#"Sulfates, nitrates, perchlorates, phosphates......."#; all of these species, and others, are polyatomic ions. Lewis structures can represent (sometimes) their ionic character: #O=N^(+)(-O)_2""^-, (O=)_2S(-O)_2^-,(O=)_3ClO^(-)#; all of these species have formal negative charges. And these are combined with metal or protic cations in the solid state. And of course, cations, too, can be polyatomic; the best example is #"ammonium ion"#, i.e. #NH_4^+#.