Types of Formulas
There are several types of formulas: molecular, structural, and empirical. Most of the formulas we have been dealing with have been molecular formulas.
Molecular formulas apply to any molecular material. A molecular formula tells you the actual number of each kind of atom within that molecule. H2 shows that two hydrogen atoms are contained in the molecule. Similarly, O2 shows that two oxygen atoms are contained in the molecule. H2O shows that a water molecule contains two hydrogen atoms and one oxygen atom. Hydrogen peroxide has the formula H2O2, meaning that its molecules each contain two hydrogen atoms and two oxygen atoms.
Structural formulas also apply to molecular materials. They not only tell you how many of each kind of atom there is but which atoms are bonded to one another. The structural formula tells you something about the arrangement of atoms within the molecule. Water, for example, can be written as H-O-H, showing that the oxygen atom is in the middle and the two hydrogen atoms are bonded to it. The structural formula for hydrogen peroxide can be written as H-O-O-H.
Empirical formulas apply to any type of compound, whether it consists of molecules or not. Empirical formulas are formulas that are derived from experimental data. (Empirical means experimental.) All that is really shown about the compound in an empirical formula is the ratio of atoms. The very nature of how you go about calculating those formulas only allows you to get the simplest ratio.
Here are some examples. H2O is both an empirical and a molecular formula. The simplest ratio of hydrogen atoms to oxygen atoms is 2:1. Therefore, H2O is an empirical formula. However, if you can isolate individual molecules of water and figure how many atoms there really are in each one, it turns out there are 2 hydrogen and 1 oxygen atoms bonded together in the cluster that forms the molecule. So H2O is a molecular formula as well. In hydrogen peroxide, the simplest ratio of hydrogen atoms to oxygen atoms is 1:1, one hydrogen atom for every oxygen atom. So the empirical formula is HO. That is different from the molecular formula, which is H2O2.
A molecule like glucose shows how exaggerated the differences can be. The structural formula shows how the 24 atoms are hooked together. The molecular formula shows that each molecule contains 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms. The empirical formula simply shows that the ratio of atoms is 1:2:1.
So these are the three different kinds of formulas. The empirical formula gives you the simplest ratio. The molecular formula applies only to molecular materials and it gives you the actual number of each kind of atom that's contained in a molecule. The structural formula shows which atoms are bonded to which.
E-mail instructor: Sue Eggling
Clackamas Community College