Bronze was the first successful metal alloy. Its properties are significantly different from the pure metals, copper and tin. Several different metals can be mixed with copper to make a useful alloy. Tin has been the most commonly used alloying element, but arsenic, antimony and lead have also been used.
Bronze is a mixture of elements, not a compound, so in theory any proportions can be made.
Bronze is harder than copper, making it useful for tools and weapons. It can be sharpened. Bronze melts at a lower temperature than pure copper. It also flows better into molds. Since it is less likely that bubbles will be trapped, the cast bronze object will be harder than cast copper.
The microscopic structure of bronze explains some of the observed properties. Small amounts of tin can be incorporated into a copper lattice as substitutional impurities. Since tin atoms are larger than copper atoms, it is hard for planes of atoms to slip past them. This reduces the flexibility of the metal. Large amounts of tin in a sample will create precipitates, having a crystal structure distinct from that of the main lattice. This type of discontinuity also makes it hard for planes of atoms to slip. 10% tin in copper is a popular ratio that results in excellent properties. Due to the inhomogeneous crystal lattice, bronze alloys have lower electrical conductivity than pure copper.
Bronze was the most important material in the world but was eventually superseded by iron, which was more common but more difficult to work with. Historians have theorized that the Bronze Age ended because tin supplies became scarce.