Why is rust different from the iron and oxygen that it is made of

For the same generic reason that table salt doesn't taste or behave

like the corrosive sodium or the poisonous chlorine that it's made of.

The physical and chemical properties of compounds in general are

very different from the characteristics of any of their constituent

elements in the pure elemental state.

- - Water doesn't act anything like Hydrogen or Oxygen.

- - A candle doesn't act anything like Carbon or Hydrogen.

- - Dry ice doesn't act anything like Carbon or Oxygen.

- - DNA doesn't act anything like Hydrogen, Oxygen, Nitrogen,

Carbon or Phosphorus. (We are lucky.)