When you see a molecule or compound described in a textbook or as part of a chemical reaction, it is usually in the form of a chemical formula. These combinations of letters and numbers – such as H₂O – can be challenging to understand if you don’t know what the individual parts mean. If you’re learning chemistry or reading about different chemicals, you need to know what chemical formulas mean and how you interpret them. You can easily pick up the basics and read a chemical formula. The letters tell you which elements are present, and the numbers tell you how many atoms of each make up a molecule.

A chemical formula tells you the specific elements included in the compound and the number of atoms of each. The letters in a chemical formula are the symbols for the specific elements. So for example, H means hydrogen, O means oxygen, S means sulfur, Cu means copper, F means fluorine, Fe means iron and Au means gold. If you don’t know what a particular symbol means, you can check a periodic table.

The numbers added as a subscript tell you how many atoms of each element are present. If no number is present, there is only one atom. You can now interpret chemical formulas. For example, many people know H₂O is water, but now you know that the H means hydrogen and the O means oxygen, and the number 2 after the H means that there are two hydrogen atoms for every oxygen atom.

Another example is H₂SO₄. This is the formula for sulfuric acid. The letters show that it contains hydrogen, sulfur and oxygen, and the numbers show there are two atoms of hydrogen, one atom of sulfur and four atoms of oxygen per molecule. You can interpret the majority of chemical formulas like this.

You’ll occasionally encounter a chemical formula like this: Mg(OH)₂. The Mg is magnesium, and we know what O and H are, but the brackets are a new feature. They indicate that the number outside the brackets is applied to all the elements inside the brackets. So, the formula above – for magnesium hydroxide – contains one atom of magnesium, two of oxygen and two of hydrogen. They’re grouped like this because the oxygen and hydrogen atom are often paired together as a “hydroxyl” group. The details of why don’t matter for interpreting the formula, but it tells you that there are two of these groups, each containing one atom of oxygen and hydrogen in the molecule.

Ionic compounds are represented in the same way as covalently bonded compounds, but the meaning is a bit different. There are no individual units of ionic compounds comparable to molecules, so the formula tells you the ratio of one atom to another. For example, NaCl is sodium chloride (table salt), and this means it exists in a ratio of one sodium atom to one chlorine atom.

For a single ion, a + or – symbol is used to tell you the charge. So Na⁺ is the positively charged sodium ion and Cl⁻ is the negatively charged chloride ion.

Chemical formulas are usually used, but in some cases, you might encounter a structural formula. These don’t include numbers; instead, they represent the molecule through an arrangement of symbols linked by lines that represent the bonds between atoms. A single line represents a single bond, and a double line represents a double bond. For example, carbon dioxide (CO₂) has a single carbon atom double-bonded to oxygen atoms on each side, so it would be represented as:

O = C = O

These can get more complicated, but the basic rules are easy to understand.