For a C-13 nucleus, the ground state is 1/2- (odd parity). In other words it is 1s(1/2)^2 1p(3/2)^4 1p(1/2)^1. The next state is 1/2+. I'm a little confused here. My book seems to suggest that this state is written as 1s(1/2)^1 1p(3/2)^4 1p(1/2)^2, but it seems strange that the excited nucleon would be the one of the two in the lowest energy level. Isn't it the most tightly bound?