Resistance to Oxidation

The Mechanism of Resistance

Metals positioned at the top of the reactivity series, such as potassium and sodium, have a very high tendency to lose electrons. Because they form cations so readily, they are easily oxidized and have an extremely low resistance to oxidation.

The general half-equation for a metal oxidizing shows this loss of electrons:

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

For instance, a highly reactive metal like magnesium loses electrons easily:

$$Mg \rightarrow Mg^{2+} + 2e^-$$

Conversely, metals at the bottom of the series, like gold and platinum, hold onto their electrons very tightly. They have a very low tendency to lose electrons, meaning they possess a high resistance to oxidation. Because these bottom metals do not react with atmospheric oxygen or water, they are found in the Earth's crust as pure, uncombined native metals.

The Aluminium Exception

You might expect aluminium to break down rapidly since it sits high up in the reactivity series, but aluminium objects like window frames can survive decades of harsh weather. Aluminium is indeed a highly reactive metal, but it appears to have a high resistance to corrosion (the gradual destruction of metals by environmental substances).

This happens because aluminium reacts almost instantaneously with oxygen to form a thin, extremely tough layer of aluminium oxide ($Al_2O_3$).

$$4Al(s) + 3O_2(g) \rightarrow 2Al_2O_3(s)$$

Unlike rust, this oxide layer is not porous and does not flake off. It adheres strongly to the surface, acting as a physical barrier that prevents any further oxygen or water from reaching the unreacted metal beneath.

Corrosion vs. Rusting

It is crucial to distinguish between the general breakdown of metals and the specific breakdown of iron. Some metals undergo tarnishing, which is a slow surface oxidation that forms a dark layer of oxide or sulfide, acting as a visible indicator of their resistance to oxidation.

Rusting is a specific term reserved exclusively for the corrosion of iron (and its alloy, steel), and it requires both oxygen and water to occur. When iron oxidizes, the resulting iron(III) oxide is structurally weak, porous, and flaky. It peels away from the surface, continuously exposing fresh iron to the environment until the entire metal structure is completely compromised.

Gold Purity and Practical Applications

Understanding a metal's resistance to oxidation allows jewelers and engineers to choose the right material. Gold is ideal for jewelry and electronic connections because it retains its electrons strongly, remaining shiny, conductive, and entirely resistant to tarnishing.

However, pure gold is very soft and is often alloyed with other metals to increase its strength. The purity of gold is measured in carats, where 24-carat gold is considered completely pure. You can calculate the percentage purity of a gold item using a simple formula.

Worked Example

A jeweler crafts a ring out of 9-carat gold. Calculate the percentage of pure gold in the ring.

Step 1: State the formula for gold purity.

• $\text{Percentage of gold} = \frac{\text{Number of carats}}{24} \times 100$

Step 2: Substitute the given values into the formula.

• $\text{Percentage of gold} = \frac{9}{24} \times 100$

Step 3: Calculate the final percentage.

• $\text{Percentage of gold} = 0.375 \times 100 = 37.5\%$