What is the Difference Between Atoms, Molecules, Ions, Elements and Compounds?

Atoms, molecules, ions, elements and compounds are very often confused because they are all related. They are used to describe particles of matter – knowing and understanding the differences and relationships between these particles is essential to understanding what chemistry is all about but you need to think really, really small.

Atoms

The ancient Greeks came up with the idea of atoms over 2000 years ago. A philosopher (thinker) called Democritus is credited with the idea. Imagine you have a piece of stone and you hit it hard with a hammer. It will shatter into pieces. Then take one of those pieces and hit it again. It will break into even smaller pieces. Eventually, you will end up with pieces so small that they can’t be broken into anything smaller – they are ‘indivisible‘. The Ancient Greek word for indivisible was ‘atomos‘ so he called the particles ‘atoms’.

The Greeks did not do any scientific experiments, all their science was done in the minds of the philosophers so there was no hard evidence to support it so the theory was never really developed. However, in the 18th century, a scientist called John Dalton realised that Democritus’ idea might be useful in helping to understand the world around us. He suggested that all materials could be reduced to extremely small basic particles – atoms – which could not be broken down any further and could not be created or destroyed.

These days we know that atoms can be broken down into even smaller particles, but for most of the time in chemistry, thinking of them as Dalton did (small hard spheres with no electrical charge) really does work well.

Molecules

Molecules are what we call particles made from 2 or more atoms that are bonded together. They have no electrical charge either.

molecules

Ions

Sometimes, atoms and molecules can have an electric charge, either plus or minus (positive or negative). When they do, we don’t call them atoms or molecules any more, we call them ions. Ions can group together to form large regular shaped groups that we call ‘ionic lattices’. If they become big enough, we see them as crystals.

Elements

An element is a substance that is made up from only one sort of atom. They are the simplest of the chemicals that you use. No matter how hard you look, if you have an element, you will find all of the atoms are the same. The atoms of different elements, however, are different to each other.

The periodic table

The periodic table via sciencenotes.org.

The elements are grouped together on the periodic table, all of the elements of a group have similar properties.

Some elements are made from molecules, some are just atoms.

Compounds

A compound is a substance that is made up from two or more different types of atom that are joined together. In other words, if you analyse a substance and find that it is made from three elements (therefore 3 different types of atom) all joined together, then it is called a compound.

Water, seen here in three states of liquid, solid and gas is a common compound.

Water, seen here in three states of liquid, solid and gas is a common compound.

Some compounds are made from molecules, others are made from ionic lattices.

water-as-compound

How to Use These Words

The words atom, molecule, ions and ionic lattice are used to describe elements and compounds but there are strict rules on how you can combine the words.  Here are the rules and examples:

An element in its normal state can be described as being either atoms or molecules but not ions. For example, you can say a molecule of oxygen, an atom of iron, an iodine molecule.

In chemical reactions, atoms can become charged so then you can call them ions. For example, you can say a sodium ion, an ion of chlorine.

A compound in its normal state can be described as being either molecules or an ionic lattice (or you can simply say made from ions) but never atoms. For example you can say salt is an ionic lattice, salt is made from ions, a molecule of carbon dioxide.

The whole situation can seem complicated at first but as you learn more about chemistry and chemical formulae, understanding what each of these five words means is incredibly useful.

Taught science for 16 years at a secondary school in the East Midlands.