Iridium at a glance
- Atomic number
- 77
- Symbol
- Ir
- Atomic mass
- 192.22 u
- Category
- Transition metal
- Group / period
- 9 / 6
- Electron configuration
- [Xe] 4f 14 5d 7 6s 2
- State at 25 °C
- Solid
- Melting point
- 2446 °C
- Boiling point
- 4428 °C
- Density
- 22.56 g/cm³
- Electronegativity
- 2.2
- 1st ionization energy
- 880 kJ/mol
- Oxidation states
- +3, +4
- Discovered
- 1803
Atomic structure
A neutral iridium atom has 77 protons and 77 electrons, and its most common isotope carries about 115 neutrons. One atom weighs about 3.19 × 10⁻²⁵ kg, so a mole of iridium (6.022 × 10²³ atoms) weighs 192.22 g. That nucleus holds 1.49 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 15, P 2), which gives the configuration [Xe] 4f 14 5d 7 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 2 stable isotopes: Ir-191 and Ir-193. Its most common isotope, Ir-192, has the same mass number as Os-192 and Pt-192: isobars, nuclei of different elements with 192 nucleons each.
Physical properties
At room temperature (25 °C) iridium is a solid. It melts at 2446 °C and boils at 4428 °C, so it stays liquid across a 1982-degree range. It stays solid in molten iron (1538 °C) but would melt in a welding arc. That is the 8th highest melting point of any element. Its density is 22.56 g/cm³, 22.6 times as dense as water, the 7th densest element. Its atoms have a radius of about 135 pm (Osmium next door is 130 pm). In the solid, its atoms pack into a face-centred cubic lattice.
Chemical behaviour
Its electronegativity is 2.2 on the Pauling scale. Pulling off its outermost electron takes 880 kJ/mol, more than Rhodium above it (720 kJ/mol). A bond between iridium and chlorine would be polar covalent, with an electronegativity difference of 0.96. It releases 151 kJ/mol when it gains an extra electron. Its common oxidation states are +3, +4, so in compounds you will usually meet it as Ir³⁺ and Ir⁴⁺. Group 9 also holds Cobalt, Rhodium and Meitnerium, which share its outer-shell pattern.
Where it comes from
It makes up 0.001 mg/kg of Earth's crust, the 80th most abundant element there. It was first identified in 1803, making it the 41st element to be discovered (counting the 11 known since antiquity together).