question: How are protons and neutrons held together?
comment: Protons and neutrons are made of particles called *quarks*. Quarks are, as far as scientists know, fundamental particles - meaning, quarks aren't made of even smaller particles. Quarks are made of energy, concentrated in the quark field. Quarks have something called *color charge*. To be absolutely clear: **color charge has nothing to do with visible color or photons or light**. It's just the name they chose to give it because it's a convenient analogy that helps explain sort of how it works. You know how white is really just how we perceive a mixture of all visible colors? (If not, [peruse some of these topics]( URL_1 ).) Specifically, because our eyes only detect red, green, and blue, white light is a combination of those. Quarks cannot exist alone. It just isn't possible. There is a reason, it's not important here. What *is* important is that when quarks are together, they must be "color neutral" or "white" - again, I want to emphasize that this doesn't mean literally visibly white, it's just a name, a concept. Protons and neutrons are made out of three quarks each (but slightly different ["flavors" of quarks]( URL_0 )). To be color neutral, a proton or neutron has to have a "red" quark and a "green" quark and a "blue" quark, all together. Regardless of what kind of quarks they are, one of them will have red color charge, one will have green color charge, and one will have blue color charge. [Side note: some rare and short-lived particles have *two* quarks, not three. Instead of red+green+blue, they have red+antired, or green+antigreen, etc. And of course there are particles that have antired+antigreen+antiblue. That's fine. As long as it either has a color+anticolor pair, or a three color set. And somewhat recently, scientists discovered a *very* rare, *very* short lived particle that had a three color set *and* a color+anticolor pair, for a total of five quarks! Crazy stuff.] But, they don't all stay the same color all the time. They will randomly change colors. But their color charge has to *go* somewhere. Kind of like when a positively charged proton absorbs a negatively charged electron, it becomes a neutron with no charge. When a neutron becomes a proton, that extra negative charge has to *go* somewhere, so the neutron emits an electron and becomes a proton. Likewise, when, say, a red quark becomes a green quark, it emits a particle that carries its red charge. That particle is called a *gluon*. Well, now this proton has two green quarks and a blue quark - it isn't color neutral! So the blue quark absorbs the gluon and becomes a red quark! Except now it's two greens and a red quark! The blue-turned-red quark still has to send its blue charge somewhere, though, so *it* emits a gluon. One of the green quarks absorbs it and becomes a blue quark. Huzzah! All three color charges are present, it's color neutral! Except that green quark has to send *its* green charge somewhere, so it sends out a gluon, and the cycle continues. As long as the quarks are sharing gluons between each other, they stick very close to each other and the particle doesn't fall apart. This is called the *color force* and it's the force that holds protons and neutrons together as protons and neutrons. **This is the important part for how protons and neutrons stick to _each other_**: usually the gluons get absorbed by other quarks within that same proton or neutron. *Occasionally*, they end up outside of it and get absorbed by quarks in a nearby proton or neutron. That little bit of sharing of gluons between nearby protons and neutrons is what holds them together. It's the same force - what we called color force a moment ago - sort of "leaking out" of the individual particles to affect the ones next to them. That little leak of color force is strong enough to overcome the repulsive force of the electromagnetic force trying to make all the positively charged protons fly away from each other. In the context of sharing gluons between protons and neutrons, this is called the *strong nuclear force*.
passages: Neither the proton nor neutron is an elementary particle, meaning each is composed of smaller parts, namely three quarks each. A proton is composed of two up quarks and one down quark, while the neutron has one up quark and two down quarks. Quarks are held together by the strong force, or equivalently, by gluons, which mediate the strong force.
   Like protons, the quarks of the neutron are held together by the strong force, mediated by gluons. The nuclear force results from secondary effects of the more fundamental strong force.
  
