📘 Definition
An alloy is a mixture of a metal with one or more other elements (usually metals or carbon). The atoms of different sizes disrupt the regular arrangement of the lattice.
Types of Alloy
Substitutional Alloy
Atoms of the added element are similar in size to the host metal and replace some host atoms in the lattice.
E.g. Bronze (Cu + Sn), Brass (Cu + Zn)
Interstitial Alloy
Atoms of the added element are much smaller and fit into the gaps (interstices) between host metal atoms.
E.g. Steel (Fe + C) – carbon atoms fill gaps in the iron lattice
Why Alloys Are Harder
In a pure metal, identical-sized atoms are arranged in regular layers that can slide easily. Introducing atoms of different sizes destroys the regular packing. The layers can no longer slide smoothly past each other → the alloy is harder and less malleable than the pure metal.
For full marks: You must explicitly state that different-sized atoms prevent the layers of cations from sliding over each other. Simply saying "it disrupts the lattice" is not enough.
⚠️ Examiner Trap
Alloys are not compounds. They are mixtures. No chemical bonds form between the two metals. They still have metallic bonding throughout. Also, alloying typically decreases electrical conductivity because the irregular lattice scatters electron flow.