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An atom typically consists of electrons, protons, and neutrons. Electrons are negatively charged and participate in bonding to stabilize the atom.

Conversely, protons are positively charged and balance the charge of the atom. In addition, their positive charge attracts the negatively charged electrons.

What role do neutrons play in an atom?

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    $\begingroup$ You're only considering the electro-magnetic force. Protons and neutrons interact via the strong force, which is far stronger than electro-magnetism - that's how the nucleus can hold together against the electro-magnetic repulsion of the individual protons in the first place. Of course, reality is even more complex - you can't just keep adding neutrons to make atoms more stable :) $\endgroup$ – Luaan Jan 19 '17 at 9:27
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    $\begingroup$ "electrons participate in bonding to stabilize the atom" doesn't make very much sense. The stability of a molecule is determined by its electrons, but each atomic nucleus in that molecule is, AFAIK, no more or less stable than it would be if stripped of all its electrons and floating in the vacuum. $\endgroup$ – zwol Jan 19 '17 at 22:04
  • $\begingroup$ It is a strangely anthropomorphic question. None if the particles has job description. $\endgroup$ – Greg Jan 25 '17 at 16:51
  • $\begingroup$ Sorry I don't have enough reputation to add a comment, but I think you can find you answer from here $\endgroup$ – Ash Jan 28 '17 at 15:37
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Neutrons bind with protons and one another in the nucleus through the strong force, effectively moderating the repulsive forces between the protons and stabilizing the nucleus.$^{[1]}$

$\ce{^2He}$ (2 protons, 0 neutrons) is extremely unstable, though according to theoretical calculations would be much more stable if the strong force were 2% stronger. Its instability is due to spin–spin interactions in the strong force, and the Pauli exclusion principle, which forces the two protons to have anti-aligned spins and gives the $\ce{^2He}$ nucleus a negative binding energy. $\ce{^3He}$ (2 protons, 1 neutron), on the other hand, is stable, and is also the only stable isotope other than $\ce{^1H}$ with more protons than neutrons.$^{[2]}$


$^{[1]}$ Wikipedia, Neutron, Beta Decay and the Stability of the Nucleus
$^{[2]}$ Wikipedia, Isotopes of Helium

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    $\begingroup$ Also I's point out that (1) Neutrons confounded the relationship between atomic number and atomic mass as chemistry was being understood as a science. (2) In general the mass of different isotopes has a very small effect on chemical reactivity. $\endgroup$ – MaxW Jan 19 '17 at 19:52
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In a few more words, physicists right now are confident in saying that there are four fundamental things that happen:

  1. Protons and neutrons stick together. (The "strong nuclear interaction".)
  2. Neutrons sometimes "fall apart" into a proton, electron, and antineutrino. Sometimes this can happen in reverse, too. (The "weak nuclear interaction", also known as "beta decay" or "radioactivity".)
  3. Positive charges repel other positive charges and attract negative charges. (The "electromagnetic interaction", also known as "chemistry" and "light".)
  4. Things fall down. ("The gravitational interaction".)

(I said that these were in order of "decreasing everyday strength," but that's not very precise given that these things scale differently with distance etc. But this is the rough order that you should be thinking about the problem you're interested in.)

Everything else you are used to is caused by these 4 fundamental interactions. For example when you are sitting in a chair, secretly the force holding you up is a force of electron clouds around nuclei repelling each other, so this is chiefly the "electromagnetic" forces at play opposing the "gravitational" forces pulling you down.

There is also a slight subtlety which these 4 interactions do not completely cover, but every physicist knows about it: it says that "two identical particles cannot remain in an identical state." This usually means that those particles have to occupy higher and higher-energy states. It turns out a lot of the structure of the periodic table comes from this rule! This rule ultimately says that the number of columns you add (when you add columns to the periodic table) must be twice the next odd number: so you see that we start off adding 2 columns, then we add 6 columns, then we add 10 columns, then we add 14 columns; the physics says that the next number of columns to add would be 18 and that the pattern is seen when you divide by 2, you first add one pair, then three pairs, then five pairs, then seven pairs: increasing odd numbers. And this is just because each new electron needs to be (a) orbiting further away and (b) possibly spinning faster.

