Introduction

Everything around us that occupies space is matter, and matter appears in the forms solid, liquid and gas. Different materials behave very differently from one another - water, petrol, oxygen, stainless steel - and to understand why their physical and chemical properties differ, physicists needed to identify the fundamental building blocks that make them up.

The idea of an indivisible smallest particle goes back to ancient Greek thought: the word "atom" itself comes from the Greek for "without division." When ordinary matter is divided again and again, there comes a point beyond which further division is not possible in the same way - the leftover speck is what was named the atom. Atoms are extraordinarily small: a hydrogen atom, the simplest of all atoms, has a radius of about $10^{-10}$ m. Richard Feynman's analogy captures just how small this is - if a single atom were magnified to the size of an apple, that same apple, scaled up by the identical factor, would become the size of the Earth.

This unit builds up the modern understanding of matter in stages. First, the historical models of the atom (Thomson, Rutherford, Bohr) are studied to understand how the atom's own internal structure was worked out experimentally. It later became clear that the atom itself is not fundamental - it consists of a central nucleus surrounded by electrons - and by around 1930 it was established that the nucleus itself is built from protons and neutrons. Even more fundamentally, protons and neutrons are now known to be made of quarks. The remainder of the unit is devoted to the structure and properties of the nucleus itself, and to how nuclear energy is produced and used.