The elementary particles are the particles that make up everything that we know and understand about our world and our universe. The majority of the particles that we know of are described by a wonderful masterpiece called the Standard Model. This model describes all of the known forces (except gravity) and particles (except the theoretical graviton). While the Standard Model is not entirely perfect in describing the way that the particles behave, it has, in its own way, an excellent method of grouping the particles to make it easier to understand their behaviors with other particles in the immediate vicinity.
The Standard Model divides all the particles in the universe into to categories: the fermions and the bosons. Fermions are the particles that have a 1/2 angular momentum (like 1/2, 3/2, 5/2, etc.). The bosons are the particles that have angular momentums that are integers (like 1, 2, 3, etc.). The fermions are further split into two major categories: the leptons and the quarks. Leptons are fundamental particles, and they include the electron, the muon, the tau particle, and their respective neutrinos (the electron neutrino, the muon neutrino, you get the idea). One of these six, as you may be able to recognize, is one of the particles in the atom. The electron is actually the only particle of the atom that is a fundamental particle (you will see why in a minute). It is also the most commonly occurring of the leptons, as the others break down quickly and are unstable.
Quarks are fermions that have fractional charges (meaning that instead of having a +1 charge, they have a +2/3 charge). There are six different types of quarks: the up quark, the down quark, the charm quark, the strange quark, the top quark, and the bottom quark. The up, charm, and top have a +2/3 charge, and the down, strange, and bottom quark all have a -1/3 charge. The only differences between each type of quark is that the charm quark is more massive than the up quark, the top quark is more massive than the charm quark, and the same applies for the other set (in the respective order).
Do you remember how I mentioned earlier that the electron is the only fundamental particle in the atom? Well, that is because the neutron and the proton are both formed of these quarks. The proton is composed of two up quarks and one down quark (resulting in an overall charge of +1), and the neutron is composed of one up quark and two down quarks (thus resulting in an overall charge of 0). The addition of the charges of the respective quarks results in the creation of two particles that are essential to the formation of macroscopic matter.
So what are the other particles that I was mentioning earlier? The particles called bosons are those with integral angular momentum. The major bosons described by the Standard Model are the ones that contribute to the interactions between particles. The particles described by the Standard Model are the photon, the gluon, and the W and Z bosons. The last major boson is the graviton, but since it has not been “seen”yet, it, as well as the force of gravity, is not described by the Standard Model.
The first boson to describe is the photon. This boson is responsible and is the carrier for the electromagnetic force (the force that keeps the electrons around the protons). The second is the gluon, which is the carrier of the strong force (the force that is responsible for keeping an atom’s nucleus together). The third set of bosons are the W and Z bosons. These are very massive particles, and they are responsible for the weak nuclear force (the force that is responsible for radioactive decay).
One of the main differences between fermions and bosons is that the fermions must follow the Pauli Exclusion Principle. This principle states that no two fermions may have the exact same four quantum numbers at the same time (meaning that no two fermions may be located in the same space at the same time. However, bosons do not have to follow this rule, so you could (hypothetically) have two bosons that have the exact same four quantum numbers (they are in the same location).
So what about the elusive gravitational force and graviton? One of the reasons why the graviton is not included in the Standard Model (as well as the gravitational force as a whole) is because of the incompatabiliy of the two theories of quantum mechanics and the general theory of relativity. These two theories operate at two very different scales (the former being the micro-micro-scopic scale, the other being the macro-macro-scopic scale). These two theories have various ideas as to what gravity is, so the force of gravity and the carrier of the force (the graviton) are not included in the Standard Model.
Although this article looks long, I am not done yet. All of the above particles also have a twin. This twin is known as their antiparticle. The antiparticle has the exact same mass as its respective particle, but it has an opposite electrical charge. However, the antiparticles anihilate their respective particles when they meet each other, and in the process, photons are created and released.
The Standard Model is a model full of complexities and interesting things. It is composed of so many particles that it is often hard even for particle physicists to keep track of them all. However, the Standard Model helps to explain the interactions that these particles have with other particles, and it helps us to keep track of all of them so we may organize them to better understand them.