So as a nucleus gets bigger and bigger, a similar story happens. The neutrons and protons cooperate due to the strong nuclear interaction. They, it turns out, really like to stick to each other! But then the second effect takes hold: if a nucleus has too many protons, they have to be in really spinny high-energy states in the nucleus, because the lower-energy states are already occupied by other protons! But there are lower-energy neutron states which are unoccupied. At some point it becomes energetically favorable for a proton to reverse-beta-decay into a positron plus a neutron plus a neutrino, so that the neutron can fall into that lowest-energy state.

So that's what the neutrons do in the nucleus: they are "about as sticky" as the protons but they are different particles that can occupy the other states.

Now you might also think, "oh, those protons are repelling each other also, due to the electromagnetic interaction." And that is true, but it is a weaker effect than either of these. That effect basically balances out at a certain atomic number, which happens to be Iron. All of the smaller atoms are driven more by the strong nuclear force to want to "fuse" together into larger atoms, trying to be iron. And all of the bigger atoms are driven more by the electromagnetic repulsion to want to "fission" apart into smaller atoms. (But of course until you get to intrinsically-unstable atomic numbers like Uranium, you can still have little stable states of larger-than-iron atoms, where if you fire a neutron into the nucleus it might fall apart but for right now it's jiggling around safely.)

See also: Wikipedia's articles on stable nuclides and nuclear binding energies.

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  • $\begingroup$ Good answer, but electromagnetism is much stronger than weak interactions. $\endgroup$ – J.G. Jan 19 '17 at 18:00
  • $\begingroup$ @JG I mean, that's true in a technical sense (the mass of the W boson makes the weak force between two nucleons much smaller than the electromagnetic force between them), but I struggle with how to better convey this idea of "looking at each nucleon individually, the first thing you should think about is its ability to stick to other nucleons, the second thing is its ability to transmute into other nucleons, and finally you should think about its electromagnetic repulsion of other nucleons." Maybe I'll just delete the aside of "decreasing everyday strength". $\endgroup$ – CR Drost Jan 19 '17 at 18:10
  • $\begingroup$ I think the issue is that the stability of a nucleus hinges on the competition between strong and electromagnetic forces, and weak interactions occur when they would increase the resulting binding energy. Weak forces aren't taken into account in the semi-empirical mass formula. $\endgroup$ – J.G. Jan 19 '17 at 18:14
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In chemistry, the neutrons are important as they determine the spin of a nucleus which determines if and how it is observable by NMR. Like 1H is spin 1/2 and 2H is spin 1. The neutron also adds mass to the atom.

In chemical reactions however, the nucleus is not involved and nuclear reactions are more a topic in physics.

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Neutrons play very important role in an atom. They provide stability to the atom and also prevent protons from repelling one another.

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    $\begingroup$ The answer would be more complete if you explained at least briefly how the neutron provides stability and prevents protons from repelling each other. $\endgroup$ – airhuff Jan 20 '17 at 8:47
  • $\begingroup$ OK Sir, why not. If we drive an empty car, we will feel that the vibrating very much because of unstable, and if we keep some mass in the car then we will feel comfortable to drive and the car will be moving very smooth. Because mass provide the car stability. In physics We can say that mass is inertia and inertia always providing stability. Proton having positive charge and same charges repel each other. Here neutrons play a role of a boundary between the protons to prevent the repulsion. $\endgroup$ – user40151 Jan 20 '17 at 9:29
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    $\begingroup$ You have good points on your answer, could you edit it and perhaps expound on it so that others can better understand how neutrons provide stability and prevent protons from repelling one another? Also, big warm welcome to Chemistry.SE! $\endgroup$ – Melanie Shebel Jan 22 '17 at 9:09

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