← Atoms and Molecules

Lesson 5 of 7 · Textbook 8.3

Atoms and molecules

Atoms rarely sit alone. Join them and you get a molecule — but whether that molecule is an element or a compound depends on one thing only.

What a molecule is

An atom can combine with one or more atoms of the same element or of different elements to form molecules.

Definition A molecule is made up of two or more atoms that are chemically combined.

Drawing them with labelled circles

A simple way to represent atoms and molecules is with labelled circles.

What you drawWhat it means
One circle, labelled with a chemical symbolAn atom of that element. One circle labelled H is a hydrogen atom.
Two circles joined, both labelled HA hydrogen molecule — atoms of the same element combined. Total atoms: 2.
Two H circles and one O circle, joinedA water molecule — atoms of different elements combined. Total atoms: 3.
A limitation of the model, which is examinable Circles of different colours and sizes are used so that we can tell atoms apart easily. In fact, atoms do not have any colour. This is one limitation of scientific models — they are simplified representations, and are not always real and accurate.

If a question asks for a limitation of the labelled-circle model, this is the answer it wants.

Element or compound?

The one test A molecule of an element consists of only one type of atom chemically combined — nitrogen (N₂), oxygen (O₂).
A molecule of a compound consists of two or more types of atom chemically combined — nitrogen dioxide (NO₂).

Build a molecule

Chemical formula—
Total atoms0
Element or compound?—

The five presets are exactly the Learning Point on page 75. Build each one and check the atom counts against your own answers.
The trap worth naming O₂ is a molecule, but it is still an element. Two atoms joined does not make a compound — the atoms have to be different kinds. Joining two oxygen atoms gives you oxygen, not something new.

This is the same point Chapter 3 made about the particle diagrams. If it caught you there, it will catch you here.

Check yourself