Resistance to Oxidation

$2Mg + O_2 \rightarrow 2MgO$

Oxidation can also be described in terms of electrons. Metal atoms form positive ions by losing electrons:

$M \rightarrow M^{n+} + ne^-$

These descriptions are connected: when a metal forms an ionic oxide, its atoms have lost electrons to oxygen. A metal that loses electrons readily is therefore oxidised readily.

Connecting oxidation to the reactivity series

The reactivity series places metals in order of their tendency to react and form positive ions. In this course, the order is:

potassium, sodium, calcium, magnesium, aluminium, (carbon), zinc, iron, (hydrogen), copper, silver, gold

Carbon and hydrogen are included as useful reference points, but they are not metals.

Metals near the top lose electrons and become positive ions more readily. They are more reactive, are oxidised more easily and have less resistance to oxidation. Metals near the bottom lose electrons less readily. They are less reactive and have greater resistance to oxidation.

This is a relative comparison. For example, iron is more resistant to oxidation than magnesium, but less resistant than copper. Saying that one metal is “more resistant” does not necessarily mean that it can never be oxidised; it means that it is less easily oxidised than the metal being compared with it.

Using oxygen transfer to compare metals

A more reactive metal can remove oxygen from the oxide of a less reactive metal. When heated, iron reacts with copper(II) oxide:

$iron + copper\ oxide \rightarrow iron\ oxide + copper$

Iron gains oxygen, so iron is oxidised. Copper oxide loses oxygen, so it is reduced. This reaction shows that iron has a greater tendency to be oxidised than copper, placing iron above copper in the reactivity series. Copper is therefore more resistant to oxidation than iron.

Iron does not remove oxygen from magnesium oxide under the same sort of test. Magnesium holds oxygen more strongly because magnesium is more reactive and more readily oxidised than iron. The observations therefore place the metals in the order:

$magnesium > iron > copper$

Moving from magnesium towards copper in this order, reactivity and ease of oxidation decrease, while resistance to oxidation increases.

Making a complete explanation

To explain the relationship, link the ideas in a chain:

1. A metal higher in the reactivity series has a greater tendency to form positive ions.
2. Forming positive ions involves losing electrons, so the metal is oxidised more readily.
3. It therefore has lower resistance to oxidation.
4. A metal lower in the series loses electrons less readily, so it has greater resistance to oxidation.

This explains why highly reactive metals are commonly found in compounds, whereas very unreactive metals such as gold may occur as uncombined elements in the Earth’s crust.