Molybdenum (Mo)

Molybdenum has the sixth-highest melting point of any element at 2,623 °C, and unlike tungsten it can be worked without extreme difficulty, which is why furnace interiors are built from it. Nitrogen-fixing bacteria use a molybdenum-iron cofactor to break the N≡N bond at ordinary temperature and pressure — the reaction industry needs 450 °C and 200 atmospheres to force. Its disulfide is structurally like graphite, sheets that slide over one another, and it keeps working in a vacuum where graphite fails because graphite's lubricity secretly depends on adsorbed water.

Properties

Atomic number

42

Symbol

Mo

Standard atomic weight

95.95 u

Category

Transition metal

Group

6

Period

5

Block

d-block

Electron configuration

[Kr] 5s¹ 4d⁵

Electrons per shell

2, 8, 18, 13, 1

Melting point

2,896 K — 2622.8 °C — 4753.1 °F

Boiling point

4,912 K — 4638.9 °C — 8381.9 °F

Density

10.2 g/cm³

Electronegativity

2.16 (Pauling)

Oxidation states

+6, +4, +3

Discovered

1778 by Carl Wilhelm Scheele; isolated by Peter Jacob Hjelm in 1781

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s¹ 4d⁵

Noble gas shorthand

[Kr] 5s¹ 4d⁵

Shells

K: 2 · L: 8 · M: 18 · N: 13 · O: 1

What it is used for

• High-strength steel for pressure vessels, gun barrels and drill pipe
• Molybdenum disulfide as a dry lubricant in vacuum and high heat
• Furnace heating elements and glass-melting electrodes
• Molybdenum-99, whose decay product runs most nuclear medicine scans
• Hydrodesulfurisation catalysts in oil refineries

Anomalous electron configuration: [Kr] 5s¹ 4d⁵ rather than the predicted 5s² 4d⁴ — the same half-filled-subshell effect as chromium, directly above it.

Neighbours in the table

Group 6

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Molybdenum sits in group 6 with chromium, tungsten, seaborgium.