2.3 Compounds
Summary: How much water have you used today? A water meter, like that in Figure 1, can tell you. Tags: science oxford-science student-book-7 chemistry Created: 2026-05-19 Last Updated: 2026-05-19
After this topic, you will be able to:
Think back
- What is an atom?
- How many types of atom are there in an element?
- How are the particles arranged in a substance in the gas state?
Key idea A compound is a substance made up of atoms of two or more elements. A compound has different properties to the elements whose atoms are in it.
Key words compound, molecule
How much water have you used today? A water meter, like that in Figure 1**, can tell you.**
Figure 1A water meter measures the amount of water used in a home. Full text alternative to the content
What is water?
Water is important. But what is water? Water is made from the atoms of two elements, hydrogen and oxygen. This means that water is a compound. A compound is a substance made up of atoms of two or more elements, with the atoms joined together strongly. The properties of a compound are different from the properties of the elements whose atoms are in it.
A Explain why water is a compound.
Hydrogen, oxygen, and water particles are all different. This explains why the three substances have different properties.
Hydrogen is a gas at room temperature. If mixed with air and ignited with a spark (set on fire), hydrogen explodes.
Hydrogen atoms go round in pairs (see Figure 2). These two-atom particles are Molecules of hydrogen. A molecule is a group of two or more atoms strongly joined together.
Figure 2A hydrogen molecule consists of two hydrogen atoms.
Oxygen is a gas at room temperature. You cannot see or smell it. Oxygen exists as molecules. Each molecule is made up of two oxygen atoms (see Figure 3). In the air, oxygen molecules spread out. They move quickly from place to place.
Figure 3An oxygen molecule consists of two oxygen atoms.
Water particles are molecules made up of three atoms (see Figure 4). Each water molecule is made up of one oxygen atom and two hydrogen atoms. This means that water is a compound.
Figure 4A water molecule has one oxygen atom joined to two hydrogen atoms. Full text alternative to the content
One property that is different for hydrogen, oxygen, and water is boiling point. In liquid hydrogen, weak Forces hold the molecules close to each other. In liquid water, stronger forces hold the molecules close to each other. This means that it takes less Energy to separate hydrogen molecules from each other than to separate water molecules from each other. Hydrogen has a lower boiling point than water.
B Explain why a compound has different properties to the elements whose atoms the compound is made from.
Maths skills One oxygen molecule is made up of two oxygen atoms. Calculate the number of oxygen atoms in 25 oxygen molecules.
Do you add salt to food?
Figure 5Many people add salt to food.
Salt is a compound. Its scientific name is sodium chloride. It contains particles of two elements, sodium and chlorine.
- Sodium is a shiny, silver-coloured metal that fizzes (bubbles) in water.
- Chlorine is a smelly, green poisonous gas at room temperature.
So why doesn’t salt smell? Or poison you? Or fizz in your mouth?
In salt, sodium and chlorine particles are not simply mixed up. They are joined together to make one substance – sodium chloride. The compound has different properties to the elements whose atoms are in it.
C Describe one difference in properties between sodium chloride and sodium.
Stretch zone
- Copy and complete the sentences. A compound is a substance made up of atoms of one/two or more elements. The properties of a compound are the same as/different from the properties of the elements it is made of. A molecule is a group of two/three or more atoms weakly/strongly joined together.
- Look at Figure 6. For each diagram, write down whether it shows an element or a compound. Explain why. Figure 6 Full text alternative to the content
Stretch zone
- Suggest why water has a higher boiling point than oxygen.