Reading a Chemical Formula

Introduction

A chemical formula is chemistry's shorthand. Just a few characters, such as H₂O or CO₂, tell a chemist which elements are present and how many atoms of each there are. Once you can read a formula, you can describe a substance in words, compare it with others and later use it in calculations. This page shows how to decode the two main parts of any formula: the symbols and the small numbers after them.

Core explanation

Symbols name the elements. Every element has a symbol of one or two letters. The first letter is always a capital; a second letter, if there is one, is always lower case. So Co is cobalt, a single element, while CO contains two elements, carbon and oxygen. Reading capitals carefully is the first skill.

Subscripts count atoms. A small number written after and slightly below a symbol is a subscript . It tells you how many atoms of the element immediately before it are present. - H₂O: 2 hydrogen atoms, 1 oxygen atom. - CO₂: 1 carbon atom, 2 oxygen atoms. - CH₄ (methane): 1 carbon atom, 4 hydrogen atoms.

No subscript means one. The number 1 is never written. In NaCl there is one sodium for each chlorine; in CO there is one carbon and one oxygen.

A subscript belongs to one symbol only. In H₂SO₄, the 2 applies only to H and the 4 only to O. There is one sulfur atom. Totalling: 2 + 1 + 4 = 7 atoms in one molecule of sulfuric acid.

Changing the subscript changes the substance. H₂O is water; H₂O₂ is hydrogen peroxide, a different substance with different properties. CO is carbon monoxide, a toxic gas; CO₂ is carbon dioxide. The subscripts are fixed by the substance's structure and cannot be altered to suit a calculation.

Molecules versus giant structures. For a molecular substance, the formula is a molecular formula : it gives the actual atoms in one molecule. C₆H₁₂O₆ (glucose) means each molecule has 6 C, 12 H and 6 O. For a giant structure, the formula shows the simplest ratio, called a formula unit . NaCl means Na⁺ and Cl⁻ in a 1 : 1 ratio; Al₂O₃ means 2 aluminium for every 3 oxygen.

Formulae for elements. Elements that form molecules show the atoms per molecule: O₂, N₂, Cl₂, P₄, S₈. Metals, noble gases and giant covalent elements are written with the symbol alone: Fe, He, C.

Charges. A superscript such as the ⁺ in Na⁺ shows an electric charge, not a number of atoms. Do not confuse superscripts with subscripts.

Step-by-step reasoning

To read any simple formula:

1. Split it into symbols by finding each capital letter. 2. Name the element for each symbol. 3. Read the subscript after each symbol; if none, the number is 1. 4. List the number of atoms of each element, then add them for a total.

Visual explanation

Write NH₃ in large letters. Draw an arrow from N to the words "1 nitrogen" and from the small 3 to "3 hydrogen". Then show a model of ammonia in the atom simulator: one blue sphere joined to three white spheres, matching the formula exactly.

Real-world analogy

A formula is like a recipe card that lists ingredients with quantities: "2 eggs, 1 cup flour". The symbol is the ingredient and the subscript is the quantity. If no quantity is written, you use one. Change a quantity and you get a different dish.

Real-world example

Food labels and medicine packets often list ingredients by chemical name, and pharmacists and chemists use formulae to tell similar substances apart. Sodium hydrogencarbonate, NaHCO₃, is baking soda, while sodium carbonate, Na₂CO₃, is washing soda. One extra sodium and one fewer hydrogen make a very different product.

Why?

Why must a subscript follow the symbol it counts? A formula needs a clear, universal rule so every chemist reads it the same way. By agreement, the subscript applies only to the symbol immediately in front of it, so H₂O can mean only two hydrogens and one oxygen, never anything else.

Common misconception

"CO and Co are the same thing." Co, with a lower-case o, is the metal cobalt. CO, with two capitals, is carbon monoxide, a compound of carbon and oxygen. Always check whether each letter is a capital.

Worked example

Question: Describe the formula C₂H₅OH (ethanol) in words and give the total number of atoms.

Reasoning: Capitals: C, H, O, H. C has subscript 2, the first H has 5, O has none (1), the last H has none (1). Hydrogen appears twice, so add: 5 + 1 = 6.

Answer: One molecule contains 2 carbon, 6 hydrogen and 1 oxygen atom: 9 atoms in total.

Quick check

1. How many atoms of each element are in one molecule of CO₂? Answer: One carbon atom and two oxygen atoms.

Exam focus

Examiners test whether you can state the number of each type of atom in a formula and the total. Watch for elements that appear twice, such as H in C₂H₅OH, and for missing subscripts meaning 1. Never change subscripts when balancing equations.

Advanced insight

Chemists use several kinds of formula. An empirical formula shows only the simplest ratio: glucose, C₆H₁₂O₆, has the empirical formula CH₂O. A structural formula shows how the atoms are joined, which matters because different molecules can share the same molecular formula; ethanol and methoxymethane are both C₂H₆O.

Summary

A chemical formula uses element symbols to show which elements are present and subscripts to show how many atoms of each. A subscript counts only the symbol before it, and no subscript means one. Molecular formulae give the atoms in one molecule; formulae of giant structures give the simplest ratio.

Practice questions

1. How many atoms of each element are in one molecule of H₂SO₄? Answer: 2 hydrogen, 1 sulfur and 4 oxygen atoms, making 7 atoms in total. 2. What does the formula CaCl₂ tell you about calcium chloride? Answer: It contains calcium and chlorine in a ratio of 1 calcium to 2 chlorine. 3. Explain why H₂O and H₂O₂ are different substances. Answer: They contain different numbers of oxygen atoms per molecule, so their molecules and properties are different; H₂O is water and H₂O₂ is hydrogen peroxide. 4. How many elements and how many atoms are in one molecule of glucose, C₆H₁₂O₆? Answer: Three elements (carbon, hydrogen and oxygen) and 6 + 12 + 6 = 24 atoms.