Conserved quantities imply some underlying symmetry enjoyed by the interactions.
Space-Time symmetries lead to conservation laws of momentum, angular momentum, and energy. Isospin and internal symmetries Soon after the discovery of the neutron it was realized that neutrons and protons behaved very much the same. The forces they exerted on each other were nearly independent of whether the interaction was between neutrons, protons, or neutrons and protons. The electrical charge of the protons spoils the near perfect identity of neutrons and protons. The nucleon can exist in either of two states, as a proton or a neutron.