The properties of a metal change substantially when you mix it with another metal or with a nonmetal, to form an alloy. Bronze, a mixture of copper and tin, was the first alloy to be used widely. Probably by accident, an ancient coppersmith mixed a little tin together with copper (tin deposits are found near copper deposits) and discovered that the resulting alloy was harder and stronger the either metal alone, and that the alloy could hold an edge. For half a millennium, a period historians call the Bronze Age (ca. 2500-2000 B.C.E), people used bronze to make swords, battle-axes, and metal-tipped plows and spades.

We can understand why bronze behaves differently from copper by comparing their respective crystal structures. Recall that copper crystals consists of wafer-like layers of atoms that slip easily past on another. Tin atoms don’t fit perfectly into the lattice of the copper crystals, so their presence prevents slippage between planes.

Steel, the most widely used metal today, is also an alloy, a mixture of iron and carbon. The properties of steel closely reflect the carbon content; the more carbon, the harder the steel. Mixing chrome with steel produces another alloy, stainless steel, which resists corrosion.

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