Why neutrons are neutral? [closed]








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If electrons have a negative charge and protons have a positive charge
then how come neutrons have zero charge without consisting of protons and electrons?




Protons and neutrons are both [baryons](http://en.wikipedia.org/wiki/Baryon), subatomic particles composed of three quarks. [Quarks](http://en.wikipedia.org/wiki/Quark) are smaller fermionic particles that make up protons, neutrons, and other [hadrons](http://en.wikipedia.org/wiki/Hadron).


Electrons are [leptons](http://en.wikipedia.org/wiki/Lepton), fermions that are not composed of quarks.


Quarks have mass, spin, charge, and (confusingly) color charge. Color charge is a quantum property that quarks have, and all hadrons need to have a net zero color charge. 


The proton is composed of three quarks: two up quarks and one down quark. Each up quark has a spin of $+\frac{1}{2}$, a mass of $2.3~\text{MeV/c}^2$, and a charge of $+\frac{2}{3}$. Each down quark has a spin of $-\frac{1}{2}$, a mass of $4.8~\text{MeV/c}^2$, and a charge of $-\frac{1}{3}$. The total charge of the proton is $2(+\frac{2}{3})+(-\frac{1}{3})=+1$. The total spin of the proton is $2(+\frac{1}{2})+(-\frac{1}{2})=+\frac{1}{2}$. The masses of the quarks do not add up because most of the proton's mass is in its confinement energy.


We can do the same analysis for a neutron, which is composed of one up quark and two down quarks. The charge of the neutron is $(+\frac{2}{3})+2(-\frac{1}{3})=0$. The total spin of a neutron is $(\frac{1}{2})+2(-\frac{1}{2})=-\frac{1}{2}$. Again the masses do not add up. 


Protons and neutrons are different because the are composed of different quarks. However... enough physics. Now for the nuclear chemistry. 



> 
> Interconversion of protons, neutrons, and electrons
> 
> 
> 


The proton is a [stable particle](http://en.wikipedia.org/wiki/Proton#Stability) with a lifetime at least $2.1\times 10^{29}$ years.


However, protons can be converted into neutrons (and electron neutrinos) by electron capture:


$$\ce{p+ + e- -> n +\nu}\_e$$


This requires an investment of energy because the rest mass of the neutron is higher than the rest mass of a proton by $1.29~\text{MeV/c}^2$.


Free neutrons are [not stable](http://en.wikipedia.org/wiki/Neutron#Beta_decay_and_the_stability_of_the_nucleus), undergoing beta decay to a proton, electron, and electron antineutrino with a halflife of about 10 minutes.


$$\ce{n -> p+ + e- +\bar{\nu}}\_e$$


Most importantly, and fortunately, most neutrons bound in nuclei are stable. The protons and neutrons in a nucleus can be treated quantum mechanically analogously to electrons from several atoms bound in a molecule. There are not a collection of independent particles that do not interact. The nucleons occupy quantum mechanical energy levels. Neutrons can only decay if the proton it would become can occupy a lower energy level, which is usually not possible except for those radioisotopes which tend to undergo beta decay, thanks to the Pauli Exclusion Principle! Thus, the bound neutron is stabilized.


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