# Why neutrons are neutral? [closed]

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?

• Nice question. I'm not sure if it's physics or chemistry, though. Wikipedia says that the fact that neutron has no charge is proven by experiments and is empirical, no proofs or explanations. Jan 26 '15 at 12:57
• Neutrons, are neutral but they are composed of quarks that have partial charges that balance Jan 26 '15 at 19:56
• this question is off-topic, it belongs to physics May 10 '17 at 20:09
• I'm voting to close this question as off-topic because it's not really about chemistry and would be better suited for Physics.SE. May 10 '17 at 20:31

Protons and neutrons are both baryons, subatomic particles composed of three quarks. Quarks are smaller fermionic particles that make up protons, neutrons, and other hadrons.

Electrons are leptons, 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 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, 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.

• What I understand from you explanation that protons are not positive charge but they're composed of quarks and those quarks give the proton its positive charge and neutron a neutral charge. Thanks sir for your time, I promise you that I'll check the references you give me so I can completely understand your answer Jan 26 '15 at 13:43
• I don't think it is accurate to say up quarks have spin 1/2, down quarks -1/2 , and that this determines the spin of the proton and neutron. I think both up and down have spin 1/2 and, just as two or more spin 1/2 electrons could be parallel or antiparallel, so it with quarks. The old theory was that such alignment of the quarks resulted in the spins of the proton and neutron, but even this is not the current theory: scientificamerican.com/article/… Jan 26 '15 at 19:15
• @DavePhD - I will admit to not being an expert on particle physics. I went with what was in the Wikipedia articles and mapped them to my much better understanding of electrons Jan 26 '15 at 19:39
• @BenNorris protons, neutrons, electrons, and all types of quarks (up, down, strange, etc.) all have spin 1/2 even according to Wikipedia. en.wikipedia.org/wiki/Spin_(physics) and en.wikipedia.org/wiki/Spin-%C2%BD Their anti-particles are also spin 1/2. Apr 1 '16 at 17:00