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This question already has an answer here:

An atom is very small so it is probably difficult to focus exactly on a single atom. I wonder how scientists count the exact number of electrons, protons, neutrons in an atom of a certain element.

Could you explain it?

In other words, how do we know that for example, $\ce{^{19}_9F}$ has nine protons and ten neutrons and nine electrons?

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marked as duplicate by Mithoron, Jon Custer, M.A.R., airhuff, Tyberius Mar 8 '18 at 0:00

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  • $\begingroup$ @inɒzɘmɒЯ.A.M I'm not sure if OP didn't maybe ask about single atoms not atoms in general? This meaning could stem from OP's misconception, but such question would be actually much more interesting. $\endgroup$ – Mithoron Aug 26 '15 at 14:04
  • $\begingroup$ @Mith at first read, I didn't understand what they ask. I was gonna VTC, but then it struck me that they want to know how scientists find out how many protons are there in an atom. They could verify or correct me though. And yes, I think they're saying generally, but I figured an example would help. $\endgroup$ – M.A.R. Aug 26 '15 at 14:10
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For charge neutrality, the number of electrons has to equal the number of protons. So, the question really is how to know how many protons and how many electrons. This is actually a very interesting question dating back to the early days (and even before) of nuclear physics and chemistry.

Now, one can do it fairly easily with an ion accelerator and a magnet. With the right set up, one can get a beam of mono-energetic ions out of the accelerator, send them through a stripper to remove electrons, and then send them through a magnet to separate the components out by velocity (which depends on the ion mass through the energy). From the magnet deflections one can get a charge-to-mass ratios, and get back to number of protons (charge on a fully stripped ion) and number of neutrons (from number of protons and the mass). This is the heart of mass spectrometry (although one can use time-of-flight techniques rather than, or in addition to, magnetic separation).

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  • $\begingroup$ So, this means we only know the ratio between the elements, but not exactly the amount of electrons/protons? It's sounds to me that scientist all over the world agreed that H has 1 positive and 1 negative particle and based on ratio we can say that other elements like Na, has x times more mass than H and so it should have x times more electrons/protons. Am I right? $\endgroup$ – AndaluZ Jan 10 '17 at 10:39
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    $\begingroup$ @AndaluZ - Basically, but it is a little more subtle than that - every ratio yields an integer number of protons. So, the proton could be a composite particle, which it in fact is at high enough energies. But no matter how you parse an atom, you get an integer number of units of +1 charge. $\endgroup$ – Jon Custer Jan 10 '17 at 14:10

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