How many elements are there for single bond molecules?
single bond refers to bond_type = '-';
SELECT COUNT(DISTINCT T.element) FROM ( SELECT DISTINCT T2.molecule_id, T1.element FROM atom AS T1 INNER JOIN molecule AS T2 ON T1.molecule_id = T2.molecule_id INNER JOIN bond AS T3 ON T2.molecule_id = T3.molecule_id WHERE T3.bond_type = '-' ) AS T