The Greeks first proposed that matter could be cut up into little bits. There would come a stage they thought at which the bits could not be made any smaller. The smallest bits that matter could be cut into they called atoms.
Today we know that all ordinary matter is made up of atoms. There are over a hundred different types of atoms, that make up the periodic table. Today we know that atoms are not the smallest possible particles. Atoms are made up of smaller particles – protons, neutrons and electrons. The protons and neutrons are in the nucleus of the atom, and electrons orbit around the outside like little planets.
It is now known that neutrons and protons are also not fundamental. Neutrons and protons are made up of smaller particles called quarks. The quarks may or may not be made up of smaller particles.
Particles that are not made up of smaller particles are called fundamental. The electron is believed to be a fundamental particle. No electron has ever been split into smaller particles and as far as we know, the electron is a point particle, so has no size and cannot be split.