Atoms and molecules

Molecules

We are learning how to:

• represent the atoms of a molecule

• work out the number of atoms in a molecule.

A molecule is a group of atoms bonded together. Some elements are made up of atoms bonded together – oxygen gas consists of molecules with two atoms of oxygen bonded together.

Compounds may also be made up of molecules. Water, for example, is not an element – it is a compound of hydrogen and oxygen. A water molecule has two hydrogen atoms bonded to a single oxygen atom.

Activity 9.2

Here is what you should do:

1. Discuss these points in your group.

a) What are elements made up of?

b) When a chemical equation is written, such as Fe + S FeS what is represented by the Fe and the S? c) What do you think is happening to the atoms in this equation? d) How many atoms of Fe and how many of S were used to make FeS?

Drawing models of molecules

We can represent molecules using models and diagrams. Simple ball shapes are used for the atoms and, since there are bonds between the atoms, sticks or lines are used for these.

Fig 9.5 shows how we represent molecules of water (H2O), carbon dioxide (CO2), ammonia (NH3) and methane (CH4).

How many atoms make a molecule?

Each molecule is made up of a specific number of atoms. The formula for water is H2O. This shows that it takes two atoms of hydrogen (H) and one atom of oxygen (O) to make one molecule of water. A water molecule consists of three atoms.

Vinegar, for example, is mostly ethanoic acid. Ethanoic acid has the formula C2H4O2. This means that each molecule of ethanoic acid is made up of two atoms of carbon, four atoms of hydrogen and two atoms of oxygen.

Some compounds have many atoms.

Check your understanding

1. Look at the models in Fig 9.5. Copy them and label them. Which colour ball is used for: a) oxygen (O)? b) hydrogen (H)? c) carbon (C)?

2. Identify the number of atoms in the molecules with the following formulas: a) CO2 b) NH3 c) SO2 d) PCl5 e) N2O5 f) C2H6O.

Key terms molecule one unit of a compound bonded chemically connected formula a symbolic representation of the atoms within one molecule of a substance