The smallest parts of an atom are fermions, the elementary particles that form the subatomic components in every atom. Atoms, the smallest particle of an element that still retains the properties of the element, are composed of protons, neutrons and electrons. Protons and neutrons can be broken down further into quarks, one type of fermion. Electrons are a class of leptons, the other type of fermion.
Fermions are the building blocks of matter and energy in the universe, and are thought to be the smallest particles that exist. Fermions make up most of what is observed in the universe.
Fermions are divided into two classes: quarks and leptons. There are six types of quarks: up, down, charm, strange, top and bottom. Protons and neutrons are made up of three quarks each. A proton has two up quarks and one down quark, while a neutron has two down quarks and one up quark. Quarks have a fractional electrical charge, but when they combine into hadrons such as protons and neutrons they do so in a formation that results in a net integer electrical charge.
Leptons also have six types, one of which is the electron. Unlike quarks, which always are found in groups, the leptons are solitary particles.