2.4 Chemical formulae
Summary: Are the windows closed? If so, there is probably more carbon dioxide in the room now than there was 10 minutes ago. Every cell in your body makes carbon dioxide, which you breathe out. Carbon dioxide… 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 a molecule?
- What is a compound?
- Name the change of state from gas to liquid.
Key idea A chemical formula gives the relative number of atoms of each element in a substance.
Key words chemical formula, relative number
Are the windows closed? If so, there is probably more carbon dioxide in the room now than there was 10 minutes ago. Every cell in your body makes carbon dioxide, which you breathe out. Carbon dioxide is a compound. It is made up atoms of two elements – carbon and oxygen.
Carbon monoxide is another compound. It also consists of atoms of carbon and oxygen. But carbon monoxide is poisonous. It can be very bad for you if you breathe it in.
Why are carbon–oxygen compounds different?
You already know that the properties of a compound depend on the elements whose atoms are in it. The number of atoms of each element also make a difference.
Carbon dioxide has two oxygen atoms for every carbon (see Figure 1). Carbon monoxide has one oxygen atom for every carbon (see Figure 2).
Figure 1A carbon dioxide molecule has one carbon atom and two oxygen atoms. Full text alternative to the content
Figure 2A carbon monoxide molecule has one carbon atom and one oxygen atom.
A Give the number and types of atoms in one carbon dioxide molecule.
How do we name compounds?
Compounds made up of oxygen and one other element have two-word names. The second word is oxide. Table 1 gives some examples. Table 1 The names of some compounds containing oxygen. Elements in compound
Name of compound
aluminium and oxygen
aluminium oxide
zinc and oxygen
zinc oxide
As you know, some elements form more than one oxide. Table 2 gives some examples. Table 2 The names of some oxides. Compound molecule made up of …
Name of compound
1 carbon atom and 1 oxygen atom
carbon monoxide
1 carbon atom and 2 oxygen atoms
carbon dioxide
1 sulfur atom and 2 oxygen atoms
sulfur dioxide
1 sulfur atom and 3 oxygen atoms
sulfur trioxide
Figure 3A sulfur dioxide molecule has one sulfur atom and two oxygen atoms. You can represent it with a 2D or a 3D diagram. Full text alternative to the content
Figure 4A sulfur trioxide molecule has one sulfur atom and three oxygen atoms. You can represent it with a 2D or 3D diagram. Full text alternative to the content
The compound of sodium and chlorine is called sodium chloride. Chlorine becomes chloride. In any compound of a metal with a non-metal, the end of the name of the non-metal becomes –ide.
B Name the compound that has Molecules with one sulfur atom joined to three oxygen atoms.
What is a chemical formula?
Every element and compound has its own chemical formula. The chemical formula for a substance shows the relative number of atoms of each element that are in it. ‘Relative number’ means how many atoms of one type there are compared to another type.
For example:
- The chemical formula of oxygen gas is O2. This shows that its molecules have two oxygen atoms.
- The chemical formula of carbon dioxide is CO2. This shows that there is one carbon atom for every two oxygen atoms.
- The chemical formula for carbon monoxide is CO. This shows that there is one carbon atom for every oxygen atom.
When you are writing Chemical Formulae, write the numbers:
- to the right of their chemical symbol, just below the line
- smaller than the Chemical Symbols.
C A compound has one sulfur atom for every two oxygen atoms. Write the formula of the compound, with the chemical symbol for sulfur first.
Working scientifically Give the correct names for the compounds with these chemical formulae. Choose from the list of names below. a CO2 b SO3 c CO d SO2 List of names: carbon monoxide, carbon dioxide, sulfur dioxide, sulfur trioxide.
Stretch zone a) Copy and complete the sentences. A compound is made up of atoms of sodium and chlorine. Its name could be sodium chloride/sodium oxide. A compound has one atom of nitrogen to two atoms of oxygen. Its name is nitrogen dioxide/nitrogen trioxide. b) Figure 5 shows one molecule of a compound. Write its formula. Figure 5
Stretch zone
- Paracetamol reduces pain. Its molecules are made up of these atoms: 8 carbon atoms, 9 hydrogen atoms, 1 nitrogen atom, and 2 oxygen atoms. Write the chemical formula of paracetamol.