Break anything in the world into smaller and smaller pieces and you eventually reach a piece that can be broken no further by ordinary means: an atom. Everything — you, the air, a star — is built from these unimaginably tiny bricks. And yet an atom is itself almost entirely empty space.
Right at the centre sits a minute, incredibly dense core called the nucleus, a tight bundle of protons (positively charged) and neutrons (no charge at all). Whizzing around it, far out in the emptiness, are the lightweight electrons (negatively charged). If the nucleus were the size of a marble, the nearest electrons would be the length of a football field away.
Play with the two knobs below. Adding protons changes which element you have — one proton is hydrogen, six is carbon, eight is oxygen. The number of protons is the atomic number, Z. Adding neutrons makes the nucleus heavier without changing the element: protons plus neutrons together give the mass number, A. Watch the electrons fill their shells as the atom grows.
Most nuclei are perfectly content to sit still forever. But some are unstable: they have too many neutrons, or simply too many particles crammed together, and sooner or later they break apart, flinging out radiation. We call this being radioactive. It is how the Sun's warmth begins, how we date ancient bones, and how a hospital scanner sees inside you.
Start by meeting the atom itself and its three particles in