Isospin quantum number for light nuclei Does anyone  know where I can find the Isospin values for light nuclei (H, C,N, O, S, Cl, ..) in their ground state? 
 A: The best place to look is the Evaluated Nuclear Structure Data File, hosted by the National Nuclear Data Center at Brookhaven National Lab.  For example, the data file for helium-4,

shows that the first two states with isospin $T\neq 0$ are the negative-parity states centered at 23.3 and 23.6 MeV.
Be warned that at modest proton number $Z$ the assumption underlying isospin symmetry, namely that protons and neutrons can be treated symmetrically inside the nucleus, starts to break down and it may no longer be possible to assign a definite isospin to a state.  For example neon-20, an "alpha-cluster" nucleus, is assigned a $T=0$ ground state, but neon-21 has isospin assignments only for some excited states and neon-22 seems to have no isospin-assigned states at all.  The next alpha-cluster nucleus, magnesium-24, has no isospin assignment in the ground state and several excited states labeled "T=0 and 1".  You can compare this to the orbital angular momentum mixing that makes the ground states for deuterium and helium-4 into complicated mixtures of $S$ and $D$ waves.
