Edexcel GCSE Chemistry (1CH0) — Transition Metals, Quantitative Analysis and Equilibria
You can easily bend a pure copper pipe, but try bending a brass door handle made from the same metal mixed with zinc. This dramatic change in strength comes down to the microscopic arrangement of particles inside the material.
Pure metals consist of a giant lattice entirely made of atoms (or ions) of the exact same size. Because pure metals lack variation in atomic size, their uniform particles arrange into perfectly regular, repeating layers.
When a physical force is applied, these even layers easily slide over each other. This sliding mechanism is what gives pure metals their malleability and ductility.
An alloy, however, is a mixture of a metal with at least one other element. The crucial difference is that the added element has a different atomic radius (size) compared to the original metal.
For example, pure iron consists of atoms with a radius of . When smaller carbon atoms (radius ) are added to make steel, these differently sized atoms cause , breaking up the neat arrangement.
Because the structure is now uneven, the layers of atoms can no longer slide over each other easily. This inability of the layers to slip past one another drastically increases the hardness of the material and makes the alloy significantly less malleable than the pure metal.
How do engineers actually prove that a new alloy is stronger than a pure metal? They use a standardised physical test called the indentation method to directly compare their properties.
To conduct this investigation, follow these steps:
When comparing the data, the pure metal will display a noticeably deeper and wider indentation than the alloy.
This provides macroscopic evidence of the microscopic structure. In the pure aluminium, the regular layers of identical atoms slipped under the pressure of the weight, permanently changing the metal's shape. In the duralumin alloy, the distorted lattice successfully prevented the atomic layers from sliding, resisting the force. This resistance results in a much shallower dent, proving the alloy is harder and less malleable.
Students often state that alloys are harder because "the bonds are stronger." This is incorrect and scores zero marks; you must explain that the structure is distorted, preventing the atomic layers from sliding.
In Edexcel "Explain" questions about alloys, always use the specific words "atoms" or "ions" — mark schemes frequently penalise the vague term "particles".
When describing the indentation practical, ensure you explicitly state your control variables (such as dropping the same mass from the exact same height) to prove your method is a fair test.
To get full marks for why an alloy is harder, you must mention both the different size of the added atoms and the resulting inability of layers to slide.
Atoms
The basic building blocks of a substance; pure metals consist of a giant lattice of identical atoms.
Ions
An atom that has lost or gained electrons, becoming electrically charged; metal lattices consist of positive ions in a sea of delocalised electrons.
Malleability
The ability of a material to be bent, hammered, or rolled into thin sheets without shattering.
Ductility
The ability of a substance to be drawn out into a long, thin wire.
Alloy
A mixture of two or more elements, where at least one of those elements is a metal.
Atomic radius
The distance from the center of an atom's nucleus to its outermost electron shell, representing the size of the atom.
Lattice distortion
The disruption of the regular, layered arrangement of atoms in a metal, caused by the introduction of atoms of different sizes.
Hardness
A measure of how resistant solid matter is to permanent shape change (such as indentation) when a compressive force is applied.
Giant lattice
A large-scale 3D structure of atoms or ions bonded together in a regular, repeating pattern.
Put your knowledge into practice — try past paper questions for Chemistry
Atoms
The basic building blocks of a substance; pure metals consist of a giant lattice of identical atoms.
Ions
An atom that has lost or gained electrons, becoming electrically charged; metal lattices consist of positive ions in a sea of delocalised electrons.
Malleability
The ability of a material to be bent, hammered, or rolled into thin sheets without shattering.
Ductility
The ability of a substance to be drawn out into a long, thin wire.
Alloy
A mixture of two or more elements, where at least one of those elements is a metal.
Atomic radius
The distance from the center of an atom's nucleus to its outermost electron shell, representing the size of the atom.
Lattice distortion
The disruption of the regular, layered arrangement of atoms in a metal, caused by the introduction of atoms of different sizes.
Hardness
A measure of how resistant solid matter is to permanent shape change (such as indentation) when a compressive force is applied.
Giant lattice
A large-scale 3D structure of atoms or ions bonded together in a regular, repeating pattern.
