Definition. A subatomic particle is a unit of matter or energy that's the fundamental makeup of all matter. According in modern atomic theory, an atom has a nucleus, which is its center, or core. The nucleus contains subatomic particles: protons and neutrons. Protons are positively-charged particles. Neutrons are neutral particles. Surrounding the nucleus is a cloud of very small subatomic particles, called electrons. Electrons are negatively-charged particles.