How do I know when to use brackets in a compound with polyatomic ions?

Question

Shewolf · Accepted Answer

Thanks drwls. :)

Shewolf · Answer

For example, sodium nitrite, I believe the answer is Na(NOsubscript2)

drwls · Answer

You don't need brackets for NaNO2 because it contains only one NO2 ion per molecule. If it were something like Ca(NO2)2, you would need two ions and would put the number of ions outside the bracket.