Subatomic particle
A subatomic particle is a particle smaller than an atom. This means it is very, very small.  Like atoms and molecules, a subatomic particle is far too small to be seen with the eye.  They are also very interesting to physicists who study atoms.  The subatomic particles that make atoms: protons, neutrons, and electrons are the most commonly studied ones.  The study of subatomic particles is called particle physics.