A molecular forumla is a series of letters and numbers that indicate which atoms, and how many of them, are in a molecule.
The letters are the symbol of an element, and indicate which atom(s) are in the molecule. The numbers are shown in subscript, and represent how many of that atom are in the molecule.
What Is an Atomic Symbol?
A symbol is an abbreviation for an element (you could think of the word "element" as meaning a specific type of atom). Symbols are one or two letters, and always have the first letter capitalized, and the second letter, if present, lower-case.
Some common atomic symbols are C for carbon, H for hydrogen, O for oxygen, and N for nitrogen. Two examples of symbols that are more than one letter are Na for sodium and Cl for chlorine.
The Molecular Formula of Water
An example of a molecular formula would be the one for water, which is H2O. The "2" that is subscripted immediately after the H indicates that there are two hydrogens. The fact that no number is subscriped after the O indicates that there is only one oxygen.
Note that the number indicating how many of an atom are in a molecule must always be subscripted. A number that is not subscripted indicates the number of molecules. For example, 3H2O would mean three molecules of water, each having two hydrogens and one oxygen.
The Molecular Formula of Cholesterol
Sometimes certain functional groups — a small group of atoms within the molecule that gives the molecule a certain function or classification — are shown separately in larger molecules to give more information about the molecule's structure.
For example, the molecular formula of cholesterol could be written as C27H45OH, which gives us more information than writing it as C27H46O.
In both cases, there are 46 hydrogens, but the first way of writing the formula shows us that there is an OH, or "hydroxyl" group, which makes cholesterol an alcohol. The OH group also has other important properties that are discussed in later lessons.