The substance is sodium hypochlorite.
Right from the start, you know that you're dealing with the sodium cation,
Sodium has an atomic number equal to 11, which means that a neutral sodium atom has 11 electrons surrounding its nucleus.
Sodium will lose one electron from its outermost shell to form the
Now, from the information given to you I think that it's safe to assume that your ionic compound only contains one sodium cation.
This means that the overall charge on the polyatomic anion must be
This polyatomic anion contains a total of 26 electrons, out of which 1 comes from the negative charge, and 8 come from the oxygen atom.
Now, you can assume that the polyatomic anions contains a single oxygen atom, since the problem didn't say anything about multiple oxygen atoms being a part of the anion.
If that's the case, the remaining atom(s) must contain a total of
#"no. of e"^(-) = 26 - 1 - 8 = "17 e"^(-)#
As it turns out, a neutral chlorine atom, which has a total of 17 electrons, matches this description perfectly. You are thus dealing with the hypochlorite anion,
The ionic compound is called sodium hypochlorite,
SIDE NOTE Even if you don't assume that the anion only contains one oxyge natom, the only valid option would still be the hypochlorite anion.
This is the only polyatomic ion that has a