Which procedure did early chemists use to determine which substances were elements? . a. they mixed several substances together and observed the results.. b. they broke down mixtures by chemical means.. c. they physically broke down the elements they discovered.. d. they observed the substances that made up individual atoms.. . why do elements in a group on the periodic table have similar chemical properties? . a. similar atomic masses. b. similar atomic numbers. c. same number of protons. d. same number of outermost electrons.