# Structure of Atoms

When I was in high school (70s) we learned about protons, neutrons, and electrons.

But now we know about quarks, leptons, bosons, etc.

Is it possible that there are even smaller particles than quarks, etc. or is that it?

Turtles. Turtles, all the way down.

In your example, leptons and bosons are classes of particles, not particles themselves. Bosons and fermions are the two primary classes of particles. Within fermions, there are hadrons and leptons–the electron is the lightest lepton. Quarks are a type of fermion.

To answer your question, no. As far as we know, there is no internal structure to quarks or electrons. They are both elementary particles.

There is some talk of “preons”, but they are at this point theoretical and not certainly necessary to explain any particular effect. There are some “input parameters” to the standard model (e.g. particle masses) that would nice to explain in terms of something else, but no specific reason to believe that that something else necessarily is a bunch of more fundamental particles.

You have things kind of mixed up:

Electrons, quarks, and leptons are elementary particles. Meaning we don’t know how to divide them into small units.

Protons, neutrons and bosons are not elementary particles.

A few fixes…

“Quarks” is as much a class of particles as “leptons”. No reason to remove that from the OP’s list.

Once you start talking about composite particles it’s not the most obvious grouping.

A hadron is not an elementary particle (maybe you didn’t mean so, but worth clarifying), and hadrons are not a subset of fermions by any means. For elementary particles, quarks and leptons are fermions. Some hadrons also happen to be fermions, but that just means they have half-integer spin (intrinsic angular momentum).

The electron is the fourth lightest lepton, with the three neutrinos being lighter leptons.

For clarity, since you list both: electrons are a type of lepton.

Some bosons are elementary particles.

Here’s a quick summary of the jargon…

If you take a look at the diagram at the top of this wiki page on the Standard Model:

• Everything in that table is fundamental as far as we know, and everything that we know that is fundamental is in that table.

• In purple are the quarks. In green are the leptons. In red and yellow are the fundamental bosons.

• If a particle has spin 0, 1, 2, … or any integer (times Planck’s constant), it is called a boson. This is why the gluon, photon, Z, W, and Higgs are all bosons. Composite particles can also have integer spin, and when they do they are also called bosons, just not fundamental ones.

• If a particle has spin 1/2, 3/2, 5/2, … or any half-integer (times Planck’s constant), it is called a fermions. This is why the quarks and leptons are all fermions. Composite particles can also have half-integer spin, and when they do they are also called fermions, just not fundamental ones.

• Quarks (or more nitpickily, quarks and gluons) combine together to form composite particles. These composite particles are collectively termed hadrons. Neutrons and protons are hadrons consisting of three quarks each. Pions and kaons are less well known particles that consist of a quark plus an anti-quark. There are hundreds more of each variety, and the three-quark hadrons are called baryons and the quark+antiquark hadrons are called mesons.

• No evidence has been found for substructure of any of the fundamental particles, but this is an active area of research both experimentally (looking for hints) and theoretically (coming up with ideas for what the underlying math might look like).

Strictly speaking, quarks are a subset of the hadrons, since the hadrons are just the particles which are subject to the Strong Force. Usually, though, when we speak of hadrons, we mean the composite particles composed of quarks.

And the Standard Model does not include gravitons, which are presumed to exist and which would presumably be just as fundamental as photons etc., but we know very little about them (certainly very little in the language of particle physics).

Hadrons are strictly the composite particles made up of quarks and gluons. Quarks are not themselves hadrons. (From the Greek “hadros”, meaning “bulky”. The etymology matches actual usage.)

When someone describes a particle’s “spin” is this meant literally? As in the particle is rotating like the earth around its axis? Or is spin just a word used to describe an unrelated mathematical property?

The latter: it’s a property that we can’t really observe or understand. “Spin” is just a convenient way to visualize it, like the + and - of protons and electrons, or the colors of quarks.

It’s true that quantum spin is best thought of as an abstract mathematical property, and definitely shouldn’t be thought of like a spinning planet. However, quantum spin and macroscopic spin both carry angular momentum, so they aren’t completely unrelated. A charged particle with spin also has a magnetic moment, just as if we moved a charged object in a circle.

And in atoms, electron-spin angular momentum (typically written as S) interacts with the orbital angular momentum (L) to observable effect. Oftentimes, an electron is in a state of definite combined angular momentum (J=L+S) that has undetermined values of L and S individually.

Spin angular momentum is conserved with regular angular momentum. If you created a disk out of some matter with all of the electrons being spin up, and then snapped your fingers and switched all of the electrons to spin down, then the disc would start to rotate to compensate with regular angular momentum for the change in spin angular momentum.

Well, maybe not “just as”. The magnetic moment is approximately twice as large (slightly over twice) as one would think from the analogy with classical angular momentum.

Isn’t spin also related to symmetry functions? I thought you had to rotate a spin 1/2 fermion (like an electron) around 720 degrees to have it “look the same”.

The spin state of a spin-1/2 particle is described by a mathematical structure (and He-man villain) called a spinor. Spinors have different properties under rotation than geometric vectors in that they invert under 360 degree rotations instead of returning to their original state. How much that translates to “look the same” depends on how you define “look”.

I recently learned that some isotopes, e.g. Bismuth-210, have two different forms. One form of [sup]210[/sup]Bi decays via β-particle with half-life 5 days; the other form decays via α-particle, half-life 3 million years.

How well is this understood? I guess the nucleons form a shell structure much like electrons do. But an atom with excited electron will soon decay into the base state, right?

Has String Theory lost it’s appeal now?

Don’t know if it has lost its appeal but I have heard that it has failed to make any testable predictions that can be confirmed (or not) via experiment

From what I understand, it’s kind of like how it is hard to throw something into the sun. One think, “inside the sun is a much lower energy than 1AU away is, so it should just drift in that direction”, but in reality that lost energy and angular momentum has to go somewhere and in space there’s precious little to take it away.

Similarly the higher energy nuclear isomer state and the lower energy nuclear isomer state have a significantly different spin, and there aren’t any particles it can emit to take all of that spin away in one go, so it has to happen through a multi-particle interaction which is rare as fuck.