Chemical formulae of compounds
A compoundA substance formed by the chemical union of two or more elements. is a substance that contains two or more elementA substance made of one type of atom only. that are chemically combined. The elements in a compound are present in fixed proportions. For example, carbon dioxide always has 12 g of carbon for every 32 g of oxygen.
A chemical formulaA combination of symbols that indicates the chemical composition of a substance. can be used to represent a compound. The formula shows:
- the symbols for each element in the compound
- the number of atomThe smallest part of an element that can exist. of each element in a unit of the compound
For example, magnesium oxide is made up of two elements, magnesium and oxygen. Its formula is MgO. This shows that it has one atom of magnesium for every one atom of oxygen.
Here are some more examples of compounds and their formulae. The subscriptA letter or number written below the line. number in a formula shows if there is more than one atom of an element.
| Name of compound | Formula |
| Sodium chloride | NaCl |
| Potassium bromide | KBr |
| Magnesium iodide | MgI2 |
| Carbon dioxide | CO2 |
| Carbon monoxide | CO |
| Sulfur trioxide | SO3 |
| Water | H2O |
| Ammonia | NH3 |
| Methane | CH4 |
| Name of compound | Sodium chloride |
|---|---|
| Formula | NaCl |
| Name of compound | Potassium bromide |
|---|---|
| Formula | KBr |
| Name of compound | Magnesium iodide |
|---|---|
| Formula | MgI2 |
| Name of compound | Carbon dioxide |
|---|---|
| Formula | CO2 |
| Name of compound | Carbon monoxide |
|---|---|
| Formula | CO |
| Name of compound | Sulfur trioxide |
|---|---|
| Formula | SO3 |
| Name of compound | Water |
|---|---|
| Formula | H2O |
| Name of compound | Ammonia |
|---|---|
| Formula | NH3 |
| Name of compound | Methane |
|---|---|
| Formula | CH4 |
Many compounds exist naturally. They can also be formed from their elements in chemical reactionA change in which atoms are rearranged and joined together differently to form new substances. They often involve energy changes.. In a chemical reaction, one or more new substances are formed. Most chemical reactions involve energy changes.
It is not easy to split up a compound into its elements - the only way to do this is in chemical reactions.