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How many real elementary particles (not hypothetical) make up an atom or can be in an atom?

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How many real elementary particles (not hypothetical) make up an atom or can be in an atom?

This is tricky, because of the inclusion of the word "real". Let's say we're talking about a helium atom, and we're talking about how many different types of elementary particles there are. The helium atom is comprised of protons, neutrons, and electrons. OK, now let's say we turf out a neutron. A free neutron normally decays to a proton, an electron, and an antineutrino. However a small fraction also emit a gamma ray:

$$n^0 → p^+ + e^− + \barν_e + γ$$

So we can count the electron, the antineutrino, and the gamma photon. That's three real elementary particles. (I won't distinguish between particles and antiparticles). So far so good. It's when we turn to the proton that things get tricky. If you look at the Wikipedia gluon article you can read "as opposed to virtual ones found in ordinary hadrons". The gluons in the proton are virtual, not real, so we can't count them! And then we come to the quarks. We usually say there's two up quarks and a down quark in a proton, and they're different enough to bring the total up to five. They don't annihilate like an electron and a positron.

But we've never actually seen those quarks. Nobody has ever seen a free quark. The proton definitely has some kind of tripartite structure, but I can't prove that those quarks are real like an electron. I can't show you a track. And IMHO it would be wrong to think of the proton as a bag containing three quarks as per this hyperphysics depiction. Or as a bag full of myriad quarks and gluons as per Matt Strassler's depiction. What's the bag made of and where does it go? If quark confinement is such a big deal, how come we have no problem turfing out two quarks? Where do the quarks and gluons go in low-energy proton-antiproton annihilation to gamma photons? How come the only stable baryon is the proton? These are such tough questions that I err towards Feynman's partons mixed in with TQFT, wherein the quarks are the parts of the proton and are more like lowest common numerator rather than actual real particles. But the proton is real, and whilst people say it isn't elementary, there's a lot of hydrogen and helium out there. So my answer is four.

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The number depends on the type of atom, but all of them are mostly made of up and down quarks (which make up the proton and neutron) and electrons.

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You can make an atom with heavier generations of quark if you want to count that, for example, a top quark has the same spin and charge as an up quark and you can "construct" a proton from a charm or top quark - not sure how stable it would be.

http://www.particleadventure.org/three_gen.html

But as Gabriel said, Up quark, Down quark and the Electron are the basic building blocks of all atoms. - just those 3. (anti matter would be the anti particle equivalent of those 3).

Now "hypothetical" is a kind of a non-scientific term. The strong and electromagnetic force carriers (Photon and Gluon) are no more hypothetical than the quarks are, maybe less so since we see photons every day. With that in mind, so I'm tempted to say you need 5 fundamental particles if you count the 2 force particles.

see picture of structure of the atom here: http://www.fromquarkstoquasars.com/standard-model-an-overview-of-particle-physics/

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    $\begingroup$ The top actually decays so fast that it doesn't settle into hadrons. That is the reason the PDG lists it as having its own decay modes with branching ratios rather than listing the decay of mesons and baryons containing it as with the bottom, charm and lighter quarks. $\endgroup$ Jun 9, 2015 at 14:57
  • $\begingroup$ If thats the case than the force carriers of the weak nuclear force W and Z bosons are required as well. Thats makes it seven. $\endgroup$
    – Horus
    Jun 9, 2015 at 14:57
  • $\begingroup$ I considered including them, but they're not always present and as I understand it, when they are present, they're temporary. The Photon and Gluon actually hold the atom together. But then, I only have a kind of superficial understanding of this stuff. $\endgroup$
    – userLTK
    Jun 9, 2015 at 15:06

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