Atoms, elements and compounds: All substances are made of atoms. An atom is the smallest part of an element that can exist.

The idea

This statement establishes the particle model that underpins all of chemistry. Every substance, whether an element, a compound or a mixture, is built from atoms, so the properties of a material come from the atoms it contains and how they are arranged. An element contains only one type of atom, and that single atom is the smallest piece of the element that can exist. For example, a piece of copper is made of countless copper atoms; if you kept dividing it, you would eventually reach one copper atom, and splitting that atom would give particles that are no longer copper. Atoms are extremely small, roughly 1 × 10⁻¹⁰ m across, so they cannot be seen with a school light microscope. They are not indivisible, however: each atom has a nucleus containing protons and neutrons, surrounded by electrons.

What earns the marks

• States that all substances are made of atoms, including elements, compounds and mixtures.
• Defines an atom as the smallest part of an element that can exist.
• Recognises that an element contains only one type of atom.
• Explains that atoms are the building blocks of all materials and are too small to see with a light microscope.
• Uses the idea that atoms themselves contain smaller particles, such as protons, neutrons and electrons, so they are not indivisible.

Common mistakes and how to put them right

• Saying an atom is the smallest particle that exists.

  Put it right by specifying the smallest part of an element, because atoms contain protons, neutrons and electrons.

• Confusing atoms with molecules.

  Put it right by noting that a molecule is two or more atoms bonded together, whereas an atom is a single particle of an element.

• Thinking compounds contain only one type of atom.

  Put it right by stating that a compound contains atoms of two or more elements chemically combined.

Exam tips

• Use the exact phrase 'smallest part of an element' when defining an atom, because vague wording such as 'smallest thing' loses credit.
• Link the definition to a named element, for example copper or oxygen, to show the idea applies to a real substance.
• If asked why atoms cannot be seen, refer to their very small size, roughly 1 × 10⁻¹⁰ m, rather than saying they are invisible.