This is a quite elementary question, but I still can't wrap my head around it.
I know how to calculate the formal charges of an ion or a molecule, but I don't understand what happens when oxygen (when sharing a single covalent bond with sulfur in a sulfate ion) has a lone electron that doesn't bond nor pair. I don't understand why this puts a -1 charge on the atom. What happens?
I've attached an image which explains what I'm trying to figure out:
See those two red dots? What do they mean? I understand that they symbolize the lack of a single electron needed to complete an octet. But why does it give the atom a negative charge?