Protons, neutrons and electrons are composed of even smaller particles such as quarks and leptons.
Atoms , since 2014, are not the smallest unit of matter. Particles called quarks and leptons are now generally acknowledged to be the fundamental building blocks. Protons and neutrons are made of three quarks each. The known types of quarks are called top, bottom, up, down, strange and charm, while the leptons are classified into electron neutrino, muon neutrino, tau neutrino, electron, muon and tau.
Three quarks make up each proton — two "up" quarks (each with a 2/3 positive charge) and one "down" quark (with a 1/3 negative charge) — and they are held together by other subatomic particles called gluons, which are massless.
Like protons, neutrons are also made of quarks — one "up" quark (with a positive 2/3 charge) and two "down" quarks (each with a negative 1/3 charge).
Electrons are not made of quarks. They are in a separate "family" of particles known as LEPTONS.