An atom is the smallest particle of an element that retains the chemical properties of the element. That is a nice simple definition.

So the definition of an atom is closely ties to the concept of an element. However, remember there are some elements for which the simplest particle which retains the properties of the element is a molecule. For example hydrogen contains diatomic molecules, which we can represent using the formula H₂. Hydrogen atoms (a single atom of hydrogen) has different properties compared to the diatomic molecule, H₂.

Representing an atom pictorially we would draw a circle () on a piece of paper, but we would imagine a sphere (a ball) since we can imagine in 3-dimensions.

Another statement that is often made is that matter is composed of atoms. This statement does not refer to a specific example of matter, but of matter in general. And in general matter is composed of atoms. If the example of matter is an element we know that it is made up of only one kind of atom. If the example of matter is a compound we know there are two or more kinds of atoms which make up the sample. And we know that the different atoms are chemically 'bonded' to one another in a compound. If the sample of matter is a mixture it may consist of a mixture of elements, a mixture of an element and a compound or a mixture of compounds. Since elements are made of atoms and compounds are made of atoms, therefore the mixture is made of atoms.