We present a simple model for understanding the bonding of endohedral atoms in small fullerene cages, based on their approximate spherical shape. Previous work has shown that the one-electron wave functions of a fullerene cage can be assigned angular momentum quantum numbers which describe their overall angular character. These quantum numbers form the basis for approximate selection rules which govern the bonding with endohedral atoms. With this model we successfully address the very different bonding of various tetravalent elements in C-28 and the remarkably strong bonding of U in this small fullerene. We also make several specific predictions regarding the stability of other endohedral complexes.