# Protons and neutrons the same mass?

1. Dec 6, 2012

I was wondering, how can both a proton and a neutron weight 1u?

An Up quark has a mass of 1.7–3.1 MeV (according to Wikipedia).
Down has a mass of 4.1-5.7 MeV.
Average Up: 2.4
Average Down: 4.9

If a proton is Up+Up+Down (2.4+2.4+4.9) it has a mass of 9.7 MeV.
A neutron is Down+Down+Up (4.9+4.9+2.4), and it has a mass of 12.2 MeV.

How can protons and neutrons have the same mass?

Sorry if it's kind of confusing.

2. Dec 6, 2012

### Staff: Mentor

They do not. This is just an approximation.

Proton: 1.007276 u = 938.3 MeV
Neutron: 1.008665 u = 939.6 MeV

Most of their mass comes from binding energy in QCD, but the small mass difference in the quarks can be seen in the total mass.

3. Dec 6, 2012

### K^2

For starters, protons and neutrons don't consist of three quarks. They consist of a boatload of quarks, anti-quarks, and various bosons holding that mess together.

There are three valence quarks in each nucleon. What that means is that if for every quark in the nucleon you find an anti-quark, you'll end up with three quarks left without a pair. They will be uud for a proton and udd for a neutron. They are entirely responsible for nucleon's charge for example, as well as some other quantum numbers, but they make up a very small fraction of the nucleon's mass.