Fermions

Fermions are matter particles. Fermions have half-integer spin and follow Fermi–Dirac statistics; this just means that fermions follow the Pauli Exclusion Principle, which states that no two identical half-integer spin particles may occupy the same quantum state simultaneously.

Fermions can be fundamental matter particles, such as the electron, which cannot be broken down further, and they can also be composite matter particles— such as the familiar proton and neutron, which can be broken down into smaller fundamental fermions, such as quarks. We will be focusing on fundamental fermions— the smallest particles of matter.

Quarks

Quarks represent one of the two types of fundamental fermions (the other being Leptons, see below). Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons– the familiar components of atomic nuclei.

Quarks are subject to all four fundamental forces, unlike leptons, which do not interact with the strong force. Because quarks interact with the strong force (aka color force) they possess color charge— red, green, or blue; the strong force also keeps quarks bound together in the form of hadrons (protons, neutrons, etc.). Due to a phenomenon known as color confinement, quarks are never directly observed or found in isolation; they can be found only within hadrons. For this reason, much of what is known about quarks has been drawn from observations of the hadrons themselves. Leptons on the other hand, do not interact with the strong force. This is why the electron (a type of Lepton) flies freely around the outside of an atomic nucleus, while quarks remain bound together within the protons and neutrons that make up the core of an atom.

Quarks have fractional electric charge values – either 1⁄3 or 2⁄3 e, depending on the kind of quark. There are two main classes of quarks: up-type (up, charm, and top quarks), which have an electric charge of +2⁄3, and down-type (down, strange, and bottom quarks) which have an electric charge of −1⁄3.

As discussed in the forces section, the weak force (weak interaction or weak nuclear force) is one of the four fundamental forces of nature and is the mechanism responsible for both radioactive decay and nuclear fusion of subatomic particles. Quarks come in generations— up and down quarks are &lsdquo;first generation&rsdquo; which means they are the least massive and most commonly observed, charm and strange are &lsdquo;second generation,&rsdquo; and top and bottom are ‘third generation’ and are the most massive and rare. Through interaction with the weak force, second and third generation quarks decay into first generation quarks.

Second and third generations of quarks (charms, stranges, tops, bottoms) do not occur in everyday matter and are only seen in extremely high-energy environments such as cosmic rays or particle accelerators. They quickly decay to up and down quarks which are the fundamental constituents of the matter that composes our daily environment.

Leptons

Leptons are fundamental, spin 1⁄2 particles (fermions) that unlike quarks, do not interact with the strong force, but like all fermions, are subject to the Pauli Exclusion Principle. The most familiar of all the leptons is the electron, which governs nearly all of chemistry as it is found in atoms and is directly tied to all chemical properties.

Two main classes of leptons exist: charged leptons (also known as the electron-like leptons), and neutral leptons (better known as neutrinos). Neutrinos have no electric charge (0), but charged leptons like the electron, carry an integer charge of -1 e (their antiparticles, such as the electron's antipartlce—the positron— carry a +1 charge). Charged leptons can combine with other particles to form various composite particles such as atoms, while neutrinos rarely interact with anything, and are consequently rarely observed.

Unlike quarks, leptons are not subject to the strong force (aka color force) and thus do not have any color charge. Charged leptons participate in the other three fundamental interactions: the weak force, gravitation, and electromagnetism, while neutral leptons (neutrinos) are electrically neutral and therefore only interact with the weak force and gravitation. The electromagnetic force governs most of the phenomena we experience in daily life and in normal chemical reactions- if the strong or weak forces becomes involved in a reaction, such as in radioactive decay-then it is a nuclear and not a chemical reaction. So neutrinos, as electromagnetically neutral fermions, are really only observed in nuclear reactions, which is why electron neutrinos are much less prevalent than electrons.

Like quarks, leptons can also be grouped by generation— electrons and electron neutrinos are &lsdquo;first generation&rsdquo; which means they are the least massive and most commonly observed, muon and muon neutrinos are &lsdquo;second generation,&rsdquo; and tau and tau neutrinos are ‘third generation’ and are the most massive and rare. Through interaction with the weak force, second and third generation leptons decay into first generation leptons.

Second and third generations of leptons (muons, taus, and their associated neutrinos) do not occur in everyday matter and are only seen in extremely high-energy environments such as cosmic rays or particle accelerators. They quickly decay to first generation leptons.