Resources | Subject Notes | Chemistry
This page explains why aluminium appears unreactive in many chemical situations, focusing on the formation of a protective oxide layer. This is a key concept in understanding the reactivity of metals.
The reactivity series is a list of metals arranged in order of their reactivity. A metal higher up the series is more reactive than a metal lower down. For example, potassium is more reactive than copper.
A simplified reactivity series is shown below:
| Metal | Reactivity |
|---|---|
| Sodium (Na) | Very High |
| Magnesium (Mg) | High |
| Aluminium (Al) | Medium |
| Iron (Fe) | Low |
| Copper (Cu) | Very Low |
| Zinc (Zn) | Medium |
| Hydrogen (H) | Reference |
This series shows that sodium and magnesium are more reactive than aluminium, while iron, copper, and zinc are less reactive.
Aluminium (Al) is a metal that is not as reactive as many other metals. This is because it readily reacts with oxygen in the air to form a thin, tough, and adherent layer of aluminium oxide ($Al_2O_3$).
When aluminium is exposed to air, the aluminium atoms react with oxygen to form aluminium oxide:
$$4Al(s) + 3O_2(g) \rightarrow 2Al_2O_3(s)$$
This aluminium oxide layer is very hard and tightly adheres to the surface of the aluminium. It acts as a barrier, preventing further reaction between the aluminium and the environment.
The aluminium oxide layer protects the underlying metal from rusting or other forms of corrosion. This is why aluminium is used in many applications where resistance to corrosion is important, such as in cans, aircraft, and buildings.
When other metals are present, the aluminium oxide layer will corrode preferentially, protecting the underlying metal. This is known as galvanic protection.
Because the aluminium oxide layer is so protective, aluminium appears unreactive in many situations. It will not readily react with acids, bases, or other corrosive substances. The oxide layer must be damaged or removed for the aluminium to become reactive.
The apparent unreactivity of aluminium is due to the formation of a protective aluminium oxide layer. This layer prevents further reaction between the aluminium and its environment, making it a very useful metal in many applications.