Osmium at a glance
- Atomic number
- 76
- Symbol
- Os
- Atomic mass
- 190.23 u
- Category
- Transition metal
- Group / period
- 8 / 6
- Electron configuration
- [Xe] 4f 14 5d 6 6s 2
- State at 25 °C
- Solid
- Melting point
- 3033 °C
- Boiling point
- 5012 °C
- Density
- 22.59 g/cm³
- Electronegativity
- 2.2
- 1st ionization energy
- 840 kJ/mol
- Oxidation states
- +4, +8
- Discovered
- 1803
Atomic structure
A neutral osmium atom has 76 protons and 76 electrons, and its most common isotope carries about 114 neutrons. One atom weighs about 3.16 × 10⁻²⁵ kg, so a mole of osmium (6.022 × 10²³ atoms) weighs 190.23 g. That nucleus holds 1.5 neutrons for every proton, typical of heavy nuclei, which need extra neutrons to hold together against the protons' repulsion. The electrons fill 6 shells (K 2, L 8, M 18, N 32, O 14, P 2), which gives the configuration [Xe] 4f 14 5d 6 6s 2. 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. It has 6 stable isotopes: Os-184, Os-187, Os-188, Os-189, Os-190 and Os-192. No other element has a stable isotope with mass number 190, so Os-190 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) osmium is a solid. It melts at 3033 °C and boils at 5012 °C, so it stays liquid across a 1979-degree range. It stays solid in a welding arc but would melt in the surface of the Sun. That is the 4th highest melting point of any element. Its density is 22.59 g/cm³, 22.6 times as dense as water, the 6th densest element. Its atoms have a radius of about 130 pm (Rhenium next door is 135 pm). In the solid, its atoms pack into a hexagonal close-packed lattice.
Chemical behaviour
Its electronegativity is 2.2 on the Pauling scale. Pulling off its outermost electron takes 840 kJ/mol, more than Ruthenium above it (710 kJ/mol). A bond between osmium and chlorine would be polar covalent, with an electronegativity difference of 0.96. It releases 104 kJ/mol when it gains an extra electron. Its common oxidation states are +4, +8, so in compounds you will usually meet it as Os⁴⁺ and Os⁸⁺. Group 8 also holds Iron, Ruthenium and Hassium, which share its outer-shell pattern. Handle with care: as a pure element it is toxic.
Where it comes from
It makes up 0.0015 mg/kg of Earth's crust, the 76th most abundant element there. It was first identified in 1803, making it the 40th element to be discovered (counting the 11 known since antiquity together).