What Chemical Formulas Mean

In order to understand nomenclature (as well as lots of other things) you need to understand the meaning of a chemical formula.

Lets start with a simple example: H₂O Most people have seen the formula for water before. The number 2 means that there are two atoms of hydrogen. Since there is no number after the oxygen, we assume that there is one atom of oxygen.

The Rule: A subscripted number always gives the number of the thing that appears right before it.

Another Rule: If no subscript is written, it is assumed to be 1.

Using those two simple ideas, formula can get pretty complicated. Let's look at a tough one.

(NH₄)₃PO₄

 
It will be easiest, in this case, if we work backwards. The 4 at the end, applies to the oxygen and since there is no number on the phosphorus we must have 1.

The 3 applies to what comes right before it. However, in this case, what comes right before it is a set of parentheses. That means that this compound contains 3 "NH₄" groups.

Each of those groups, in turn, is comprised of a single nitrogen and 4 hydrogen atoms.

That means that this compound contains 12 hydrogen atoms (3 groups, each containing 4 H's).

Here's another ugly example: Al(HCr₂O₇)₃
This formula indicates:

• 1 Al
• 3 H
• 6 Cr
• 21 O