In metals, the electrons leave the outer shells of metal atoms, forming positive metal ions and a ‘sea’ of delocalised electrons. The structure of a solid metal consists of closely packed metal ions, arranged in a regular way to form a metallic lattice structure.
Metallic lattices do not contain fixed numbers of atoms. This means that the chemical formula used for a metal is simply the symbol for the element. For example, the formula for iron is Fe, even though a piece of iron contains very many iron atoms.