Metals consist of giant structures with enormous numbers of atoms linked together in a 3-D lattice (lattice: an array of points in space).
For a typical metal, M, the elemental formula would effectively be M   . 
Convenience dictates that this is written simply as M but there is certainly no implication that the metal atoms are monatomic, e.g., sodium is written as Na, magnesium as Mg and copper as Cu.