<P> Ionic compounds do not exist as individual molecules; a formula unit thus indicates the lowest reduced ratio of ions in the compound . </P> <P> A chemical formula shows the kinds and numbers of atoms in the smallest representative unit of a substance . </P> <P> In mineralogy, as minerals are almost exclusively either ionic or network solids, the formula unit is used . The number of formula units (Z) and the dimensions of the crystallographic axes are used in defining the unit cell . </P>

When is a particle classified as an atom molecule or a formula unit