A formula unit is the exact make up of a molecule, or a group of bonded atoms, and is the smallest ratio of atoms in a compound, or group of loosely connected ions. The formula unit for any non-ionic molecule — one that has strong chemical bonds, like water — is called the molecular formula, which is H₂0 for water. In ionic compounds, no strong chemical bond is formed, and the formula units that are used to represent these compounds are called the empirical formula, or the smallest atomic ratio.

The formula unit can be confusing without a basic knowledge of how molecules are formed. In non-ionic molecules like ethane (C₂H₆), a reduction to the atomic ratio gives little meaning or representation of the molecule. The strong chemical bonds cause these atoms to always stay together as C₂H₆. Using the empirical formula of CH₃ does little good and might create confusion with other non-ionic molecules that have the same empirical formula.

Ionic compounds behave much differently because no strong chemical bond can form. The loose connections formed, called ionic bonds, by each atom behave like magnets ready to attract, and this force holds ionic compounds together. The ionic compound takes on many complex forms of the same pattern. Therefore, to study the properties, only the empirical formula — or the most reduced atomic ratio, as with fractions — is used when trying to calculate formula units. With sodium chloride (NaCl), the ionic structure produces cubes with the positive sodium ions, or cations, looking for the negative chlorine ions, or anions, in a 1:1 ratio.

Slightly different is aluminum oxide, Al₂O₃. Here, aluminum ions are searching for three electrons from oxygen ions, which only offer two. Therefore, if two aluminum ions match up with three oxygen ions, the charges are balanced, forming the ionic compound. The new formula unit Al₂O₃ is used to represent the ionic compound.

The formula unit becomes most helpful when determining the structure and behavior of any given molecule, whether it is ionic or non-ionic. With a little practice and a periodic table of elements, nearly everything that there is to know about a molecule can be determined simply from the formula unit. For ionic compounds, the empirical formula is the formula unit. The formula NaCl, for example, will tell someone exactly how much of each atom is in a given weight, how easy it will be to do certain chemical experiments and even how much electricity can be generated.

For non-ionic compounds, the molecular formula is the formula unit. Benzene’s formula unit, C₆H₆, for example, will allow for a complete analysis of the molecule. It will even show the fact the benzene forms only in a circular pattern, called a cyclic molecule.