IB Chemistry SLTopic 2 — From Models to MaterialsPaper 1 & 2Materials~10 min read
Alloys and Their Properties
Almost no metal object you own is made of a pure metal. Pure copper is too soft for a door handle, pure iron bends and rusts, pure gold scratches with a fingernail. The fix is to deliberately spoil the metal’s neat internal arrangement — and the reason that works comes straight out of the metallic bonding model.
📘 What you need to know
An alloy is a mixture of a metal with one or more other elements — usually another metal, sometimes a non-metal such as carbon.
The elements are physically mixed, not chemically bonded into a compound, so an alloy has no fixed formula.
Metallic bonding is non-directional, so atoms of a different size can slot into the lattice without breaking anything.
The delocalised electrons are still there, so an alloy still conducts electricity and heat.
Different-sized atoms disrupt the regular layers, so the layers can no longer slide over each other easily. The alloy is harder and stronger, and less malleable.
Alloys can also be more corrosion resistant or better at surviving extreme temperatures.
You do not need to memorise compositions. You do need to explain hardness using atomic size and lattice disruption.
What an alloy actually is
Start with what an alloy is not. It is not a compound. When you mix zinc into molten copper, no electrons are transferred and no new substance with a fixed formula is created. You end up with one metallic lattice containing two kinds of cation, sharing one pool of delocalised electrons. That is why brass is written as “about 70% copper, 30% zinc” rather than as a formula — you can dial the ratio to whatever the job needs.
Making one is crude in principle: melt the metals together, stir, let it cool. Because the mixing happens in the liquid, the second element ends up spread evenly through the solid lattice rather than sitting in lumps.
The word “mixture” is doing real work here. If an exam question asks whether an alloy is a compound, the answer is no — and the reason is that the components keep their own identity and can be present in any proportion.
Why metallic bonding allows it
This is the bit worth understanding properly. In a covalent solid, bonds point in fixed directions at fixed angles — push a foreign atom in and you snap something. In an ionic lattice, positions alternate by charge, so a wrongly sized or wrongly charged ion wrecks the pattern.
Metallic bonding has no such fussiness. Each cation is simply attracted to the delocalised electrons all around it, in every direction equally. Swap one cation for a bigger or smaller one and the attraction still works. That single property — non-directional bonding — is what makes the entire field of alloys possible.
The second element does not need to fit perfectly. Because metallic bonding pulls equally in all directions, an atom of the wrong size is tolerated — it just leaves the rows slightly bent.
Why alloys are harder
Malleability in a pure metal comes from the layers. Because every cation is identical and the rows are regular, a whole layer can shuffle sideways past its neighbour and immediately find a new, equally comfortable position. Hammer a pure metal and the layers slip; the metal changes shape instead of cracking.
Now jam in some atoms of a different size. The rows are no longer flat and evenly spaced, so a layer trying to slide has to climb over the obstruction. Far more force is needed for the same movement, which is exactly what “harder” and “stronger” mean at the atomic level.
This is the diagram to reproduce in an exam answer. Regular rows and a sliding arrow on one side; a mis-sized atom and a blocked arrow on the other.
What does not change
Conductivity. The delocalised electrons are untouched, so alloys still conduct. In practice they conduct slightly less well than the pure metal, because the uneven lattice scatters the moving electrons, but they are still good conductors.
The type of bonding. It is still metallic. On the bonding triangle an alloy sits in or near the metallic corner, because both elements have low electronegativities and the difference between them is small.
The state of matter and general look. An alloy is a solid metal with a metallic lustre, just a tougher one.
One thing that often does change: the melting point. Solder is chosen precisely because a tin–lead mixture melts lower than either pure metal, which is what lets you join components without cooking them.
Alloys worth recognising
Alloy
Made from
Useful property
Typical use
Brass
Copper and zinc
Hard, resists corrosion, works well as a casting
Door fittings, instrument parts
Bronze
Copper and tin
Hard and corrosion resistant
Sculpture, medals, bearings
Steel
Iron with a little carbon
Very strong under load
Construction, bridges, vehicles
Stainless steel
Iron, chromium, nickel, carbon
Forms a protective oxide layer, so it resists rust
Cutlery, surgical tools, cookware
Solder
Tin and lead (or tin and silver)
Melts at a low temperature
Joining electrical components
Do not spend revision time memorising this table. Exam questions hand you the composition and ask you to explain the property — the marks are in the explanation, not in the recall.
Worked examples
WORKED EXAMPLE
Brass is harder than pure copper. Explain why, in terms of structure and bonding. [3]
Mark 1: describe the pure metal
In copper the cations are all the same size and sit in regular layers, so layers slide over each other easily.
Mark 2: describe the change
Zinc atoms are a different size, and they are spread through the lattice, so the layers are no longer regular.
Mark 3: link to the property
The layers can no longer slide past each other easily, so more force is needed to deform the metal.
Brass is harder because the disrupted lattice resists layers slippingnotice all three marks are about layers sliding. That is the whole answer.
WORKED EXAMPLE
Explain why brass still conducts electricity even though it is a mixture of two metals.
Step 1: what conduction needs
Charged particles that are free to move through the solid.
Step 2: what the mixing did and did not change
Both copper and zinc release outer electrons into the shared pool, so the sea of delocalised electrons is still there. Only the arrangement of cations changed.
Delocalised electrons still move through the lattice, so brass conductsa small drop in conductivity is expected, because the uneven lattice scatters the electrons
WORKED EXAMPLE
A solder is 60.0% tin and 40.0% lead by mass. What percentage of the atoms in the lattice are tin?
Use M(Sn) = 118.71 g mol−1 and M(Pb) = 207.2 g mol−1. Work with 100 g of solder.
Step 1: moles of each metal in 100 gn(Sn) = 60.0 ÷ 118.71 = 0.5054 moln(Pb) = 40.0 ÷ 207.2 = 0.1931 molStep 2: total and fractiontotal = 0.5054 + 0.1931 = 0.6985 mol0.5054 ÷ 0.6985 = 0.723672.4% of the atoms are tin (3 s.f.)by mass it was 60%, by atoms it is 72% — lead atoms are heavier, so fewer of them are needed to make up 40 g
💡 Exam tip
The magic words are different sizes, layers and slide. If your answer contains all three linked together, you are almost certainly on full marks.
Say the alloy is a mixture, and that the bonding is still metallic. Both are commonly credited.
If asked for a diagram, draw two lattices side by side with clearly different circle sizes in the alloy. Label the sliding layers.
Link every property back to a use: harder → door handles and tools; corrosion resistant → cutlery; low melting point → solder.
Non-directional bonding is a strong phrase to use when explaining why atoms of a different size can be accommodated at all.
If given percentages by mass and asked about atoms, convert to moles first. Mass percentage and atom percentage are not the same number.
⚠ Common mix-up
Calling an alloy a compound. There is no chemical reaction and no fixed formula — it is a mixture held together by metallic bonding.
Saying alloys are harder because they have stronger bonds. They do not. The bonding is the same; the geometry is worse.
Claiming alloys do not conduct. They do. The delocalised electrons never went anywhere.
Drawing the second element sitting on top of the lattice. The foreign atoms sit inside the lattice, replacing or squeezing between the original cations.
Confusing hard with strong with brittle. Alloys are harder and less malleable; that is not the same as being easy to snap.
Writing about “molecules” in a metal. There are no molecules — only cations and a sea of electrons.
Up next: Polymers — we leave the metallic corner and head to the covalent one, where the useful trick is not mixing atoms but joining thousands of small molecules into one enormous chain.
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