- In a pure metal:
- All atoms are the same size.
- They are arranged in regular, closely packed layers (a metallic lattice).
- Positive metal ions are surrounded by a “sea” of delocalised electrons that hold the lattice together.
- Because the atoms are all identical and neatly arranged, the layers can slide easily over each other when a force is applied.
- That’s why many pure metals (like pure copper or pure iron) are relatively soft and malleable.
- An alloy is a mixture of a metal with:
- another metal, or
- a non-metal (often a small atom like carbon).
- When we add these new atoms into the metallic lattice:
- The added atoms are usually a different size from the main metal atoms (either larger or smaller).
- This distorts the regular layers.
- The layers can no longer slide past each other as easily → the metal becomes harder and stronger.
Key idea: Alloying ≈ “messing up” the perfect layers, so they can’t slip.
- The added atoms are similar in size to the host metal atoms.
- They replace some of the original metal atoms in the lattice.
- Brass = copper + zinc.
- Some Cu atoms are substituted by Zn atoms.
- Zn atoms are a different size, so they distort the lattice and make sliding more difficult.
- The added atoms are small and fit into the spaces (interstices) between metal atoms.
- This significantly blocks movement of the layers.
- Steel = iron + carbon.
- Small C atoms sit in the gaps between Fe atoms.
- This makes steel much harder and stronger than pure iron.
Changing the atomic arrangement changes the properties.
- In pure metals, layers of identical atoms slide easily → softer, more malleable.
- In alloys, distorted layers can’t move as easily → harder and stronger.
So:
- Brass (Cu + Zn) is harder and stronger than pure copper.
- Steel (Fe + C) is harder and stronger than pure iron.
Some alloys are designed to resist corrosion.
- Stainless steel = iron + carbon + chromium (and sometimes nickel).
- Chromium forms a thin, strong, adherent oxide layer on the surface.
- This oxide layer protects the metal underneath from rusting.
Compare:
- Pure iron: rusts easily (iron(III) oxide forms and flakes off).
- Stainless steel: much more corrosion-resistant, used in cutlery, sinks, medical tools.
- Density: Because alloy atoms have different masses and sizes, the density of an alloy can be different from its pure metal.
- Brass has a slightly different density from pure copper.
- Solder (tin + lead) has a different density from either pure tin or pure lead.
- Electrical conductivity:
- Pure metals: very good conductors (regular lattice, electrons move easily).
- Alloys: usually lower conductivity than the pure metal.
- Distortions in the lattice scatter electrons, so they don’t flow as freely.
Important: Alloys still conduct, but generally less well than pure metals.
Suppose brass is 70% copper and 30% zinc by mass.
- Molar mass of Cu ≈ 63.55 g mol⁻¹
- Molar mass of Zn ≈ 65.38 g mol⁻¹
Average atomic mass of atoms in the alloy (per “average atom”):
$$\begin{gathered}
M_{\mathrm{avg}}=0.70 \times 63.55+0.30 \times 65.38 \\
M_{\mathrm{avg}} \approx 44.485+19.614 \approx 64.10 \mathrm{~g} \mathrm{~mol}^{-1}
\end{gathered}$$
So the “average” atom in this brass alloy has a molar mass of about 64.1 g mol⁻¹, slightly higher than pure copper.
You don’t need this level of detail for basic bonding, but it’s a nice link to quantitative chemistry.
Alloys let us tune the properties of a metal to match a job. We almost never want a metal “as it comes” – we want:
- stronger,
- lighter,
- more corrosion-resistant,
- sometimes cheaper or easier to shape.
- Structure effect
- Zn atoms substitute for Cu → distorted lattice → harder, more durable.
- Still has delocalized electrons → good conductor.
- Uses
- Musical instruments (trumpets, saxophones) – good acoustic properties and corrosion resistance.
- Plumbing fittings – doesn’t corrode quickly in water.
- Decorative items – attractive gold-like colour, doesn’t tarnish as quickly as pure copper.
- Structure effect
- C atoms in interstices → interstitial alloy → very strong and hard.
- Different steels adjust carbon and other elements (Cr, Ni, Mn) to tune hardness, toughness, and corrosion resistance.
- Uses
- Building frameworks (beams, girders).
- Car bodies and machinery.
- Tools and knives.
- Bridges, pipelines, ships.
- Structure effect
- Chromium forms a protective oxide film that prevents rust.
- Strong, hard, corrosion-resistant.
- Uses
- Cutlery and kitchen equipment.
- Medical instruments.
- Chemical plants and food processing equipment.
Alloys are crucial because they:
- Improve safety – stronger steels in bridges, cars, and buildings reduce failures.
- Increase durability – corrosion-resistant alloys last longer, reducing waste and cost.
- Enable technology – specialized alloys in electronics, turbines, batteries, and medical implants.
- Support sustainability – longer-lasting materials mean fewer replacements and less resource use.
In short, alloy design is a big part of materials science and green chemistry:
we want materials that are strong, safe, and as environmentally friendly as possible.
- What changes in the atomic arrangement when a pure metal becomes an alloy?
- Why are alloys usually harder and stronger than the pure metals they are made from?
- How does an interstitial alloy differ from a substitutional alloy? Give one example of each.
- Why is stainless steel more resistant to corrosion than pure iron?
- Why do alloys usually have lower electrical conductivity than pure metals?
- Choose one alloy (e.g. brass, bronze, steel, stainless steel) and explain how its structure makes it suitable for its main uses.