An interesting fact: free neutrons [0] are unstable and will decay into protons [1] with mean lifetime of ~15 minutes. That is one down quark will decay into an

An interesting fact: free neutrons [0] are unstable and will decay into protons [1] with mean lifetime of ~15 minutes. That is one down quark will decay into an up quark (neutron is composed out of udd quarks, proton: uud) because down quarks have bigger mass than up quarks (see this nice Feynman diagram [2]). In the nucleus on the other hand Pauli exclusion principle makes two protons less favorable than a proton and neutron. Warning: this is what I understood from Wikipedia, IANAP (I am not a physicist). [0]: https://en.wikipedia.org/wiki/Neutron https://en.wikipedia.org/wiki/Neutron [1]: And a couple of other particles, see: https://en.wikipedia.org/wiki/Neutron#Description https://en.wikipedia.org/wiki/Neutron#Description [2]: https://commons.wikimedia.org/wiki/File:Beta_Negative_Decay.svg https://commons.wikimedia.org/wiki/File:Beta_Negative_Decay....