why is orbital moment quenched while atoms forming solid

atom has well defined spin(up and down) and orbital(s,p,d,etc) momentum, but when forming crystals, why the spin degree continues to be good quantum number while orbital momentum is quenched?

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Orbital angular momentum is a good quantum number for the atomic problem because the Coulomb potential between the electron and nucleus is rotationally invariant, but the potential an electron feels in a crystal is not. A non-spherically-symmetric potential can couple states with different $l_z$, and so if $\psi_{l_z}$ were the eigenstate of the spherically symmetric problem with angular momentum $l_z$, then the correct atomic eigenstates in, for example, a cubic or tetragonally symmetric potential separate into linear combinations like $\psi_{l_z} \pm \psi_{-l_z}$ which measures out to total $l_z=0$.