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Neon 21 has 10 protons and 11 neutrons, therefore using the nuclear shell model (i.e. https://upload.wikimedia.org/wikipedia/commons/thumb/e/e5/Shells.png/220px-Shells.png) it seems it should have 2 protons and 3 neutrons in the $1d_{(5/2)}$ subshell.

Due to the even number of protons, they should have zero spin overall and the first two neutrons should also give zero spin, but then the remaining neutron should give the overall spin of the atom, i.e. 5/2. Hence it seems to me that the spin parity of the ground state should be $(\frac{5}{2})^+$. However it is given as $(\frac{3}{2})^+$ (http://www.nndc.bnl.gov/chart/). Why is this?

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