1Compounds have new properties
When atoms of different elements join, they make a compound: a new substance with new properties. Sodium + chlorine → sodium chloride (salt).
compound
Lesson 5.2 notes
Compounds and formulae
A compound is made when atoms of different elements join. Its formula tells you which atoms, and how many.
2Reading a formula
A formula shows the atoms in a substance. A small number counts the atom just before it. No number means one.
formula
3Big numbers count molecules
A big number in front counts whole molecules. 2CO₂ is two carbon dioxide molecules: 2 carbon atoms and 4 oxygen atoms.
molecule
4Molecules of compounds
Every molecule of a compound has the same atoms in the same ratio. Change the ratio and you get a different substance.
ratio
5Giant lattices
Some compounds, like salt, are giant lattices, not molecules. The formula gives the ratio: NaCl has one sodium for every chlorine.
lattice
6Naming compounds
With two elements, the name ends in -ide. With oxygen as well, it often ends in -ate: copper sulfate is CuSO₄.
-ide-ate
Explore: Formula builder
Choose a formula and watch its atoms build the molecule.
Common mistake: The small number is not the number of molecules
H₂O is one molecule with 2 hydrogen atoms. Two water molecules are written 2H₂O, with the big number in front.
Remember
- A compound is two or more elements chemically joined. It has new properties.
- A formula shows which atoms, and how many: CO₂ is 1 carbon and 2 oxygen.
- Two elements: the name ends in -ide. With oxygen as well, it often ends in -ate.
Going further at GCSE
GCSE: Chemical formulae and equations → GCSE: Counting atoms and balancing →