Molecules and Reading Chemical Formulas

Core Idea

A molecule is a group of two or more atoms bonded together. A chemical formula uses element symbols and subscript numbers to show exactly which atoms are in a molecule and how many of each. In H2O, the H stands for hydrogen, the O stands for oxygen, the subscript 2 means there are two hydrogen atoms, and the absence of a subscript after O means there is one oxygen atom. Learning to read chemical formulas is like learning to read words — it lets you understand what substances are made of.

How It's Best Learned

Practice reading formulas with physical or digital models. Start with simple, familiar molecules — H2O (water), CO2 (carbon dioxide), NaCl (table salt), O2 (oxygen gas) — and count the atoms of each element. Then try slightly more complex ones like C6H12O6 (glucose).

Common Misconceptions

• A subscript applies only to the element immediately before it — in H2O, the 2 belongs to hydrogen, not to oxygen.
• Not all molecules are compounds — O2 is a molecule but not a compound because it contains only one element.
• Chemical formulas show the ratio of atoms, not the physical arrangement — H2O tells you the count but not the shape of the molecule.
• The subscript "1" is never written — if no subscript appears after a symbol, it means there is exactly one atom of that element.

Explainer

You know that elements are made of single types of atoms and that compounds combine different types of atoms with chemical bonds. The next step is learning how chemists write down what is in a molecule using a chemical formula — a compact notation built from element symbols and numbers.

A molecule is a group of two or more atoms held together by chemical bonds. Some molecules are simple: an oxygen molecule (O2) is just two oxygen atoms bonded together. Others are more complex: a glucose molecule (C6H12O6) contains 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms, for a total of 24 atoms bonded into one structure. Whether simple or complex, every molecule can be described by its chemical formula.

Reading a chemical formula is straightforward once you know the rules. Each element symbol tells you which element is present. The small number written slightly below and to the right of a symbol is called a subscript, and it tells you how many atoms of that element are in one molecule. If there is no subscript, the count is one. In H2O: H has a subscript of 2 (two hydrogen atoms), and O has no subscript (one oxygen atom). In CO2: C has no subscript (one carbon), and O has a subscript of 2 (two oxygens). The subscript always belongs to the element immediately before it — never to the element after it.

It is worth noting that not every molecule is a compound. O2 (oxygen gas) is a molecule because it has two bonded atoms, but it is not a compound because both atoms are the same element. H2O (water) is both a molecule and a compound because it contains atoms of two different elements bonded together. This distinction can be confusing at first, but the rule is simple: a compound must involve more than one element.

Chemical formulas are the foundation of the language of chemistry. Once you can read a formula, you can determine what elements make up a substance, how many atoms of each are present, and even begin to calculate ratios and proportions. Think of formulas as recipes — they tell you the exact ingredients and quantities in every molecule. As you advance, you will use formulas to write chemical equations, calculate masses, and predict the outcomes of reactions.