Let's take a simple example: #"sodium sulfate,"# #Na_2SO_4#. As a salt, this is clearly electrostatically neutral, however, we can dig a bit deeper, and represent its Lewis structure.
For the positive sodium ions, we have #2xxNa^+#; the individual sodium ion has 10 electrons, and is thus a cation. Why?
For #"sulfate dianion"#, we have #6+4xx6+2# electrons to distribute, and this represents the 6 valence electrons from the 5 chalcogen atoms, plus the 2 electrons that constitute the negative charge.
A Lewis structure of #(O=)_2S(-O^(-))_2# in which the neutral atoms are each associated with 6 valence electrons, and the anionic oxygens with 7 valence electrons (and hence the negative charges) is commonly invoked. Such a structure implies the equivalence of ALL of the oxygen atoms, in that we can draw resonance structures in which the negative charges can reside on any two oxygen atoms.
A representation as #""^(2+)S(-O)""_4^(-)# is contraindicated by standard rules of Lewis structure determination, even tho it is a fact that all the oxygen atoms are equivalent. Representation of sulfate as #(O=)_2S(-O^(-))_2#, and the parent sulfuric acid as #(O=)_2S(-OH)_2# are what you will generally find in a text. The Lewis representation of #"sulfuric acid"#, and #"nitric acid"#, #O=N^(+)(-O^(-))(OH)#, are thus both a bit problematic.
