What is a chemical formula in simple words?

A chemical formula is a code that shows which elements are in a substance and how many atoms of each element are joined together. It uses element symbols and small numbers to describe the makeup of one particle of that substance.

How do you read a chemical formula like H2O?

To read H2O, say "H two O." The H stands for hydrogen and the small 2 means there are 2 hydrogen atoms. The O stands for oxygen, and with no number after it, that means there is 1 oxygen atom. So H2O is 2 hydrogen atoms and 1 oxygen atom joined together.

What is a chemical formula used for in chemistry?

A chemical formula helps chemists quickly see what a substance is made of and compare it to other substances. It is also used in equations for chemical reactions to keep track of how many atoms of each element are present before and after a reaction.

What is a chemical formula vs a chemical name?

A chemical name is the word name of a substance, like "water" or "sodium chloride." A chemical formula is the symbol version, like H2O for water or NaCl for sodium chloride, that shows exactly which elements and how many atoms of each are in one unit of the substance.

What Is a Chemical Formula? Definition and Examples

Kid-friendly explanation

A chemical formula is like a recipe code for a substance. The letters tell which elements are inside, and the small numbers tell how many atoms of each kind are stuck together. For example, H₂O means water has 2 hydrogen atoms and 1 oxygen atom. It is a quick way for scientists to read what something is made of!

Worked examples

Example 1 (simpler): Reading H₂O

Look at the chemical formula H₂O.

The letter H stands for the element hydrogen.
The small 2 after H means there are 2 hydrogen atoms.
The letter O stands for the element oxygen.
There is no small number after O, which means there is 1 oxygen atom.

So H₂O tells us that one particle (one molecule) of water has 2 hydrogen atoms and 1 oxygen atom.

Example 2 (more advanced): Interpreting 2NaCl and C₆H₁₂O₆

First, look at NaCl, the chemical formula for table salt.

Na stands for sodium, and there is 1 sodium atom.
Cl stands for chlorine, and there is 1 chlorine atom.
NaCl means 1 sodium atom joined to 1 chlorine atom.

If you see 2NaCl in a chemical equation, the 2 in front is called a coefficient. It means there are 2 units of NaCl.

2NaCl has 2 sodium atoms and 2 chlorine atoms in total.

Now look at C₆H₁₂O₆, a chemical formula for a type of sugar (glucose).

C stands for carbon, and the 6 means there are 6 carbon atoms.
H stands for hydrogen, and the 12 means there are 12 hydrogen atoms.
O stands for oxygen, and the 6 means there are 6 oxygen atoms.

This chemical formula shows that one glucose molecule has 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms.

Going deeper

Chemical formulas are built from element symbols, which are 1- or 2-letter codes for each type of atom. The number of protons in an atom, called its atomic number, decides which element symbol it gets. For example, all atoms with 1 proton are hydrogen and use the symbol H.

Chemical formulas also reflect how atoms share or transfer valence electrons, which are the outermost electrons that take part in bonding. Different formulas represent different substances, even if they use the same elements. For instance, H₂O (water) and H₂O₂ (hydrogen peroxide) both contain hydrogen and oxygen, but in different ratios, so they behave very differently.

In later grades, your child will see chemical formulas inside full chemical reaction equations. Coefficients in front of formulas show how many units of each substance react or are produced, and balancing those equations helps track how many atoms of each element are present before and after the reaction.

Common mistakes

Mixing up subscripts and coefficients. A small number after a symbol (subscript) tells how many atoms are in one unit, while a number in front (coefficient) tells how many units there are in a reaction.
Thinking no subscript means zero atoms. When there is no small number after an element symbol, it means there is 1 atom of that element, not zero.
Ignoring groupings in parentheses. In formulas like Ca(OH)₂, the subscript 2 applies to everything in the parentheses, so there are 2 oxygen atoms and 2 hydrogen atoms, not just 2 of one of them.

Everyday connection

Chemical formulas are printed on many things around your home, such as cleaning products, food labels, and medicine bottles. When a label lists H₂O or NaCl, it is using chemical formulas to show exactly what substances are inside, the same way a recipe lists ingredients for a dish.

Related terms to explore

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What is a chemical formula?