Alloys are harder than pure metals because their molecular structure prevents the metal atoms from sliding over one another. Conversely, alloys have lower electrical and thermal conductivity than pure metals.
While most alloys are synthetic, in rare instances, they can also occur in nature. Meteoric iron is a natural alloy of iron and nickel, and is the first alloy used by humans. Meteoric iron is the metal component of meteorites; the iron and nickel combine into an alloy due to radioactive processes in space. Another example of a naturally-occurring alloy is electrum, a natural alloy of gold, silver and copper. Its use in jewelry dates back to ancient Greece.
The earliest recorded synthetic alloys are bronze, brass and pewter. Bronze is an alloy of copper and tin, brass is an alloy of copper and zinc, and pewter is a range of alloys consisting of tin and a second component metal. Lead, copper, antimony and bismuth are all used as secondary components of pewter. Steel alloys and mercury amalgam also date back thousands of years.
Not all alloy components are metals, though all alloys do contain at least two metals. All steel alloys contain carbon, as do non-alloy forms of iron such as cast iron and pig iron. These forms of iron are not considered alloys, because the only metal present in them is iron.