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I was wondering about how many elementary particles form up one molecule of $H_2O$.

What I thought of is the following:

Hydrogen : 0 neutrons, 1 electron, 1 proton; electron is elementary and proton is formed of 3 quarks so hydrogen (H) is 4 particles and hence $H_2$ is 8 particles. Same thought for Oxygen 8 neutrons each 3 quarks, 8 protons each 3 quarks, 8 electrons the total for oxygen is 56.

So Total number of particles in $H_2O$ is 64 fundamental particles.

Is that correct?

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    $\begingroup$ That's what I get. $\endgroup$ – lemon Jul 25 '18 at 8:03
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    $\begingroup$ For the real constituents this is accurate. But note that only about 1% of the protons mass is generated by the constituent quarks and therefore the Higgs mechanism. The other 99% are kinetic energy of the quarks and the binding energy of the gluon field. So you could say that there also countless gluons within a proton. $\endgroup$ – DomDoe Jul 25 '18 at 8:19
  • $\begingroup$ Physics SE: What's inside a proton? A similar logic would apply to neutrons. $\endgroup$ – user191954 Jul 25 '18 at 10:51
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Saying $H_2O$ is 64 fundamental particles is largely right.

One trivial problem is that sometimes the oxygen is another stable isotope like $^{17}O$ or $^{18}O$ with 1 or 2 extra neutrons, adding 3 or 6 quarks.

A more profound issue is that when quarks are bound into hadrons they do not exist as neat separate particles but as components in a complex mess of virtual quarks and gluons. Now, one can say that virtual particles are not real and should not be counted: they are just a mathematical contrivance to describe the behaviour of the actually real thing, the quantum fields. But in a sense real elementary particles are also mere excitations of the fields. One could say there are no particles at all and just a handful of fields extending across the universe, but this is likely not a useful answer to the question.

As always, the answer to the question depends a bit on what it is supposed to do. In some applications protons and neutrons count as fundamental particles, in others quarks.

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