Atom, Element and Compound: What's the Difference?
Learn the difference between an atom, an element and a compound for AQA GCSE Chemistry, with oxygen and water examples and a common exam mistake to avoid.
This article covers 4.1.1.1 of AQA GCSE Chemistry 8462 (separate science), for Foundation and Higher Tier.
An atom is a single particle of an element. An element is a substance made of only one type of atom. A compound is a substance made of two or more different elements chemically combined in fixed proportions.
Atom and element
An atom is the smallest part of an element that can exist. For example, O\mathrm{O}O represents one oxygen atom. The element oxygen contains only oxygen atoms: it does not contain atoms of another element.
An element can contain more than one atom in each particle and still be an element. Oxygen gas consists of molecules with the formula O2\mathrm{O}_2O2. Each molecule contains two oxygen atoms, but only one type of atom, so oxygen gas is not a compound.
Compound
A compound forms when atoms of different elements chemically combine. Water, H2O\mathrm{H}_2\mathrm{O}H2O, is a compound because it contains hydrogen and oxygen atoms. Its formula shows a fixed proportion: two hydrogen atoms for every one oxygen atom.
Compounds have properties different from those of the elements that form them. Their elements cannot be separated by a physical process such as filtration; separating a compound into elements requires a chemical reaction. This is different from a mixture, whose substances are not chemically combined.
To check a formula, count the different element symbols. In O2\mathrm{O}_2O2, there is only oxygen, so it is an element. In H2O\mathrm{H}_2\mathrm{O}H2O, there are two symbols, hydrogen and oxygen, so it is a compound. The small numbers tell you how many atoms of each are present.
Exam mistake to avoid: do not say that a compound is simply “two or more atoms joined together”. A molecule of O2\mathrm{O}_2O2 has two atoms joined together, but it is an element because both atoms are oxygen. For a compound, the atoms must come from different elements.
