Protons and Neutrons Trivia Quiz

Answer: They're both present in atomic nuclei.

The word "nucleon" is a generic term for a proton or a neutron, the basic components of atomic nuclei. The mass of an atom is nearly all in its nucleus, which is made up of protons and neutrons bound together by the strong force. The number of protons determines which element it is, while the number of neutrons gives the specific isotope of that element; some isotopes are stable, and some are not.

In a neutral atom, the positive charge of the nucleus is offset by negatively charged electrons, which orbit the nucleus in comparatively vast clouds. Chemical reactions occur when different atoms interact through their electrons.

Answer: Neutron

The neutron is named for the fact that it's electrically neutral: neither positive nor negative, an electromagnetic cipher. (It does have a nonzero electric field, since it's made out of charged particles, but you have to get very close to see it, and the net charge is still zero.) Its lack of charge is why the neutron was discovered decades after the proton was: it's harder to manipulate with electromagnetic fields, and it's harder to detect. For the same reason, particle accelerators like the Large Hadron Collider use electromagnetic fields to accelerate and guide protons and anti-protons, not neutrons and anti-neutrons.

The proton charge of +e is equal to 1.602 x 10^(-19) Coulombs, the SI unit for charge. Like the other SI units, the Coulomb was first defined based on tabletop experiments, so it's quite oversized in the particle world!

Answer: Up quarks and down quarks

Quarks are elementary particles that interact through all four fundamental forces. They have mass, so they feel gravity (albeit not very much). They have electric charge. They feel the strong force; in fact, that's what binds them together in composite particles like protons and neutrons. And they come in six flavors, which govern the way a quark experiences the weak force.

Each flavor -- up, down, strange, charm, bottom, and top -- has a different mass. Three of them (up, charm, and top) have an electric charge of positive 2e/3, where e is the size of the electron charge, and the other three have a charge of negative e/3. Up and down quarks are the lightest of the six, and they're the constituents of our heroes. The proton has two ups and a down (uud), while the neutron has two downs and an up (udd). See how those charges add up?

Answer: Gluon

The strong force is well-named: over short distances (like the inside of an atomic nucleus) it's far and away the strongest of the four fundamental forces. Yet it can't act over long distances because the particles that carry it also carry strong-force charge, which means they interact with each other -- and get tangled up in each other. The quarks inside a proton or a neutron are constantly exchanging gluons, which add their own signature to the mix.

Photons are the force carriers for electrodynamics; they're massless and electrically neutral. W+ bosons, along with W- and Z bosons, are very massive and carry the weak force. Poor K mesons are just bound quark-antiquark pairs, at the mercy of the other three!

Answer: Pairs of quarks and antiquarks

The space between and around the constituent quarks is rich in energy carried by gluons. A gluon may spontaneously create a quark and its antimatter counterpart, an antiquark; just as spontaneously, the pair may annihilate each other and thereby recreate the gluon. Lighter flavors of (anti)quarks are more likely to be created this way, so the sea quarks are mostly ups and downs, with a dash of stranges.

The pairs may be fleeting, but as a collective they contribute quite a lot to the mass and spin of both protons and neutrons.

Answer: The particle must be colorless.

In electrodynamics, there's only one type of charge, and it can be positive or negative. The strong force, however, sees three types of charge, each with a positive and negative sense. By convention, physicists refer to these as color charges: red, blue, green, anti-red, anti-blue, and anti-green. When you combine three different colors, or a color and its anti-color, you end up with something colorless, just as you can make white light by combining light in each of the primary colors.

It turns out that nature abhors color: you will never observe a particle with a bare color charge. They're always "confined" inside a colorless particle. If you try to pull a single quark out of a proton, for example, the force you're working against increases -- and when the bond breaks, like a snapping rubber band, that gluon generates a quark at one end and an antiquark at the other, enforcing colorlessness.

Most of these particles can be described as dipoles, but this is an electromagnetic characteristic. Something with positive parity would look the same if reflected in a mirror. Exotic particles are impossible to build in the constituent-quark model: you need gluons to take a more active role.

Answer: Proton

Particle masses are measured in terms of electron Volts divided by the square of the speed of light, where an electron Volt (or eV) is the amount of energy an electron gains or loses when crossing an electrical potential of 1 V. One eV/c^2 works out to about 1.8 x 10^(-36) kg! An electron has a mass of about 511,000 eV/c^2. A proton mass is 938.2 million eV/c^2, while a neutron has a mass of 939.6 million eV/c^2. This sounds like a lot, but it's tiny compared to our everyday, macroscopic world.

Some of this mass comes from the masses of the three constituent quarks; most of it comes from the energy of their motion, from the quark-antiquark sea, and from the gluons. The seemingly small mass difference between proton and neutron has a big effect, though: as we'll see later, it makes the neutron rather an unstable character.

Answer: Both have spin 1/2.

Technically, their spin is h-bar over two, where h-bar is the modified Planck's constant. All spins are integer or half-integer multiples of h-bar, so it's a convenient (and common) shorthand to divide h-bar out and just give its coefficient. Protons and neutrons, like electrons, have spin 1/2, which makes them fermions. You can't have more than one identical fermion occupying any given quantum-mechanical state, which is why chemistry and nuclear structure are the way they are. Photons, however, have spin 1, making them bosons -- and any number of identical bosons can occupy the same state.

Since protons and neutrons are composite particles, their spins are combinations of the spins and orbital motions of their constituents: quarks (spin 1/2), antiquarks (spin 1/2), and gluons (spin 1). In quantum mechanics, summing up three spins of 1/2 to get a spin of 1/2 is no problem, so everyone assumed that the spin came from the constituent quarks -- until a pair of experiments in the 1980s showed that the quark and antiquark spins nearly cancel each other out. This "proton spin crisis" spawned a research area -- the study of proton and neutron spin structure -- that is still active and exciting. Your correspondent may be biased, though, having become Dr. CellarDoor with a thesis in this very field!

Answer: Isospin symmetry

In this picture, protons and neutrons are isospin partners: a proton has an isospin of +1/2, a neutron has an isospin of -1/2, and the strong force sees the members of the doublet in the same way. This concept doesn't just apply to nucleons. For example, the three pi mesons -- positively charged, negatively charged, and neutral -- make an isospin triplet.

We now see isospin as a type of flavor symmetry: why should the strong force see a difference between up quarks and down quarks? This argument breaks down with other flavors, whose quarks have very different masses, but isospin symmetry is a pretty good description of the world.

Answer: Weak force

The weak force changes the flavors of quarks and leptons: a strange quark might become a charm quark, or a muon might become an electron (additional particles are involved in both cases). In a radioactive beta decay -- such as free-neutron decay -- the flavor of a constituent quark changes, in a spontaneous process mediated by the weak force, if the result will be a system with lower energy. When a free neutron decays, one of its constituent down quarks transmutes to an up quark, creating and releasing an electron and an electron-flavored anti-neutrino. The free neutron has become a free proton.

When bound in a nucleus, however, the neutron is stable, with one important caveat. Some nuclei are inherently unstable, based on the numbers of protons and neutrons that are bound within. In that case, though, it could be either a proton or a neutron undergoing beta decay, depending on the nucleus.