Chemical element · reference page

42Mo

Molybdenum

Molybdenum is element 42 (Mo), located in period 5, group 6 and the d block. At room temperature its reference phase is solid. Its electron configuration is [Kr] 4d5 5s1.

Atomic number42

Standard atomic weight95.95(1)

Group6

Period5

Blockd

Electron configuration[Kr] 4d5 5s1

Electronic structure

What controls its chemistry

The d subshell participates in the chemistry of molybdenum. Transition metals can use electrons of similar energies in bonding, which is why variable oxidation states, coordination compounds and catalytic behaviour are common across the d block.

Its electron configuration, [Kr] 4d5 5s1, should be read together with the oxidation state of the compound: removing electrons changes which d orbitals are occupied and can alter colour, magnetism, bonding and reactivity.

Chemical forms

Compounds, ions and materials

For molybdenum, important chemical forms include metals and alloys, oxides, halides and coordination compounds. Variable oxidation states are common because s and d electrons have similar energies, and ligands around the metal ion can strongly alter colour, magnetism and reactivity.

Physical behaviour

State and phase changes

The reference phase at room temperature is solid. The listed density is 10.28 g/cm³. The melting point is 2 895 K (2 622 °C). The boiling point is 4 912 K (4 639 °C). Pressure, purity and crystal structure can shift measured physical properties.

Occurrence and applications

Where molybdenum is found and used

In practice, molybdenum is encountered in metallurgy and steel, catalysts and lubricants.

In practice

Examples of real uses

Metallurgy and steel

Molybdenum is used in iron and steel processing, either as an alloying element, a refining aid or part of the ore-to-metal chemistry.

Catalysts

Molybdenum is used to accelerate chemical reactions without being consumed in the overall reaction.

Lubricants

Molybdenum is used in solid or chemical lubricants that reduce friction under conditions where oils may not perform well.

Periodic position

What its neighbours tell us

Molybdenum is in period 5 and group 6. In the neighbourhood shown here, it lies between niobium and technetium in the same period; chromium is above it and tungsten below it in the same group. These positions make it possible to compare atomic size, ionisation energy and bonding behaviour with nearby elements.

Reference data

Physical and atomic properties

Classificationtransition metal

Phase at room temperatureSolid

Electron shells2 · 8 · 18 · 13 · 1

Density10.28 g/cm³

Melting point2 895 K (2 622 °C)

Boiling point4 912 K (4 639 °C)

Pauling electronegativity2.16

First ionisation energy684.3 kJ/mol

Electron affinity72.1 kJ/mol

Values describe the element or neutral atom where applicable. Physical data can depend on allotrope, pressure and measurement conditions.

History

Discovery and identification

The discovery of molybdenum is associated with Carl Wilhelm Scheele. Recognition of the element, isolation of a pure sample and assignment of its modern atomic number did not necessarily occur at the same time.

Isotopes and atomic weight

How isotope composition changes the numbers

The standard atomic weight is 95.95(1). This value refers to the isotopic composition of natural terrestrial material, not to the mass of one specific atom.

Molybdenum has at least one stable isotope. Different isotopes have the same number of protons and therefore the same element identity, but different neutron numbers and atomic masses.

Safety

Handling and exposure

The hazards associated with molybdenum depend on chemical form, dose and route of exposure. Pure molybdenum, its ions and its compounds can have very different biological and environmental effects, so safety data should be checked for the actual substance being handled.