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Mo

Molybdenum

Molybdenum (Mo) is a transition metal with atomic number 42 and an atomic mass of 95.95. Its electron configuration is [Kr] 4d5 5s1.

Molybdenum key data

Symbol Mo
Atomic number 42
Atomic mass (standard atomic weight) 95.95
Molar mass 95.95 g/mol
Protons 42
Neutrons (most abundant isotope, molybdenum-98) 56
Electrons (neutral atom) 42
Category Transition metal
Period 5
Group 6
Block d-block
Noble gas notation [Kr] 4d5 5s1
Full electron configuration 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d5 5s1
Electrons per shell 2, 8, 18, 13, 1

Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.

The values are facts and free to use; this page’s selection and presentation are © 2026 ScienceQuest.

Measured properties of molybdenum

From PubChem, the US National Institutes of Health. Public domain.

Melting point 2896 K (2622.9 °C), 6th of 103 by melting point
Boiling point 4912 K (4638.9 °C), 7th of 93 by boiling point
Density 10.2 g/cm³, 25th of 96 by density
State at room temperature Solid
Electronegativity 2.16 (Pauling), 25th of 95 by electronegativity
Van der Waals radius 209 pm, joint 48th of 99 by atomic radius
First ionisation energy 7.092 eV (684.3 kJ/mol), 51st of 102 by first ionisation energy
Electron affinity 0.746 eV (72 kJ/mol), joint 30th of 57 by electron affinity
Common oxidation states +6
Discovered 1778

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Molybdenum electron configuration

The full electron configuration of molybdenum is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d5 5s1. With the noble-gas core collapsed it is [Kr] 4d5 5s1, and the outermost subshell is 5s1.

Molybdenum does not follow the simple filling order. Its measured configuration differs from the one the aufbau rule predicts, because a half-filled d subshell sits lower in energy than the arrangement the rule would give. This is a measured fact about the atom rather than an exception to be derived.

Bohr model of molybdenum (Mo): a nucleus and five shells holding 2, 8, 18, 13 and 1 electrons from the inside out.
Bohr model of molybdenum (Mo). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.
Orbital box diagram of molybdenum (Mo), [Kr] 4d5 5s1, with the krypton core drawn as one bar: one box per orbital, filled by Hund’s rule, showing six unpaired electrons, in 4d and 5s.
Orbital diagram of molybdenum (Mo). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one molybdenum atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

  1. one atom = 95.95 / (6.02214076 × 10²³)
  2. = 1.5933 × 10⁻²² g

Atoms in one gram

  1. atoms per gram = 6.02214076 × 10²³ / 95.95
  2. = 6.2763 × 10²¹ atoms

Molybdenum on the periodic table

Molybdenum is in period 5, group 6, in the d-block.

The other elements of group 6 are chromium, tungsten and seaborgium.

The rest of period 5

Work with molybdenum