Atom

The atom is the fundamental building block of matter. A substance composed of atoms of the same kind is called a chemical element.

Composition:
- a nucleus of nucleons: protons and neutrons where practically the entire mass of the atom is concentrated,
- electrons, very small, orbiting far from the nucleus.

The size of the atom: of the order of 10^-10 m

The smallest atom is that of hydrogen: 1 single proton, 1 electron.