The function of patina as a protective layer can also be seen in the Statue of Liberty.
According to the Copper Development Association, the oxidation of the statue's copper skin is only about 0.005 inch till now.
Another example is the oxidation of aluminum, leading to formation of white, flaky layer.
Copper(II) oxide and magnesium oxide are both ionic. If you rewrite this as an ionic equation, it turns out that the oxide ions are spectator ions and you are left with: A last comment on oxidising and reducing agents If you look at the equation above, the magnesium is reducing the copper(II) ions by giving them electrons to neutralise the charge. Looking at it the other way round, the copper(II) ions are removing electrons from the magnesium to create the magnesium ions.It is to be noted that copper does not react with water.The outer green layer, formed after the oxidation of copper, is known as patina.In the above example, the iron(III) oxide is the oxidising agent. In the equation, the carbon monoxide is the reducing agent.Notice that these are exactly the opposite of the oxygen definitions.