Atoms, elements and compounds - AQAChemical formulae of compounds
Chemists use symbols and formulae to represent elements and compounds. Word equations and balanced chemical equations represent the changes that happen in chemical reactions.
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.
