How do we read H₂O, CO₂ and 2H₂O?

Support map

Teaching model · particles and distances are not to scale

Situation and question

H₂O and 2H₂O are on the board. The digit two appears in both, but means different things. How can we avoid a counting mistake?

New terms, fully explained

An element symbol names an element: H is hydrogen, O oxygen, C carbon. A subscript applies to a symbol or group inside one formula: H₂O has two H and one O. A coefficient before a formula counts whole molecules or formula units: 2H₂O represents two water molecules, four H and two O atoms in all. A missing subscript means one. A formula states composition; by itself it shows neither molecular shape nor whether a reaction occurred.

Predict first

Before reading the example, count atoms of each element in three CO₂ molecules. What do the leading 3 and the subscript ₂ after O each mean?

Investigate or analyse given data

Make a table with rows H₂O, CO₂, 2H₂O and 3CO₂. For each, separately record particle count and counts of H, C and O. Use pencil and paper cards only; do not produce any gas.

Worked example

3CO₂ means three molecules, each with one C and two O, so three C and six O in all. 2H₂O means two molecules, four H and two O. For NaCl, count formula units rather than molecules: 2NaCl gives a ratio of two Na⁺ to two Cl⁻ in the corresponding part of a crystal.

A common mistake

Mistake: reading 2H₂O as one H₄O₂ molecule. The total atom count matches, but the molecule count and composition are misrepresented. Do not apply a coefficient to the first element only. A list of formulas does not prove a reaction happened.

Transfer to a new case

Decode 4H₂O and 2CO₂ without hints. How many molecules of each kind, and how many oxygen atoms in each set?

Answer and recall

Check: 4H₂O means four molecules and four O; 2CO₂ means two molecules and four O. Equal oxygen counts do not make the substances identical. Invent a case where the subscript and coefficient lead to different counts.

Knowledge check

1. How many O atoms in 3CO₂?
2. How many molecules in 2H₂O?

Answers: Six O; two molecules.

When to revisit

Close the page and, after 10 minutes, reconstruct the map, define the terms in your own words and solve the new case. Repeat without hints after a day and a week. If an error persists, return to the worked example and write down the missing step.

Is every substance made of molecules?

The salt case: what can the particle model now explain?