All metallic elements and metal alloys form metallic crystals, including iron, silver, sodium, potassium and gold. These crystals are different from the other three primary crystalline structures in that they are formed by metallic bonds.
The other three types of crystalline structures are ionic crystals, molecular crystals and covalent crystals. Ionic and covalent crystals are formed by ionic and covalent bonds, respectively. Molecular crystals are a special category that are not held together by bonds, but instead through weak van der Waals forces.
When a metal is melted, the individual atoms join together in a perfectly symmetrical crystal lattice pattern under the right conditions, although most metals rarely occur in large uniform crystals in nature. This is partly because most metals don't occur in pure form in nature but are instead usually found in compounds, such as metal halides, oxides and sulfides.
All metallic crystals share numerous common properties, including being malleable and ductile. Most metallic elements are also quite shiny, have a fairly high melting point and are good conductors of electricity. There are also a huge number of differences between the various types of metallic crystals, which is unsurprising since metals make up approximately 80 percent of the known chemical elements.