What couse the rusting in metals?

The rusting of metals occurs when iron, oxygen, and water combine.When they combine, the iron loses electrons to oxygen atoms. This is called oxidation and when it happens it produces a chemical reaction forming Fe203 Iron Oxide otherwise known as Ferric Oxide, or more commonly known as rust.

it forms an anion that has a larger radius. anions have a negative charge and are larger than the original atom.