Inside the Atom

Matter, elements and atoms

Everything around you is matter: anything that has mass and takes up space.

• An atom is the smallest particle of an element that still behaves like that element.
• An element is a pure substance made of only one kind of atom. Examples: carbon, oxygen, gold. There are 118 known elements.
• A compound is two or more elements chemically joined in a fixed ratio, like water (${\mathrm{H}}_2\mathrm{O}$) or table salt ($\mathrm{NaCl}$).
• A molecule is two or more atoms joined together. ${\mathrm{O}}_2$ is a molecule of an element. ${\mathrm{H}}_2\mathrm{O}$ is a molecule of a compound.

Atoms are tiny. About ten million carbon atoms side by side would span one millimetre.

How our idea of the atom changed

No one has seen an atom with their eyes. Scientists built models from evidence, and changed the model when new evidence came in.

The modern picture

An atom has two parts.

1. The nucleus: in the centre, tiny, holds almost all the mass. It contains protons (positive) and neutrons (no charge).
2. The electrons: very light, negative, moving around the nucleus in shells.

The nucleus is about 100 000 times smaller than the whole atom. If an atom were the size of a football stadium, the nucleus would be a marble on the centre spot.

Quick summary

• An atom is the smallest particle of an element. An element has one kind of atom. A compound has elements chemically joined.
• Models changed with evidence: Dalton, Thomson, Rutherford, Bohr, Chadwick.
• The gold foil experiment showed the atom is mostly empty space with a tiny, dense, positive nucleus.
• The nucleus holds protons and neutrons and nearly all the mass.
• Electrons are negative, very light, and move in shells around the nucleus.

Quiz