<P> A formula unit in chemistry is the empirical formula of any ionic or covalent network solid compound used as an independent entity for stoichiometric calculations . It is the lowest whole number ratio of ions represented in an ionic compound . Examples include ionic NaCl and K O and covalent networks such as SiO and C (as diamond or graphite). </P> <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 . The formula unit is a very important unit of measurement in chemistry . </P>

The formula unit of an ionic compound is always the same as its empirical formula