Iron(III) oxide
Molar mass: 159.69 g/mol · Class: Ionic oxide
Elements: iron, oxygen
Structure
The solid is an ionic lattice with Fe3+ and O2- ions in a 2:3 ratio. The corundum-like crystal structure makes it hard and chemically stable, which is why it persists as rust.
Properties
Red-brown solid, the main component of rust
Insoluble in water; dissolves in strong acids
Weakly magnetic in the hematite form
Stable pigment that does not fade in sunlight
Applications
Red pigment in paints, bricks, and cosmetics
Iron ore (hematite) for steel production
Polishing compound (jeweler's rouge)
Magnetic recording media (maghemite form)
Low toxicity as a solid, but inhaled dust irritates lungs: wear a mask when sanding rust or handling powder. Rust itself does not cause tetanus, but dirty wounds need medical care regardless.
Data source: IUPAC standard atomic weights 2021; molar mass computed from Fe2O3