Question

What are the brackets for around the Lewis structure?

Answer 1

They allow one to unambiguously describe the charge of the overall ion. Here is an example:

For `"NH"_4^(+)`, more accurately `["NH"_4]^(+)` (as the positive formal charge does not belong to hydrogen), we have a formal charge on `"N"` of `4-3 = +1`, while each `"H"` has a formal charge of `1-1 = 0`. So, the overall charge adds up to be `+1`.

(If you recall, formal charge assumes evenly-shared valence electrons and is `"FC"` `=` `"Valence - Owned"`.)