Could a neutron be a proton and an electron?

Not really, since that particular combination is a hydrogen atom. A neutron consists of three individual quarks, one blue, one red and one green. The quark colors refer to the charge of a quark, and the three colors yeild a net charge of zero, making the neutron the particle of choice for triggering nuclear fission and fusion, since it has mass but no charge. If you tried to use a nearly equally massive proton to crash into the nucleus of an atom and trigger a chain reaction, it would never hit its target, because the like charges would repel each other.

“Free neutrons decay by emission of an electron and an electron antineutrino to become a proton, a process known as beta decay…” Source

@gasman Yes, this is true. But they are not composed of an electron and a proton. That is what happens to the quarks making them up after they decay.

Dmitri Mendeleev’s periodical table brought order to the chemical elements. It did not explain this order, but it was an enormous step forward for chemistry.

Likewise, Murray Gell-Mann and Yuval Ne’eman (independently) brought order to baryons and mesons (all the particles that “feel” the strong nuclear force) in 1962 by proposing that they fit into a mathematical structure known as SU(3). Gell-Mann called his proposal the Eightfold Way (which he came to regret later on when New Agers seized on imagined ties between eastern religion and philosophy and particle physics). In 1964 a particle with properties predicted by the model was discovered, which lent it much credibility.

So there was a pattern, but what the reason behind it? There was a fundamental representation of SU(3) that did not have particles assigned to it in the Eightfold Way. If particles were assigned to this representation and they existed, one could hypothesize that all the other baryons and mesons were simply built up from combinations of these, like atomic nuclei are built up from protons and neutrons. But there were three major problems that prevented Gell-Mann and others from jumping immediately on this attractive proposition.

(1) Fractional electric charge. The math dictated these hypothetical particles have electrical charges of +⅔ and -⅓, which had never been observed, and seemed outright crazy to most physicists at the time. Much time and effort went into complicated schemes trying to circumvent this.

(2) Statistics. Bringing these particles very closely together would violate the Pauli exclusion principle.

(3) Freedom vs. confinement. If these particles could roam free, then why had fractional charges never been observed? If they were always confined to composite structures with integral charge, then that would imply strange properties of the strong nuclear force, and it seemed doubtful that a local gauge field theory with massive bosons (force carrying particles) could even have such properties.

By 1964, Gell-Mann felt confident enough to predict that these particles were indeed “real” despite these issues. He called them quarks. (Independently and at the same time, George Zwieg was led along a different track of reasoning to propose the same thing, he called them “aces”.)

The statistics problem was solved by introducing “color” (see the link provided by @ETpro) as a new quantum number. And asymmotic freedom was discovered in massive local gauge field theories, allowing for confinement (and explaining why fractional charges had not been observed). This led to a complete theory of the strong force: quantum chromodynamics (QCD). QCD has been on the experimental rack since the 1970s. It is very hard to precisely calculate things with this theory, but it has survived all the experimental tests thrown at it so far.

[mod says] Minor typos in this question have been corrected via internal edit.

@hiphiphopflipflapflop Thanks for the clarification— nice posting!. I knew the decay involved QCD but, having never formally studied weak or strong interactions, I’m a fuzzy on details. I left physics in the early 70s, when Gell-Mann’s Eightfold Way was the rage & quarks were newly hypothetical—and before the “standard model” was established as such.

@hiphiphopflipflapflop I;d like to add my two thuymbs up vote to that of @gasman. Thanks for taking the time to explain the current understanding so clearly.

That’s about as simple as I could bear to make it in the interests if keeping it relatively short. One of the things about the 1960’s is that some odd currents developed in particle physics due to frustrations with local gauge field theories which have been largely forgotten today (string theory being the exception). The experimental side is also interesting and brings Feynman into the story (in such a way that caused considerable tension between him and Gell-Mann) when he made a visit to SLAC and had a look at their data.