The properties of metals are high thermal and electrical conductivity, malleability and ductility.
A metallic crystal can be pictured as containing spherical atoms packed together and bonded to each other equally in all directions. The closest packing model describes this by packing uniform, hard spheres in a way that most efficiently uses the available space. Shapes of most pure metals can be changed relatively easily but most metals are durable and have high melting points, showing that bonding in most metals is strong and nondirectional. Even though it is difficult to separate metal atoms, it is easy to move them.
Due to the nature of the structure and bonding of metals, other elements can be introduced into a metallic crystal to produce substances called alloys. An alloy is a substance that contains a mixture of elements and has metallic properties. There are two types of alloys.
In a substitutional alloy some of the main metal atoms are replaced by other metal atoms of similar size. An example is brass where one-third of the atoms of the host copper are replaced with zinc atoms.
An interstitial alloy is formed when some of the holes in the closest packed metal structure are occupied by small atoms. Steel is an interstitial alloy, containing carbon atoms in the holes of an iron crystal. The presence of interstitial atoms also changes the properties of the host metal. Many types of steel also contain elements in addition to iron and carbon. Such steels are called alloy steels and they can be viewed as being mixed interstitial (carbon) and substitutional (other metals) alloys. An example of something from a variety of alloy steels are bicycle frames.