An atom is mostly empty space, with tiny electrons orbiting a dense central nucleus.

The longer version

Nearly all of an atom's mass sits in a central nucleus that occupies a vanishingly small fraction of its volume. Electrons move around that core in clouds described by quantum mechanics rather than neat orbits, and they account for almost none of the mass. The rest is space with no solid substance in it.

The reason a table feels solid is electrical repulsion, not contact. Electron clouds around neighbouring atoms push back strongly, so objects resist being pushed together. Compared with the size of the nucleus, the whole atom is enormous, which is why most of it qualifies as empty.

Where this entry sits

Everyday physics and chemistry hide small wonders in plain sight, from kitchen reactions to the light in the sky. These short facts explain matter, energy, electricity, magnets, sound, and the simple units we use to measure the world. The full list sits on the Science and Physics page, where every entry is listed in order.