Molybdenum at a glance
- Atomic number
- 42
- Symbol
- Mo
- Atomic mass
- 95.95 u
- Category
- Transition metal
- Group / period
- 6 / 5
- Electron configuration
- [Kr] 4d 5 5s 1
- State at 25 °C
- Solid
- Melting point
- 2623 °C
- Boiling point
- 4639 °C
- Density
- 10.22 g/cm³
- Electronegativity
- 2.16
- 1st ionization energy
- 684 kJ/mol
- Oxidation states
- +6, +4
- Discovered
- 1778
Atomic structure
A neutral molybdenum atom has 42 protons and 42 electrons, and its most common isotope carries about 54 neutrons. One atom weighs about 1.59 × 10⁻²⁵ kg, so a mole of molybdenum (6.022 × 10²³ atoms) weighs 95.95 g. That nucleus holds 1.29 neutrons for every proton. The electrons fill 5 shells (K 2, L 8, M 18, N 13, O 1), which gives the configuration [Kr] 4d 5 5s 1. As a d-block element, both its outer s electrons and its inner d electrons can take part in bonding, which is why it shows more than one oxidation state. Its configuration breaks the simple filling order: one electron moves so that a subshell becomes half-full or full, which is more stable. It has 7 stable isotopes: Mo-92, Mo-94, Mo-95, Mo-96, Mo-97, Mo-98 and Mo-100. Its most common isotope, Mo-96, has the same mass number as Zr-96 and Ru-96: isobars, nuclei of different elements with 96 nucleons each.
Physical properties
At room temperature (25 °C) molybdenum is a solid. It melts at 2623 °C and boils at 4639 °C, so it stays liquid across a 2016-degree range. It stays solid in molten iron (1538 °C) but would melt in a welding arc. That is the 6th highest melting point of any element. Its density is 10.22 g/cm³, 10.2 times as dense as water. Its atoms have a radius of about 145 pm (Niobium next door is 145 pm). In the solid, its atoms pack into a body-centred cubic lattice.
Chemical behaviour
Its electronegativity is 2.16 on the Pauling scale. Pulling off its outermost electron takes 684 kJ/mol, more than Chromium above it (653 kJ/mol). A bond between molybdenum and chlorine would be polar covalent, with an electronegativity difference of 1. It releases 72 kJ/mol when it gains an extra electron. Its common oxidation states are +6, +4, so in compounds you will usually meet it as Mo⁶⁺ and Mo⁴⁺. Group 6 also holds Chromium, Tungsten and Seaborgium, which share its outer-shell pattern.
Where it comes from
It makes up 1.2 mg/kg of Earth's crust, the 58th most abundant element there. In the human body it accounts for about 0.00001 % of body mass. It was first identified in 1778, making it the 24th element to be discovered (counting the 11 known since antiquity together).