Molybdenum (Mo)
Molybdenum (Mo) is a transition metal with atomic number 42 and a standard atomic weight of 95.95. At room temperature it is a solid that melts at 2,622 °C and boils at 4,639 °C.
Molybdenum key facts
Molybdenum at a glance
Identity
| Symbol | Mo |
|---|---|
| Atomic number (Z) | 42 |
| Atomic weight | 95.95 ± 0.01 (CIAAW 2024) |
| Category | Transition metal |
| Group | 6 |
| Period | 5 |
| Block | d-block |
| CAS number | 7439-98-7 |
Atomic properties
| Electron configuration | [Kr] 4d5 5s1 Full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d5 5s1 |
|---|---|
| Electrons per shell | 2, 8, 18, 13, 1 |
| Oxidation states | +4, +6 Less common: -4, -2, -1, 0, +1, +2, +3, +5 |
| Electronegativity (Pauling) | 2.16 |
| First ionisation energy | 7.092 eV (684.3 kJ/mol) 2nd: 16.16 eV · 3rd: 27.13 eV |
| Electron affinity | 0.748 eV |
| Atomic radius (empirical) | 145 pm |
| Covalent radius | 138 pm |
| Van der Waals radius | 217 pm |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Melting point | 2,622 °C · 4,751.6 °F · 2,895.15 K |
| Boiling point | 4,639 °C · 8,382.2 °F · 4,912.15 K |
| Density | 10.2 g/cm³ |
| Specific heat capacity | 0.251 J/(g·K) |
| Crystal structure | Body-centred cubic |
Isotopes & abundance
| Natural occurrence | Primordial |
|---|---|
| Radioactive | No |
| Natural isotopes (abundance) | 98Mo 24.292%, 96Mo 16.673%, 95Mo 15.873%, 92Mo 14.649%, 100Mo 9.744% (radioactive), 97Mo 9.582% |
| Stable isotopes | 6 |
| Known isotopes | 39 |
| Abundance in Earth's crust | 1.2 mg/kg |
| Abundance in seawater | 0.01 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Molybdenum electron configuration
Molybdenum has 42 electrons. Written in short form, its ground-state configuration is [Kr] 4d5 5s1 — the electrons of the noble gas krypton, plus 4d5 5s1.Written out in full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d5 5s1.
An exception to the filling rule. The simple aufbau (Madelung) order predicts 5s2, 4d4, but molybdenum actually has 5s1, 4d5. Half-filled and filled subshells, and the close energies of the outer orbitals, make this arrangement more stable.
Discovery and name
1778 — Carl Wilhelm Scheele (isolated by Peter Jacob Hjelm, 1781) · Sweden.
Greek molybdos, lead.
Known for
Used in high-strength steels and as a catalyst for removing sulfur from fuels. Essential to enzymes that fix nitrogen.
Isotopes of molybdenum
Naturally occurring molybdenum is a mix of 7 isotopes. Each has 42 protons; they differ in the number of neutrons:
| Isotope | Protons | Neutrons | Natural abundance | Stability |
|---|---|---|---|---|
| 98Mo | 42 | 56 | 24.292% | stable |
| 96Mo | 42 | 54 | 16.673% | stable |
| 95Mo | 42 | 53 | 15.873% | stable |
| 92Mo | 42 | 50 | 14.649% | stable |
| 100Mo | 42 | 58 | 9.744% | radioactive · half-life 7.07 × 10¹⁸ years |
| 97Mo | 42 | 55 | 9.582% | stable |
| 94Mo | 42 | 52 | 9.187% | stable |
39 isotopes of molybdenum are known in our data set (half-lives as published in nuclear data tables).
Molybdenum compared with group 6
The other elements in group 6, from top to bottom of the table:
| Element | Z | Atomic weight | Melting point | Boiling point | Density | Electronegativity |
|---|---|---|---|---|---|---|
| Chromium | 24 | 51.996 | 1,907 °C | 2,671 °C | 7.15 g/cm³ | 1.66 |
| Molybdenum | 42 | 95.95 | 2,622 °C | 4,639 °C | 10.2 g/cm³ | 2.16 |
| Tungsten | 74 | 183.84 | 3,422 °C | 5,930 °C | 19.3 g/cm³ | 2.36 |
| Seaborgium | 106 | [269] | — | — | 35 g/cm³ (pred.) | — |