Edexcel GCSE Chemistry (1CH0) — Extracting Metals and Equilibria
You can dig up ancient gold coins that are perfectly shiny, yet an iron tool left outside can turn into a crumbling orange mess within a few months. This dramatic difference is governed by a metal's resistance to oxidation, which is its ability to remain in an elemental state without reacting with environmental oxygen.
There is a direct inverse relationship between a metal's position in the reactivity series and its resistance to oxidation. Oxidation occurs when a substance loses electrons during a chemical reaction. Therefore, a metal that is described as "more reactive" is simply one that loses its outer shell electrons more easily to form positively charged ions called cations.
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:
For instance, a highly reactive metal like magnesium loses electrons easily:
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.
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 ().
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.
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.
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.
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.
Step 2: Substitute the given values into the formula.
Step 3: Calculate the final percentage.
Students often state that aluminium does not corrode because it is unreactive. You must specify that aluminium is highly reactive, but is protected by a tough, impermeable surface oxide layer.
In 'explain' questions about metal reactivity, examiners explicitly award marks for linking a metal's higher position in the reactivity series directly to a greater ease of losing electrons to form cations.
Never use the term 'rusting' to describe the breakdown of any metal other than iron or steel; always use 'corrosion' or 'oxidation' for other metals like copper or aluminium.
Resistance to oxidation
The ability of a metal to remain in its elemental metallic state without reacting with environmental oxygen to form an oxide.
Reactivity series
A list of metals arranged in order of their ability to lose electrons and form cations.
Oxidation
The loss of electrons by a substance during a chemical reaction.
Cation
A positively charged ion formed when a metal atom loses one or more electrons.
Native metals
Metals found in nature as uncombined elements due to their extreme resistance to oxidation.
Corrosion
The gradual destruction of metals by chemical reactions with substances in the environment.
Tarnishing
The slow surface oxidation of a metal that forms a dark layer of oxide or sulfide.
Rusting
The specific corrosion of iron or steel, which strictly requires both oxygen and water to occur.
Put your knowledge into practice — try past paper questions for Chemistry
Resistance to oxidation
The ability of a metal to remain in its elemental metallic state without reacting with environmental oxygen to form an oxide.
Reactivity series
A list of metals arranged in order of their ability to lose electrons and form cations.
Oxidation
The loss of electrons by a substance during a chemical reaction.
Cation
A positively charged ion formed when a metal atom loses one or more electrons.
Native metals
Metals found in nature as uncombined elements due to their extreme resistance to oxidation.
Corrosion
The gradual destruction of metals by chemical reactions with substances in the environment.
Tarnishing
The slow surface oxidation of a metal that forms a dark layer of oxide or sulfide.
Rusting
The specific corrosion of iron or steel, which strictly requires both oxygen and water to occur.
