IB Chemistry SLTopic 2 — From Bonding Models to MaterialsPaper 1 & 2Materials~9 min read
Alloys and Their Properties
Pure metals are often too soft to be useful. Mix in a second element and the whole thing gets harder and stronger — without changing the bonding at all. Alloys are the most practical application of the metallic model you will meet.
📚 What you need to know
An alloy is a mixture of metals, or a metal mixed with a non-metal such as carbon.
The elements are physically combined, not chemically bonded — an alloy is a mixture, not a compound.
Atoms of the second element are spread through the same metallic lattice, still held by delocalised electrons.
Metallic bonding is non-directional, so atoms of different sizes can fit into the lattice.
The different-sized atoms disrupt the regular layers, so the layers can no longer slide easily.
Result: alloys are usually harder and stronger than the pure metals they are made from.
What an alloy actually is
The word “alloy” sounds like a compound, but it isn’t. Nothing new has been made and no new bonds have formed. The second element’s atoms simply sit in among the metal ions, sharing the same sea of delocalised electrons.
This is possible because metallic bonding is non-directional — a metal ion is attracted to the electrons all around it, not in any particular direction. It doesn’t care whether its neighbour is the same size as itself. Compare that with covalent bonding, where bond angles are fixed and you can’t just drop a different atom in.
The red atoms are a second element. They share the same electron sea, but they break up the neat rows.
Why alloys are harder
Remember why a pure metal is malleable: the ions sit in neat layers that slide over one another when a force is applied. That is exactly what an alloy prevents.
Adding atoms of a different size distorts the regular arrangement. Now a sliding layer runs into an atom that is too big or too small to fit the pattern, and the slip is blocked. More force is needed to deform the metal, which is what we mean by “harder” and “stronger”.
In the pure metal the slip plane is clear. In the alloy a differently sized atom sits right in the way.
Notice what has not changed. The metallic bonding is exactly the same, so the alloy still conducts electricity and heat, still has lustre and still has a high melting point. Only the ease of sliding has changed — because only the geometry of the lattice has changed.
Alloys are often improved in other ways too:
More corrosion resistant — stainless steel resists rusting in a way that pure iron never could.
Better at extreme temperatures — important for engine and turbine parts.
Tuned melting point — solder is deliberately alloyed to melt at a low temperature so it can join components without damaging them.
Common alloys
You are not expected to memorise these. They are here so you recognise them if a question uses one as an example:
Alloy
Made from
Key property
Typical uses
Brass
Copper + zinc
Strong, corrosion resistant
Door handles, hinges, musical instruments
Bronze
Copper + tin
Hard, corrosion resistant
Medals, sculptures, ship fittings
Steel
Iron + carbon
Very strong
Construction, bridges, cars
Stainless steel
Iron + chromium + nickel + carbon
Corrosion resistant
Cutlery, surgical instruments, cookware
Solder
Tin + lead
Low melting point
Joining electrical components, jewellery
Steel is the odd one out. Carbon is a non-metal, which shows that an alloy doesn’t have to be metal + metal. The carbon atoms are much smaller than iron atoms, so they slot into the gaps between them and lock the layers in place very effectively — which is why even a small amount of carbon makes iron dramatically stronger.
WORKED EXAMPLE
Explain, in terms of structure, why brass is harder than pure copper.
In pure copper all the ions are the same sizeThey form regular layers that can slide over each other when a force is applied.Brass also contains zinc, whose atoms are a different sizeThese disrupt the regular arrangement of the layers.The distorted lattice makes it harder for layers to slide past one anotherso brass is harder and stronger than pure copper
WORKED EXAMPLE
An alloy still conducts electricity well. Explain why.
The bonding in an alloy is still metallicAdding a second element does not remove the sea of delocalised electrons.Those electrons are still free to move through the latticeso the alloy still conducts electricityOnly the ability of layers to SLIDE has been affected, not the bonding itself.
💡 Exam tip
You don’t need to learn alloy names or compositions — but you must be able to explain why alloys are harder using atomic size and lattice disruption.
If the question names an alloy you’ve never met, use whatever information it gives you. The explanation is always the same.
Say “the layers can no longer slide”, not “the atoms can’t move”. Be specific.
Compare with the pure metal the question asks about, not metals in general.
⚠️ Common mix-up
An alloy is a mixture, not a compound. No new chemical bonds are formed and there is no fixed formula.
The bonding does not change. An alloy is still held together by metallic bonding — don’t say the bonds get stronger.
Harder is not the same as stronger bonds. The hardness comes from geometry blocking the slip planes.
Alloys are not always harder in every way — solder is alloyed specifically to lower its melting point.
Up next: Polymers — leaving metals behind for giant molecules built by joining thousands of small ones together.
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