Quick Facts
| Property | Value |
|---|---|
| Symbol | Mo |
| Atomic Number | 42 |
| Atomic Mass | 95.95 u |
| Category | Transition Metal |
| Period | 5 |
| Group | 6 |
| Block | d |
| Electron Configuration | [Kr] 4d⁵ 5s¹ |
Physical Properties
| Property | Value |
|---|---|
| State at 20°C | Solid |
| Density | 10.28 g/cm³ |
| Melting Point | 2623°C (2896 K) |
| Boiling Point | 4639°C (4912 K) |
| Appearance | Silvery-gray metal |
Atomic Properties
| Property | Value |
|---|---|
| Electron Configuration | [Kr] 4d⁵ 5s¹ |
| Electronegativity | 2.16 (Pauling scale) |
| First Ionization Energy | 684.3 kJ/mol |
| Atomic Radius | 139 pm |
| Covalent Radius | 154 pm |
| Van der Waals Radius | 209 pm |
History and Discovery
Discovered by: Carl Wilhelm Scheele (identified), Peter Jacob Hjelm (isolated) Year of Discovery: 1778 (identified), 1781 (isolated) Location: Sweden
Etymology
Named from the Greek "molybdos" meaning "lead," because molybdenite ore was long confused with lead ore and graphite.
Discovery Story
Carl Wilhelm Scheele identified molybdenum as a distinct element in 1778 by studying molybdenite ore. Peter Jacob Hjelm isolated the metal in 1781 by reducing molybdenum oxide with carbon. For centuries before, molybdenite had been confused with graphite and lead ore.
Isotopes
| Isotope | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|
| ⁹²Mo | 14.53% | Stable | - |
| ⁹⁴Mo | 9.15% | Stable | - |
| ⁹⁵Mo | 15.84% | Stable | - |
| ⁹⁶Mo | 16.67% | Stable | - |
| ⁹⁷Mo | 9.60% | Stable | - |
| ⁹⁸Mo | 24.39% | Stable | - |
| ¹⁰⁰Mo | 9.82% | 7.8 × 10¹⁸ years | β⁻β⁻ |
Occurrence
Natural Abundance
Molybdenum is the 54th most abundant element in Earth's crust (1.2 ppm). The primary ore is molybdenite (MoS₂). Major producers are China, the United States, and Chile. Molybdenum often occurs with copper deposits.
Extraction and Production
- Roasting: Molybdenite to molybdenum trioxide
- Reduction: With hydrogen or carbon for metal
- Sources: Primarily molybdenite mining
- Global production: about 300,000 tons annually
Applications and Uses
Steel and Alloys
- High-strength steels (most molybdenum goes here)
- Tool steels
- Stainless steels
- Superalloys for jet engines
Chemical Industry
- Catalysts (petroleum refining, petrochemicals)
- Lubricants (molybdenum disulfide)
- Pigments
Industrial Applications
- Furnace heating elements
- Electrodes
- High-temperature equipment
Other Uses
- Fertilizers (molybdenum is essential for plants)
- Electronics (thin films)
- Nuclear applications
Biological Role
Molybdenum is an essential trace element for nearly all living organisms. It is a cofactor in important enzymes.
In the Human Body
- About 5 mg in adult body
- Essential cofactor for enzymes (xanthine oxidase, sulfite oxidase)
- Required for nitrogen metabolism
- Daily requirement: 45 μg
In Plants
Essential for nitrogen fixation in legumes and for nitrate reduction.
Dietary Sources
Legumes, grains, nuts, and organ meats are good sources.
Safety and Hazards
Toxicity
Molybdenum is essential but excess can be harmful. High levels interfere with copper metabolism. Molybdenum dust and compounds can irritate mucous membranes.
Handling Precautions
- Molybdenum powder can be flammable
- Dust may irritate respiratory system
- Follow occupational exposure limits
- Generally considered safe in metallic form
Environmental Impact
Molybdenum mining has typical environmental impacts. Excess molybdenum can affect plant and animal health.
Interesting Facts
- Molybdenum has the sixth-highest melting point of all elements
- It was used in armor plate during World War I when tungsten was scarce
- Molybdenum disulfide is an excellent dry lubricant
- Legumes require molybdenum to fix nitrogen from the air
- The electron configuration [Kr] 4d⁵ 5s¹ is anomalous (expected would be 4d⁴ 5s²)