lead oxide. Poor electrical conductor relative to other metals.
• Density: High (approx. 11.34 g/cm³)
• Melting Point: Low (327.5 °C)

Electron Configuration & Bonding Behavior

Electronic Configuration

The ground state electronic configuration of Lead (Pb) is: [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p²

Valence Shell Configuration

The valence shell configuration is 6s² 6p².

Oxidation States and Bonding

• +2 Oxidation State: This is the most common and stable oxidation state for lead. The two 6p electrons are involved in bonding, while the 6s electrons remain unshared due to the inert pair effect. This effect becomes more prominent down Group 14 due to increasing nuclear charge and poor shielding by d and f electrons, making the 6s electrons more tightly held and less available for bonding.
• +4 Oxidation State: This oxidation state is less stable than +2. It involves the participation of all four valence electrons (6s² 6p²) in bonding. Compounds in the +4 state often exhibit significant covalent character (e.g., PbCl₄, PbO₂). The stability of the +4 state decreases down Group 14. For lead, Pb(IV) compounds are generally strong oxidizing agents.

Crucial Chemical Reactions

1. Reaction with Air/Oxygen

Lead tarnishes in air to form a protective layer of lead(II) oxide (PbO). 2Pb (s) + O₂ (g) → 2PbO (s)

2. Reaction with Acids

• Dilute Non-oxidizing Acids (e.g., HCl, H₂SO₄): Lead reacts slowly to form insoluble salts (PbCl₂, PbSO₄), which passivate the surface and prevent further reaction. Pb (s) + 2HCl (aq) → PbCl₂ (s) + H₂ (g) (Lead(II) chloride is sparingly soluble) Pb (s) + H₂SO₄ (aq) → PbSO₄ (s) + H₂ (g) (Lead(II) sulfate is insoluble)
• Dilute Nitric Acid: Lead reacts readily. 3Pb (s) + 8HNO₃ (dilute) → 3Pb(NO₃)₂ (aq) + 2NO (g) + 4H₂O (l)
• Concentrated Nitric Acid: Lead reacts to produce nitrogen dioxide. Pb (s) + 4HNO₃ (conc) → Pb(NO₃)₂ (aq) + 2NO₂ (g) + 2H₂O (l)

3. Reaction with Alkalies

Lead is amphoteric and reacts with strong hot alkalis to form plumbites (tetrahydroxoplumbates(II)). Pb (s) + 2NaOH (aq) + 2H₂O (l) → Na₂[Pb(OH)₄] (aq) + H₂ (g) (Sodium tetrahydroxoplumbate(II))

4. Important Compounds and their Reactions

• Red Lead (Trilead Tetroxide, Pb₃O₄): A mixed oxide of Pb(II) and Pb(IV), commonly represented as 2PbO·PbO₂.
  • Preparation: 6PbO (s) + O₂ (g) → 2Pb₃O₄ (s) (at 450-500°C)
  • Reaction with Nitric Acid: Only the Pb(II) portion reacts, leaving insoluble PbO₂. Pb₃O₄ (s) + 4HNO₃ (aq) → 2Pb(NO₃)₂ (aq) + PbO₂ (s) + 2H₂O (l)
• Lead(IV) Oxide (Lead Dioxide, PbO₂): A dark brown powder, strong oxidizing agent.
  • Reaction with HCl: PbO₂ (s) + 4HCl (aq) → PbCl₂ (s) + Cl₂ (g) + 2H₂O (l)
  • Used as a cathode in lead-acid batteries.

Industrial and Biological Importance

Industrial Importance

• Lead-Acid Batteries: The most significant use. Lead plates and lead(IV) oxide plates are immersed in sulfuric acid, producing electricity through redox reactions.
• Radiation Shielding: Its high density makes it effective for shielding X-rays and gamma radiation in medical and nuclear applications.
• Ammunition: Used in bullets and shot due to its density and low melting point.
• Alloys:
  • Solder: Low melting point alloys of lead and tin (e.g., 60% Sn, 40% Pb).
  • Bearing Metals: Alloys with tin, antimony, and copper for reduced friction.
  • Type Metal: Lead-antimony-tin alloy for printing.
• Historical Uses (now largely restricted): Pigments (white lead, lead chromate), plumbing, cable sheathing.

Biological Importance (Toxicity)

Lead has no known beneficial biological role and is highly toxic.

• Cumulative Poison: Lead accumulates in the body, primarily in bones, soft tissues, and blood.
• Neurological Effects: Particularly damaging to the developing nervous system in children, leading to cognitive impairments, behavioral problems, and reduced IQ.
• Hematopoietic System: Interferes with heme synthesis, leading to anemia.
• Kidney Damage: Can cause chronic kidney disease.
• Reproductive Effects: Can impact fertility in both men and women.
• Exposure Routes: Ingestion (contaminated water, food, old paint chips) and inhalation (lead dust, fumes).