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Q & A: Why do we have neutrons in nuclei

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Most recent answer: 09/27/2014
Q:
Why does the nucleus have neutron in it instead of electron , I mean there is a force of attraction between opposite charges so it would be easy for a proton to be with an electron than a neutron , so why didn't it happen? Pardon me if the question was silly
- Dheeraj CS (age 17)
INDIA
A:

Hi, Dheeraj! Nice question and not silly at all.

There is no simple answer to it and it requires a long discussion.

First of all, we'll say that electrons do stick to protons, and then we call them atoms! Hydrogen is an example of such "sticking". Except the electron doesn't actually fall to the proton because electrons are waves, and, moreover, it's a very light particle. At some point the electron just cannot get any closer to the proton because the "orbit" would be smaller than the wavelength of this electron. We call it the lowest energy level of a bound state of a proton and an electron (1s state).

A free neutron is not stable.   It will decay into a proton, an electron and an anti-neutrino.   In principle one can shoot a high energy electron at a proton and induce a reaction   e- + p+ --> n + v  but it would be a rare occasion.   The reason one can't directly produce a neutron with an electron beam is the conservation of a quantity called lepton number.   You need that extra v in order to meet this requirement. 

Now, the next question is why neutrons and protons like to stick together so much. We have to remind ourselves that the electromagnetic interaction (e.g. between positive and negative electrical charges) is not the only interaction that exists in the world. The binding force in nuclei is actually the strong interaction. It binds quarks together to form neutrons or protons and leaks out to hold neutrons and protons close together. So, nuclei are stable despite the electromagnetic repulsion, and not due to electromagnetic attraction! There is always a trade-off between short-ranged "strong" attraction and electromagnetic repulsion. It is easy for a neutron and a proton to form a bound state. Not so easy for 2 protons because it is much harder for the strong interaction to overcome the electromagnetic repulsion. That's why most of the times you have as many neutrons in a nucleus as protons.

For the advanced reader:

Here we should also talk about spin statistics and particles being fermions or bosons. Identical fermions (spin 1/2) cannot occupy the same quantum state due to the Pauli exclusion principle (electrons, protons, neutrons, etc). Bosons (e.g. spin 0 or 1) can form a so called Bose-Einstein condensate, where all the particles can occupy exactly the same quantum state. Neutron and proton are not identical particles; they stick together and form a boson with an integer spin, and then you can have as many such coupled particles as you want in a nucleus because all of them would be bosons in a condensate. But actually, there is also a limit, because at some point the short-ranged stong interaction starts to give in to the long-ranged electromagnetic repulsion, and you won't be able to have too large nuclei. That's why the table of chemical elements is finite. =) 

I hope this helps!   Take a look at Q-A ID# 1226 for more discussiion on this subject.

Yulia M.


(published on 09/27/2014)

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