Molybdenum (Mo): Exam-Focused Study Guide

Introduction

Molybdenum (Mo) is a crucial transition metal located in Group 6 and Period 5 of the periodic table. Its unique electronic configuration and resulting chemical properties enable it to play indispensable roles in various high-performance materials, industrial catalysis, and fundamental biological processes. Understanding Molybdenum is essential for a comprehensive grasp of d-block element chemistry and its real-world applications.

CBSE/JEE Quick Revision Notes

• Atomic Number (Z): 42
• Atomic Mass (A): 95.96 g/mol
• Group: 6
• Period: 5
• Block: d-block (Transition Element)
• Nature: Silvery-white, lustrous, high melting point (2896 K), high density (10.22 g/cm³), highly resistant to corrosion.
• Common Oxidation States: +2, +3, +4, +5, +6. The +6 oxidation state is the most stable and common.
• Electronegativity (Pauling): 2.16
• Standard Electrode Potential (Mo³⁺/Mo): -0.2 V (acidic solution)

Electron Configuration & Bonding Behavior

Ground State Electron Configuration

Molybdenum exhibits an anomalous electron configuration, achieving enhanced stability through a half-filled 4d subshell: [Kr] 4d⁵ 5s¹

Bonding Behavior

• Variable Oxidation States: Molybdenum displays a wide range of oxidation states from +2 to +6. This is characteristic of transition metals, arising from the involvement of both 5s and 4d electrons in bonding. The +6 state is particularly stable due to the formation of bonds involving all six valence electrons (one 5s and five 4d electrons).
• Covalent Character: In higher oxidation states (e.g., in MoO₃ and molybdates), Molybdenum compounds exhibit significant covalent character.
• Complex Formation: As a d-block element, Molybdenum forms numerous complex compounds with various ligands, utilizing its empty d-orbitals to accept electron pairs.

Crucial Chemical Reactions

1. Reaction with Oxygen (Formation of Molybdenum Trioxide): Molybdenum readily reacts with oxygen upon heating to form Molybdenum(VI) oxide (molybdenum trioxide), which is its most stable oxide. 2Mo(s) + 3O₂(g) → 2MoO₃(s)

2. Reaction with Acids:

   • Non-oxidizing Acids (e.g., dilute HCl, dilute H₂SO₄): Molybdenum is generally resistant to attack by non-oxidizing acids under normal conditions.
   • Oxidizing Acids (e.g., hot concentrated HNO₃, hot concentrated H₂SO₄): Molybdenum reacts with strong oxidizing acids, typically forming molybdenum trioxide or molybdic acid, which subsequently dehydrates to MoO₃. Mo(s) + 6HNO₃(conc.) → MoO₃(s) + 6NO₂(g) + 3H₂O(l)

3. Reaction with Alkalis (Formation of Molybdates): Molybdenum trioxide (MoO₃) is an acidic oxide and reacts with strong aqueous bases to form molybdates, where Molybdenum is in the +6 oxidation state. MoO₃(s) + 2NaOH(aq) → Na₂MoO₄(aq) + H₂O(l) (Sodium Molybdate)

4. Redox Behavior (Example - Reduction of Molybdate): Molybdenum compounds exhibit redox properties. For example, in acidic solutions, molybdate (Mo in +6 state) can be reduced. MoO₄²⁻(aq) + 2H⁺(aq) + 2e⁻ → MoO₃(s) + H₂O(l) (Reduction from +6 to +4)

Industrial and Biological Importance

Industrial Importance

• Alloys: Molybdenum’s primary use is in steel alloys, enhancing strength, hardness, toughness, weldability, and corrosion resistance. It is critical in high-speed steels, tool steels, and stainless steels.
• Catalysts: Molybdenum compounds, particularly oxides and sulfides, are widely used as catalysts in the petroleum industry (e.g., hydrodesulfurization to remove sulfur from fuels) and in chemical synthesis.
• Lubricants: Molybdenum disulfide (MoS₂) is an excellent solid lubricant, valued for its low friction coefficient and stability at high temperatures and pressures.
• Pigments: Lead molybdate is used as a bright orange pigment, known as molybdenum orange.
• Electronics: Molybdenum finds applications in thin-film transistors, solar cells, and as interconnects in integrated circuits.

Biological Importance

• Essential Trace Element: Molybdenum is an essential micronutrient for virtually all living organisms, playing a crucial role in various metabolic processes.
• Enzyme Cofactor: It acts as a cofactor in several vital metalloenzymes, notably:
  • Nitrogenase: Found in nitrogen-fixing bacteria, this enzyme is critical for converting atmospheric nitrogen (N₂) into ammonia (NH₃), a form usable by plants for protein synthesis.
  • Sulfite Oxidase: In animals, this enzyme detoxifies harmful sulfite by oxidizing it to sulfate.
  • Xanthine Oxidase: Involved in purine metabolism, catalyzing the oxidation of hypoxanthine and xanthine to uric acid.
• Agricultural Impact: Molybdenum is vital for crop yield, particularly in legumes, due to its role in nitrogen fixation. Molybdenum deficiency in soil can significantly reduce plant growth.