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

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

This formula indicates:

 

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