class Molecule(Graph), definition of "self.atom2valence" 

1) Why "self.atom2valence" is restlicted for several atoms only in the definition?

2) Why the vakence is not atom2valence = {1: 1,  5: 3,  6: 4,  7: 5,  8: 6,  9: 7,
                                    but atom2valence = {1: 1,  5: 3,  6: 4,  7: 3,  8: 2,  9: 1, (in the code) ... ?
    
