Molybdenum - Mo
Molybdenum (Mo) is a chemical element with atomic number 42. This page summarises its properties, uses and environmental context, with notes for water and process-treatment engineers.
Properties · Uses and occurrence · Health · Environment · Water and treatment
Chemical properties of molybdenum
| Symbol / atomic number | Mo / 42 |
|---|---|
| Group / period / block | 6 / 5 / d |
| Standard atomic weight | 95.95(1) |
| Electron configuration | [Kr] 4d5 5s1 |
| Common oxidation state(s) | +6, +5, +4, +3, +2, 0 |
| Density (20 °C) | 10.2 g cm−3 |
| Melting point | 2622 °C |
| Boiling point | 4639 °C |
| Electronegativity (Pauling) | 2.16 |
| Non-bonded atomic radius | 0.217 nm |
| First ionisation energy | 684.316 kJ mol−1 |
| Representative isotopes | 92Mo, 94Mo–98Mo, 100Mo |
| CAS registry number | 7439-98-7 |
Standard atomic weight follows CIAAW. Parentheses indicate uncertainty in the final quoted digits. Other physical and atomic data follow the Royal Society of Chemistry. Non-bonded atomic radii are method-dependent.
Molybdenum: properties, uses and occurrence
Most molybdenum is used as an alloying element in steels and other alloys. Molybdenum disulfide is used as a lubricant and molybdenum compounds are used in catalysts and pigments. Molybdenum occurs in minerals such as molybdenite. It has seven naturally occurring isotopes.
Health and safety
Molybdenum is an essential trace element for many organisms, but excess exposure can be harmful. The effects depend on concentration, chemical form and exposure pathway; workplace dust and water exposure require separate assessments.
Molybdenum in the environment
In oxic natural waters, dissolved molybdenum is often present as molybdate (MoO42−). Its distribution depends on redox conditions, pH, mineral surfaces and competing ions. Industrial releases can occur from mining and metal-processing activities.
Molybdenum in water and treatment
Molybdate is an oxyanion, so results for molybdenum should not be interpreted as if it were a dissolved metal cation. EPA's recent treatment review found limited studies, including full-scale strong-base anion exchange and bench-scale ferric coagulation. Performance depends on pH, competing anions and the water matrix; evaluate breakthrough, regeneration waste and residuals with representative testing. Conventional coagulation should not be assumed to remove molybdenum without validation.
Molybdenum page last reviewed: by Lenntech.
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