An atom is the smallest piece of an element that still acts like that element. Everything around you, including you, is made of atoms: the chair you're sitting on, the air you breathe, even the cells in your body. Atoms are incredibly tiny. A single grain of sand contains more atoms than there are stars in the entire visible universe.

For thousands of years, people wondered what would happen if you kept breaking matter into smaller and smaller pieces. Would you eventually reach something you couldn't divide further? Ancient Greek philosophers imagined this smallest piece and gave it a special name. They were right that atoms exist, though wrong about atoms being truly indivisible.

Scientists discovered that atoms themselves contain even smaller particles: a dense center called a nucleus (made of protons and neutrons) surrounded by a cloud of electrons. Despite containing these parts, an atom of gold still behaves like gold, and an atom of oxygen still behaves like oxygen. Break an atom apart further, and you no longer have that element anymore.

Different elements are atoms with different numbers of protons. Hydrogen has one proton, helium has two, and gold has 79. When atoms bond together, they form molecules, like how two hydrogen atoms and one oxygen atom combine to make water. Understanding atoms unlocked modern chemistry, physics, medicine, and technology.