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I want to know if it's possible to find out how many atoms are in something, and how one would do that. For example, how many quarks are in my brain(easy to find out once you know how many atoms there are)?

So I ask again ... how does one find out how many atoms are in something, and of what kind of atoms these are?

EXAMPLE: My brain.

Jump if not Equal
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    I give you just an hint. Recall the Avogadro constant (http://en.wikipedia.org/wiki/Avogadro_constant), that is the number of molecules in a given volume of matter... Once you know the volume of your brain and the number of atoms in each molecule, you can get a quote for the number of quark inside. – AstoundingJB Dec 02 '13 at 22:03
  • From mass and the elements of brain, you may estimate how many mole of atoms... – user26143 Dec 02 '13 at 22:06
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    @AstoundingJB: Actually the Avogadro constant is the number of atoms/molecules in a given mass of matter. Since the brain is essentially the same density as water, you can use volume successfully here. 1 gram of brain has 6.022 10^23 nucleons in it. – Ross Millikan Dec 02 '13 at 23:10
  • @Ross Millikan: wait, your statement is erroneous... Avogadro constant actually tells us that there are (about) 6 x 10^23 MOLECULES of Hydrogen in 2 g of gaseous Hydrogen at standard conditions of temperature and pressure, that is 6 x 10^23 ATOMS in 1 g, which is half a mole (11.2 litres) of gas. Besides this, your suggestion of approximating a brain as made of water is very reasonable and useful in a first approximation. – AstoundingJB Dec 03 '13 at 09:11
  • @AstoundingJB: The volume is only at standard temp and pressure. But the statement that 2g of hydrogen is 6E23 molecules is independent of the thermal state. That is why I emphasized mass, not volume. As liquid, that is about 28 cm^3 – Ross Millikan Dec 03 '13 at 14:01
  • That's right, the Avogadro constant refers to the number of molecules within a certain mass of substance, and, @Jump if not Equal, this is the point for solving your question. The fault was mine, I thought you forgot the mass of a molecule of water. Anyway, I preferred to refer to volumes since, in my opinion, is easier to guess the volume of a brain (say 1 litre) rather than its mass. Only once you have made some guess on its density, say it's like that of water, as you said, you can evaluate its weight (say 1 kg) and the number of nucleons inside it (6E23, as you said). – AstoundingJB Dec 03 '13 at 14:57
  • errata corrige: 6E23 per gram, of course! – AstoundingJB Dec 03 '13 at 15:09

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For example, how many quarks are in my brain(easy to find out once you know how many atoms there are)?

Actually it's easier to count how many atoms are in your brain than how many quarks are in your brain. As you may know there are three quarks per nucleon in your brain... but this is not the whole truth. The force the binds quarks together creates a soup of additional 'sea' quarks, which are short-lived quarks that are constantly fizzing in and out of existence in the nucleon. See http://en.wikipedia.org/wiki/Quark#Sea_quarks for more details.

As for your question... see @AstoundingJB's hint.

kd88
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  • There are three quarks in each proton and neutron, yes. As for your link ... I am unsure of the validity of this. Has research confirmed antiquarks? Show me some evidence. – Jump if not Equal Dec 02 '13 at 22:36
  • Have you heard of the Tevatron collider? It was used to discover the top-quark by colliding protons with anti-protons (http://en.wikipedia.org/wiki/Antiproton). Anti-protons are made of 3 anti-quarks (as well as all the sea-quarks and gluons associated with normal protons). Furthermore consider the LHC in CERN - which collides protons with protons. The interactions here occur by gluon-gluon, quark-gluon or quark-anti-quark interactions. Also see deep inelastic scattering (http://en.wikipedia.org/wiki/Deep_inelastic_scattering) for more details of anti-quark evidence. – kd88 Dec 02 '13 at 22:41
  • @JumpifnotEqual Related to the number of quarks in a proton, see: http://physics.stackexchange.com/questions/81190/whats-inside-a-proton/ . And antimatter was postulated in 1928 and discovered in 1955. – jinawee Dec 02 '13 at 22:42
  • @jk88: All the previous comments about quarks are correct but let me clarify and (possibly) simplify the matter. Well, at low standard energies, within the range of your everyday life phenomena, you can't see quarks (as well as nucleons and atoms...) but you can retrace their collective properties, that is, as nucleons, as Gell-Mann(http://en.wikipedia.org/wiki/Murray_Gell-Mann) did. Then you can observe that in protons and neutrons there are (at least) three valence quarks (http://en.wikipedia.org/wiki/Quark_model). – AstoundingJB Dec 03 '13 at 09:26
  • Are there any more of them? The point is that you can't tell at low energies. On the contrary, at higher energies you find that there are a lot of messy structures (http://en.wikipedia.org/wiki/Partons) inside the nucleons. Many of this structures appear and disappear in a moment, quarks are produced and die in pairs (http://en.wikipedia.org/wiki/Pair_production), so the collective properties of each pair are null. You can never tell how many quarks are there in a nucleon but you can always observe that there are three quarks whose properties are globally visible. – AstoundingJB Dec 03 '13 at 09:44