What is the Lewis structure of #SO_2#?
Well, we could write...
...which gives a bent structure, where
Because there are 5 valence electrons around sulfur, rather than 6, the sulfur has a formal positive charge. The formally singly-bound oxygen is associated with 7 valence electrons, and thus bears A FORMAL negative charge, and of course you can draw the resonance isomer that equates the oxygen atoms.