Metals, ceramics and polymers
Chemistry · Key Stage 3 · The School
Three families
METALS are shiny, conduct heat and electricity, and bend rather than shatter because their atoms sit in layers that can slide. CERAMICS — pottery, glass, brick — are hard, heat-resistant and brittle, because their rigid bonded structure cannot slide and so cracks instead. POLYMERS are long chains, usually flexible, light and poor conductors.
What a polymer is
A POLYMER is a very long molecule made by joining many small identical units, called monomers, end to end. Polythene is thousands of ethene units chained together. The chains' length and how much they tangle or cross-link decide whether the result is a flexible bag, a rigid pipe or a car tyre — same idea, very different materials.
Alloys
An ALLOY is a metal mixed with another element. Adding a different-sized atom disrupts the neat layers so they cannot slide as easily, which makes the alloy HARDER than the pure metal. Steel is iron with a little carbon; bronze is copper with tin; brass is copper with zinc. Almost every metal in daily use is an alloy rather than a pure element.
Pure copper is soft enough to scratch with a fingernail, because its identical atoms form neat layers that slide over one another. Add a little zinc and you have brass: the different-sized atoms disrupt the layers and stop them sliding, so it is far harder. Same principle in steel (iron plus carbon) and bronze (copper plus tin). Look at the pattern — every hard-wearing 'metal' object you own is an alloy.