After struggling through all of that confusing stuff about spin in my last post, here’s something that’s a bit easier to understand. Essentially, there are two types of elementary particles: fermions and bosons. Fermions, which make up the majority of the particles, are defined by theirStandard_Model_of_Elementary_Particles.svg half spin. (Don’t know what that is? Read my last post for an explanation.) That’s what makes them fermions. Fermions have to obey a law called the Pauli exclusion principle, which states that two identical fermions cannot be in the same place at the same time. That seems like intuitive logic, but bosons don’t have to follow these rules. Subatomic particles like protons are also fermions, but we’ll be focusing on the fermions that are elementary particles. These are composed of particles like neutrinos, electrons, and quarks. The chart above shows all of the elementary particles, but the purple and green groups are quarks and leptons. Those are the two main groups of fermions.

What’s a Lepton?

Aside from quarks, leptons are one of the major groups of fermions. They come in six types, also called flavors. (No one has ever said that physicists were uncreative in their naming of terms.)

Electron, Muon, and Tau Leptons

An atom with electrons orbiting around it.

Three of the flavors, the electron, the muon, and the tau, have a negative charge. An electron is more commonly known as part of the atom. However, the electron has some interesting qualities of its own. It’s tiny in terms of mass; it’s only got about 1/1837 of the mass of a proton. The muon is a lot like an electron, but is much more massive. And the tau? It’s also like the electron, but it’s more massive than either the electron or the muon.

Electron, Muon, and Tau Neutrinos

Is it just me, or do physicists come up with the cutest names for particles? Naming a particle “neutrino” is pretty cute. Neutrinos are still leptons, but they have a neutral charge, and they’re what are known as second generation leptons. This means that they’re created when electrons, muons, and taus decay. Neutrinos are so light they were once thought to have no mass. However, it has since been discovered that neutrinos have three distinct masses, but they don’t have to line up with the three flavors of neutrino. Because of its lightness, few forces can act on the neutrino, so it can move through most matter without a problem.

Quirky Quarks

I’ve always wanted to write a heading like that. And I’m being completely accurate because quarks do have some pretty bizarre qualities. For instance, quarks will never be found on their own; they’re always in groups called hadrons. Quarks are the only particles that experience all of the fundamental forces: electromagnetic force, gravity, weak nuclear force, and strong nuclear force. Up, down, strange, charm, top, and bottom are all flavors of quarks. As for magnetic charge, the up-type quarks, (that is, up, top, and charm quarks) all have a charge of +⅔. On the other hand, the down-type quarks, (down, bottom, and strange) have a charge of -⅓. This makes a little bit more sense once we look at a particle that is made of quarks: a proton. It has a charge of +1 and is made up of two up quarks and one down quark. ⅔ + ⅔ – ⅓ = 3/3 = 1. When you do the math, it adds up.

Color Charge

Quarks also have a property called color charge. This refers to the exchange of gluons (read about them in my next blog post about bosons) when the quarks get close enough to each other aNeutron_QCD_Animationnd the creation of a color force field. (Kind of like spin, it’s only called color to distinguish between the different types, but it has nothing to do with color at all.) Three color charges and three anticolor charges exist. When quarks combine, they form neutral color particles, just like when red, green, and blue combine to form white light. However, they produce different combinations, which produce different types of hadrons. That’s about all I can write about color charge without getting hopelessly lost, but maybe that was enough for you to grasp the basic concept. The animation above shows a neutron, made up of two down quarks and one up quark and the way the color force interacts through the quarks.


There you have it: a little bit about quarks and leptons, the two types of fermions. I hope you’ve learned something about them and come back next week for a post about bosons, the other kind of elementary particle. If you want, check out some links to further information below. Until next time!



If you want to get an overview of elementary particles in general, you can click to go to my previous blog post here: Elementary Particles – What Are They?

Here’s a great post about leptons from Universe Today: What Are Leptons?

This website goes more in-depth on quarks and their different flavors: Quarks

And here’s something more that really helped me understand color charge better: The Particle Adventure – Color Charge

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