Layers in a pure metal
Definition
Metallic bonding
The electrostatic attraction between the positive metal ions in a giant lattice and the sea of delocalised electrons surrounding them.
- A pure metal is a giant lattice of positive metal ions arranged in regular layers.
- The ions are surrounded by a sea of delocalised electrons that are free to move through the whole structure.
- Metallic bonding holds the structure together, and it acts in every direction around each ion.
- In a pure metal all of the atoms are the same size, so the layers are flat and evenly stacked.
- When a force is applied, whole layers can slide over each other into new positions.
- The metallic bonding is not broken by this sliding, because the delocalised electrons move with the ions and keep holding them together.
- This is why pure metals bend and stretch instead of shattering, and why many of them are too soft for everyday use.
Why alloys are harder and stronger
Definition
Hardness
The ability of a material to resist being scratched or dented.
- In an alloy, the atoms of the added element are a different size from the atoms of the main metal.
- These different-sized atoms disrupt the regular arrangement of the layers, so the layers are no longer flat and even.
- The disrupted layers can no longer slide over each other easily, because the different-sized atoms get in the way.
- A much larger force is needed to change the shape of the alloy than to change the shape of the pure metal.
- The alloy is therefore harder, because its surface resists denting and scratching, and stronger, because it can withstand a larger force before it changes shape or breaks.
- The same idea explains the hardness of the common alloys.
- In steel, small carbon atoms sit in the gaps between the much larger iron atoms and lock the layers in place.
- In brass, zinc atoms, which are a slightly different size from copper atoms, take the place of some copper atoms and break up the flat rows.
- In stainless steel, chromium and nickel atoms of different sizes are spread through the iron, while carbon atoms sit in the gaps.
Example
Pure iron compared with steel
- In pure iron, every atom is the same size, so the layers are flat and regular.
- A hammer blow makes these layers slide, so a bar of pure iron bends easily.
- In steel, carbon atoms are much smaller than iron atoms and sit between them.
- The carbon atoms distort the layers and stop them sliding, so the same hammer blow barely dents a steel bar.
Exam technique
Writing the structure explanation
- Mark schemes for this explanation look for two linked ideas: the atoms in an alloy are different sizes, and the layers cannot slide over each other.
- Use the word layers, and never describe a metal as being made of molecules.
- Join the two ideas with "so", for example "the different-sized atoms disrupt the layers, so the layers cannot slide over each other".
- If the question asks about hardness or strength, finish by stating that more force is needed to change the shape of the alloy.
Common Mistake
- Do not say that the bonds in an alloy are stronger, because the explanation depends on the layers being unable to slide.
- Do not describe metals or alloys as containing molecules, because they are giant structures.
- Do not say that the layers "cannot move at all", because they can still slide if a large enough force is used.
Self review
- What is metallic bonding?
- Why can the layers in a pure metal slide over each other?
- What effect do different-sized atoms have on the layers in an alloy?
- Why is an alloy harder than the pure metal it is made from?
- Why is steel harder than pure iron?
