What Is an Atom? The Tiny Building Block Behind Everything

Look around at any collection of objects — a glass of water, a metal spoon, the air in the room — and despite looking completely different, all of them are built from the same basic kind of building block: atoms. An atom is the smallest unit of a chemical element that still has that element’s properties, and different arrangements and combinations of atoms are what create every different substance in the world.

An atom itself has structure: at its center is a nucleus made of protons (positively charged) and neutrons (no charge), and around that nucleus, negatively charged electrons move in a cloud. The number of protons in an atom’s nucleus determines which element it is — an atom with exactly one proton is always hydrogen, one with exactly six protons is always carbon, and so on. There are only about 118 known types of atoms (elements), yet their combinations create the millions of different substances that exist.

Atoms are almost unimaginably small and almost entirely empty space — if an atom were scaled up to the size of a football stadium, its nucleus would be roughly the size of a marble at the center, with the electrons as tiny specks far out near the stadium’s outer walls. Despite that emptiness, atoms behave in ways that make solid matter feel solid, because of the electric forces between their charged particles.

Atoms rarely exist completely alone in everyday matter — they usually bond together into molecules (like two hydrogen atoms and one oxygen atom bonding to form water, H₂O) or larger structures. Almost every property of the substances people interact with daily — texture, color, whether it’s a liquid or a solid — ultimately traces back to which atoms are involved and how they’re bonded together.