Well, a polyatomic may contain a cation in it. A cation is a positively charged ion. They are usually metals. A good way to remember is: "cats" have "paws" because they are "pawsitive". Cheesy,...
Cations are always positive, whatever they're of. Ooh, or is that anions? Yeah, on second thoughts I think cations are negative. Hang on a minute, I'll check...
...nope, I was right first time....