Allowed LS terms of a p-squared configuration
= Allowed LS terms of a p-squared configuration
Two equivalent $l=1$ <electrons> require an antisymmetric complete state. Orbital exchange has <parity> $(-1)^L$ and the <spin> singlet is antisymmetric, giving allowed $L+S$ even. The terms are ${}^3P_{0,1,2}$, ${}^1D_2$ and ${}^1S_0$, with $9+5+1=15$ states, equal to $\binom62$. Hund ordering puts ${}^3P_0$ lowest for the less-than-half-filled configuration.