  Section::::Discovery.
   Nuclear force
  
  The nuclear force (or nucleon–nucleon interaction or residual strong force) is a force that acts between the protons and neutrons of atoms. Neutrons and protons, both nucleons, are affected by the nuclear force almost identically. Since protons have charge +1 "e", they experience an electric force that tends to push them apart, but at short range the attractive nuclear force is strong enough to overcome the electromagnetic force. The nuclear force binds nucleons into atomic nuclei.
   The mass of the proton and neutron is quite similar: The proton is or , while the neutron is or . The neutron is roughly 0.13% heavier. The similarity in mass can be explained roughly by the slight difference in masses of up quarks and down quarks composing the nucleons. However, a detailed explanation remains an unsolved problem in particle physics.
   The above treatment compares neutrons with protons, allowing the complex behavior of quarks to be subtracted out between models, and merely exploring what the effects would be of differing quark charges (or quark type). Such calculations are enough to show that the interior of neutrons is very much like that of protons, save for the difference in quark composition with a down quark in the neutron replacing an up quark in the proton.
   To reduce the disruptive energy, the weak interaction allows the number of neutrons to exceed that of protons—for instance, the main isotope of iron has 26 protons and 30 neutrons. Isotopes also exist where the number of neutrons differs from the most stable number for that number of nucleons. If the ratio of protons to neutrons is too far from stability, nucleons may spontaneously change from proton to neutron, or neutron to proton.
   Inside a nucleus, on the other hand, combined protons and neutrons (nucleons) can be stable or unstable depending on the nuclide, or nuclear species. Inside some nuclides, a neutron can turn into a proton (producing other particles) as described above; the reverse can happen inside other nuclides, where a proton turns into a neutron (producing other particles) through decay, or electron capture. And inside still other nuclides, both protons and neutrons are stable and do not change form.
  
  Section::::Overview.:Antinucleons.
   six protons for every neutron, but a small fraction of the neutrons decay before fusing in the next few hundred seconds, so at the end of nucleosynthesis there are about seven protons to every neutron, and almost all the neutrons are in Helium-4 nuclei. The sequence of these reaction chains is shown on the image.
   Protons and neutrons are both nucleons, which may be bound together by the nuclear force to form atomic nuclei. The nucleus of the most common isotope of the hydrogen atom (with the chemical symbol "H") is a lone proton. The nuclei of the heavy hydrogen isotopes deuterium and tritium contain one proton bound to one and two neutrons, respectively. All other types of atomic nuclei are composed of two or more protons and various numbers of neutrons.
  
  Section::::History.
   The residual strong force is effective over a very short range (usually only a few femtometres (fm); roughly one or two nucleon diameters) and causes an attraction between any pair of nucleons. For example, between protons and neutrons to form [NP] deuteron, and also between protons and protons, and neutrons and neutrons.
  
  Section::::Halo nuclei and nuclear force range limits.
   Section::::Composition.:Particles.:Hadrons.
  
  A hadron is a composite particle made of quarks held together by the strong force. Hadrons are categorized into two families: baryons (such as protons and neutrons) made of three quarks, and mesons (such as pions) made of one quark and one antiquark. Of the hadrons, protons are stable, and neutrons bound within atomic nuclei are stable. Other hadrons are unstable under ordinary conditions and are thus insignificant constituents of the modern Universe.
   The process is reversible; neutrons can convert back to protons through beta decay, a common form of radioactive decay. In fact, a free neutron decays this way, with a mean lifetime of about 15 minutes.
  
  Section::::Quarks and the mass of a proton.
   While the nuclear force is usually associated with nucleons, more generally this force is felt between hadrons, or particles composed of quarks. At small separations between nucleons (less than ~ 0.7 fm between their centers, depending upon spin alignment) the force becomes repulsive, which keeps the nucleons at a certain average separation, even if they are of different types. This repulsion arises from the Pauli exclusion force for identical nucleons (such as two neutrons or two protons). A Pauli exclusion force also occurs between quarks of the same type within nucleons, when the nucleons are different (a proton and a neutron, for example).
   Section::::Overview.
  
  Section::::Overview.:Properties.
  
  Protons and neutrons are best known in their role as nucleons, i.e., as the components of atomic nuclei, but they also exist as free particles. Free neutrons are unstable, with a half-life of around 13 minutes, but they are common in nature and have important applications (see neutron radiation and neutron scattering). Singular protons, not bound to other nucleons, are usually regarded as the nuclei of hydrogen atoms or ions, but in some extreme cases (cosmic rays, proton beams), they may be regarded as free protons.
   Neutron
  
