How do you read a chemical formula?

Symbols and subscripts

Every formula is built from element symbols. Each symbol starts with a capital letter, so you can tell where one element ends and the next begins: NaCl contains Na (sodium) and Cl (chlorine).

A subscript is the small number written just after and below a symbol. It tells you how many atoms of that element are present, and if there is no subscript, the number is 1. In CO₂, there is 1 carbon atom and 2 oxygen atoms. In glucose, C₆H₁₂O₆, there are 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms, 24 atoms in total.

A subscript belongs only to the symbol right in front of it. In H₂O, the 2 counts the hydrogen atoms, not the oxygen.

Parentheses

Some formulas put a group of atoms in parentheses, usually a polyatomic ion that appears more than once. A subscript after the closing parenthesis multiplies every atom inside it.

Calcium hydroxide, Ca(OH)₂, contains one calcium atom and two OH groups. Multiply each atom inside the parentheses by 2: oxygen 1 × 2 = 2 and hydrogen 1 × 2 = 2. So Ca(OH)₂ has 1 calcium, 2 oxygen and 2 hydrogen atoms, 5 atoms in total.

Try a bigger one: aluminum sulfate, Al₂(SO₄)₃. There are 2 aluminum atoms. Inside the parentheses are 1 sulfur and 4 oxygen atoms, and the 3 outside multiplies both, giving 3 sulfur atoms and 12 oxygen atoms. That adds up to 2 + 3 + 12 = 17 atoms.

Coefficients

In chemical equations, you will often see a full-size number in front of a formula, such as 3H₂O. This is a coefficient. It tells you how many molecules or formula units there are, and it multiplies every atom in the formula.

3H₂O means three water molecules. Each one has 2 hydrogen atoms and 1 oxygen atom, so there are 3 × 2 = 6 hydrogen atoms and 3 × 1 = 3 oxygen atoms. When a formula has both a coefficient and parentheses, multiply by both: 2Ca(OH)₂ contains 2 calcium atoms, 2 × 2 = 4 oxygen atoms and 4 hydrogen atoms.

Notice the difference between 2O and O₂. 2O means two separate oxygen atoms, while O₂ is one molecule made of two oxygen atoms bonded together.

Hydrates and the raised dot

Some crystals hold water molecules inside their structure. These compounds are called hydrates, and their formulas use a raised dot to show the attached water. Copper(II) sulfate pentahydrate is written CuSO₄·5H₂O, and the 5H₂O means there are five water molecules for every CuSO₄ unit. Here the dot does not mean multiply; it means “together with.”

To count the atoms, add the two parts. CuSO₄ gives 1 copper, 1 sulfur and 4 oxygen atoms, and 5H₂O gives 10 hydrogen and 5 oxygen atoms. Altogether that is 1 Cu, 1 S, 9 O and 10 H, or 21 atoms.

You can even see the water at work. These crystals are bright blue, but when they are heated strongly, the water is driven off and a white powder of anhydrous (water-free) CuSO₄ is left behind.

A step-by-step method

To count the atoms in any formula, work through it in order. First, list each element by its symbol. Second, read each subscript, using 1 where there is none. Third, multiply the atoms inside parentheses by the number after them. Fourth, for a hydrate, add the atoms in the part after the dot. Finally, multiply every count by the coefficient in front, if there is one.

One more tip: a small raised number with a + or − sign is a charge, not a count. The sulfate ion, SO₄²⁻, has 1 sulfur atom and 4 oxygen atoms, and the whole group carries a −2 charge.