3.3 Chemical Formula
3.3 Chemical Formula
- Define chemical formula.
- Explain how to write a chemical formula.
- Identify the ratio of different elements in a compound from its chemical formula.
You can make a simple salad dressing using just the two ingredients pictured above: oil and vinegar. Recipes for
oil-and-vinegar salad dressing vary, but they typically include about three parts oil to one part vinegar, or a ratio of
3:1. For example, if you wanted to make a cup of salad dressing, you could mix together^34 cup of oil and^14 cup
of vinegar. Chemical compounds also have “ingredients” in a certain ratio. However, unlike oil-and-vinegar salad
dressing, a chemical compound always has exactly the same ratio of elements. This ratio can be represented by a
chemical formula.
Representing Compounds
In achemical formula, the elements in a compound are represented by their chemical symbols, and the ratio of
different elements is represented by subscripts. Consider the compound water as an example. Each water molecule
contains two hydrogen atoms and one oxygen atom. Therefore, the chemical formula for water is:
H 2 O
The subscript 2 after the H shows that there are two atoms of hydrogen in the molecule. The O for oxygen has no
subscript. When there is just one atom of an element in a molecule, no subscript is used in the chemical formula.