  The neutron is a subatomic particle, symbol or , with no net electric charge and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave similarly within the nucleus, and each has a mass of approximately one atomic mass unit, they are both referred to as nucleons. Their properties and interactions are described by nuclear physics.
   Protons and neutrons behave almost identically under the influence of the nuclear force within the nucleus. The concept of isospin, in which the proton and neutron are viewed as two quantum states of the same particle, is used to model the interactions of nucleons by the nuclear or weak forces. Because of the strength of the nuclear force at short distances, the binding energy of nucleons is more than seven orders of magnitude larger than the electromagnetic energy binding electrons in atoms. Nuclear reactions (such as nuclear fission) therefore have an energy density that is more than ten million times that of chemical reactions. Because of the mass–energy equivalence, nuclear binding energies reduce the mass of nuclei. Ultimately, the ability of the nuclear force to store energy arising from the electromagnetic repulsion of nuclear components is the basis for most of the energy that makes nuclear reactors or bombs possible. In nuclear fission, the absorption of a neutron by a heavy nuclide (e.g., uranium-235) causes the nuclide to become unstable and break into light nuclides and additional neutrons. The positively charged light nuclides then repel, releasing electromagnetic potential energy.
   The nuclear forces arising between nucleons are analogous to the forces in chemistry between neutral atoms or molecules called London forces. Such forces between atoms are much weaker than the attractive electrical forces that hold the atoms themselves together (i.e., that bind electrons to the nucleus), and their range between atoms is shorter, because they arise from small separation of charges inside the neutral atom. Similarly, even though nucleons are made of quarks in combinations which cancel most gluon forces (they are "color neutral"), some combinations of quarks and gluons nevertheless leak away from nucleons, in the form of short-range nuclear force fields that extend from one nucleon to another nearby nucleon. These nuclear forces are very weak compared to direct gluon forces ("color forces" or strong forces) inside nucleons, and the nuclear forces extend only over a few nuclear diameters, falling exponentially with distance. Nevertheless, they are strong enough to bind neutrons and protons over short distances, and overcome the electrical repulsion between protons in the nucleus.
   The proton and neutron are both fermions, hadrons and baryons. The proton carries a positive net charge and the neutron carries a zero net charge; the proton's mass is only about 0.13% less than the neutron's. Thus, they can be viewed as two states of the same nucleon, and together form an isospin doublet (). In isospin space, neutrons can be transformed into protons via SU(2) symmetries, and vice versa. These nucleons are acted upon equally by the strong interaction, which is invariant under rotation in isospin space. According to the Noether theorem, isospin is conserved with respect to the strong interaction.
   The conversion of protons to neutrons is the result of another nuclear force, known as the weak (nuclear) force. The weak force, like the strong force, has a short range, but is much weaker than the strong force. The weak force tries to make the number of neutrons and protons into the most energetically stable configuration. For nuclei containing less than 40 particles, these numbers are usually about equal. Protons and neutrons are closely related and are collectively known as nucleons. As the number of particles increases toward a maximum of about 209, the number of neutrons to maintain stability begins to outstrip the number of protons, until the ratio of neutrons to protons is about three to two.
   Within the nucleus, protons and neutrons are bound together through the nuclear force. Neutrons are required for the stability of nuclei, with the exception of the single-proton hydrogen atom. Neutrons are produced copiously in nuclear fission and fusion. They are a primary contributor to the nucleosynthesis of chemical elements within stars through fission, fusion, and neutron capture processes.
   The residual effect of the strong force is called the nuclear force. The nuclear force acts between hadrons, known as mesons and baryons. This "residual strong force", acting indirectly, transmits gluons that form part of the virtual "π" and "ρ" mesons, which, in turn, transmit the force between nucleons that holds the nucleus (beyond protium) together.
   Except for the proton and neutron, all other hadrons are unstable and decay into other particles in microseconds or less. 
  
  A proton is made of two up quarks and one down quark, while the neutron is made of two down quarks and one up quark. 
  
  These commonly bind together into an atomic nucleus, e.g. a helium-4 nucleus is composed of two protons and two neutrons. 
  
  Most hadrons do not live long enough to bind into nucleus-like composites; those who do (other than the proton and neutron) form exotic nuclei.
  
  Section::::Classification.:By statistics.
   In the so-called beta decay of a neutron (see picture, above), a down quark within the neutron emits a virtual boson and is thereby converted into an up quark, converting the neutron into a proton. Because of the energy involved in the process (i.e., the mass difference between the down quark and the up quark), the boson can only be converted into an electron and an electron-antineutrino. At the quark level, the process can be represented as:
  
  Section::::Interaction types.:Neutral-current interaction.
   The nuclear force is distinct from what historically was known as the weak nuclear force. The weak interaction is one of the four fundamental interactions, and plays a role in processes such as beta decay. The weak force plays no role in the interaction of nucleons, though it is responsible for the decay of neutrons to protons and vice versa.
  
  Section::::History.
   of total electron charge. Thus, if we place two such jugs a meter apart, the electrons in one of the jugs repel those in the other jug with a force of
  
  formula_2
  