A molecule is the smallest particle that can have a separate, independent existence.
When atoms of the same element join together the result is a molecule of an element.
When atoms of different elements join together the result is a molecule of a compound. For example, a molecule of water contains two atoms of hydrogen combined with one atom of oxygen. A molecule of carbon dioxide contains one atom of carbon combined with two atoms of oxygen.
The symbols for elements can also be used to represent molecules and compounds. So, water is represented as H2O – two hydrogen atoms (H) and one oxygen atom (O). Carbon dioxide is written as CO2 – one carbon atom (C ) and two oxygen atoms (O).
‘H2O’ and ‘CO2’ are called molecular formulae or just formulae, for short.
Numbers are written after symbols as subscripts if there are two or more atoms of the same element in a molecule.
A molecular formula shows the numbers of atoms of the different elements in one molecule of a substance.
Atoms and molecules of elements
Almost all elements, for example, iron (Fe), aluminium (Al), and copper (Cu), can be represented simply by their symbols, because they contain single atoms.
But, this is not the case with oxygen, hydrogen, nitrogen, or chlorine.
These elements exist as molecules containing two atoms combined together.
So, oxygen is best represented as O2 and not O, hydrogen as H2 not H, nitrogen as N2, and chlorine as Cl2. These molecules of elements containing two atoms are described as diatomic molecules.
Simple and Complex molecules
Some molecules are very simple, such as hydrogen chloride (HCl) and water (H2O). Others are more complex, such as ethanol (C2H5OH). Others, such as chlorophyll, are very complex (C51H72O4N4Mg).