# What is the difference between a neutron and hydrogen?

Differences? They are both an electron and a proton, since the neutron decays to a proton and an electron, what's the difference between a neutron and proton + electron? so is it just a higher binding energy between the two?

• I almost downvoted this due to its assumption that a neutron is "an electron and a proton". Try rewriting it as something like "since the neutron decays to a proton and an electron, what's the difference between a neutron and a proton and an electron"? This ignores the neutrino, but at least doesn't violate modern physics. – Carl Brannen Mar 27 '11 at 5:53

A neutron is not a proton and an electron lumped together (as your question seems to suggest you think)

A hydrogen atom is a bound state of an electron and a proton (bound by the electromagnetic force) whereas a neutron is a bound state of three quarks (bound by the strong force).

You might be tempted to think that a neutron is also a bound state of an electron and a proton because a neutron can decay into an electron and a proton and the neutron is also slightly more massive than a proton. But you'd be wrong. Here's why:

Both a neutron and a proton are bound states of three quarks. Beta decay can convert a neutron to a proton like this $$udd \rightarrow uud + e^- + \bar{\nu_e}$$

Here, one of the down quarks in the neutron gets converted into an up quark, an electron and an antineutrino through the mediation of the weak force. Since the up quark has a charge of $+2/3$ and the down quark $-1/3$, that explains the difference in charge of a neutron and a proton. That's what turns a neutron into a proton, not some kind of ejection of an electron out of the neutron, but a genuine transmutation of a fundamental particle (a down quark) through the weak force.

• So is there no provision allowing for a hydrogen atom to "absorb" an antineutrino and become a neutron? – user2775 Mar 26 '11 at 23:48
• Are you thinking of the reverse of this decay? It does happen but not in the way you imagined it. Take the antineutrino to the other side so that it becomes a neutrino and you will have $p + e^- \rightarrow n + \nu_e$. This is essentially the process of electron capture (or inverse beta decay), in which a nucleus which is proton rich absorbs one of the inner orbit electrons and transforms into another element. It can't happen with lighter elements like hydrogen because it isn't energetically favourable. – dbrane Mar 27 '11 at 0:51
• I think jpearls is actually talking about the reaction $H + \bar{\nu}_e \rightarrow n$, which is in fact "possible" in that it can conserve energy, momentum, and all quantum numbers. But there are good reasons it will never be observed: 1. The rest-frame energy of the reactants would have to be exactly right to form a neutron with no extra energy. 2. One of the reactants is a neutrino, so beams are not available in the lab and the cross-section is tiny, so even seeing a resonance at the neutron energy is a pipe dream. – Keenan Pepper Mar 27 '11 at 20:17

A neutron is a fermion, a hydrogen atom is a boson. This is related to the fact that a neutron decays into three fermions rather than two which is what you seem to think.

A neutron is composed of three valence quarks, $u,d,d$, while a hydrogen atom is made out of $u,u,d,e^-$.

The internal size of a neutron is about $10^{-14}$ meters while the internal size of the hydrogen atom is about $10^{-10}$ meters, about 10,000 times larger which gives the hydrogen a trillion times greater volume.

The excited (but still bound) states of a hydrogen atom have energies that differ by at most 13.6 electronvolts. On the other hand, the excitations of a neutron have to add many MeVs, a million times higher energy.

The hydrogen atom is a stable particle - neglecting the proton decay for a while. The lifetime of a neutron is just 15 minutes.

The neutron's behavior is studied by nuclear physics; the atoms' behavior is studied by atomic physics which is something very different, referring to different length scales and energy scales.

Much more generally, it is a fallacy to imagine that if a particle X decays to YZ, then X is the "same thing" as YZ. One may say that it has the same conserved charges. But conserved charges don't fully define the identity of an object.

The neutron decays into a proton, an electron and an antineutrino. So even the end components are different from Hydrogen which is just a proton with an electron orbiting around it.

The binding forces are also different. The proton and the electron are bound by the electromagnetic force. The neutron by the strong to the rest of the nucleons in a nucleus. When it decays, it decays by the weak force.

Have a look at this presentation for teachers where also you will see what is inside a neutron and a proton and you will understand better the nature of particles.

• One way of reading this sort of suggests that the author agrees that a good model for the neutron is the proton and electron and anti-neutrino. I'm sure this is not the case, so how about changing that 2nd paragraph... – Carl Brannen Mar 27 '11 at 5:55
• thanks Carl. will do, certainly did not mean that :). – anna v Mar 27 '11 at 7:19