Inside the Atom

In short: An atom is the smallest particle of an element. It has a tiny, dense, positive nucleus of protons and neutrons that holds nearly all its mass, with light, negative electrons moving in shells around it, and the atom is mostly empty space.

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.

Two laws that led to the atom

Around 1800, careful measurements gave chemists two laws that every reaction obeyed.

1. Law of conservation of mass (Lavoisier): mass is not gained or lost in a chemical reaction.
2. Law of definite proportions (Proust): a compound always contains the same elements in the same proportions by mass. Water is always about 11% hydrogen and 89% oxygen by mass.

In 1803 John Dalton asked: what must matter be like for these laws to be true? His answer was Dalton's atomic theory.

Dalton's theory was not perfect, but it turned atoms from a philosopher's guess into a scientific idea that could be tested.

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