Rhodium at a glance
- Atomic number
- 45
- Symbol
- Rh
- Atomic mass
- 102.91 u
- Category
- Transition metal
- Group / period
- 9 / 5
- Electron configuration
- [Kr] 4d 8 5s 1
- State at 25 °C
- Solid
- Melting point
- 1964 °C
- Boiling point
- 3695 °C
- Density
- 12.41 g/cm³
- Electronegativity
- 2.28
- 1st ionization energy
- 720 kJ/mol
- Oxidation states
- +3
- Discovered
- 1803
Atomic structure
A neutral rhodium atom has 45 protons and 45 electrons, and its most common isotope carries about 58 neutrons. One atom weighs about 1.71 × 10⁻²⁵ kg, so a mole of rhodium (6.022 × 10²³ atoms) weighs 102.91 g. That nucleus holds 1.29 neutrons for every proton. The electrons fill 5 shells (K 2, L 8, M 18, N 16, O 1), which gives the configuration [Kr] 4d 8 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 only one stable isotope, Rh-103. No other element has a stable isotope with mass number 103, so Rh-103 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) rhodium is a solid. It melts at 1964 °C and boils at 3695 °C, so it stays liquid across a 1731-degree range. It stays solid in molten iron (1538 °C) but would melt in a welding arc. Its density is 12.41 g/cm³, 12.4 times as dense as water. Its atoms have a radius of about 135 pm (Ruthenium next door is 130 pm). In the solid, its atoms pack into a face-centred cubic lattice.
Chemical behaviour
Its electronegativity is 2.28 on the Pauling scale. Pulling off its outermost electron takes 720 kJ/mol, less than Cobalt above it (760 kJ/mol). A bond between rhodium and chlorine would be polar covalent, with an electronegativity difference of 0.88. It releases 110 kJ/mol when it gains an extra electron. Its common oxidation states are +3, so in compounds you will usually meet it as Rh³⁺. Group 9 also holds Cobalt, Iridium and Meitnerium, which share its outer-shell pattern.
Where it comes from
It makes up 0.001 mg/kg of Earth's crust, the 78th most abundant element there. It was first identified in 1803, making it the 37th element to be discovered (counting the 11 known since antiquity together).