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Polyatomic Ions - Can they be considered covalent compounds?


I had a student in class who asked whether a polyatomic ion (such as sulfate, SO42-) could be considered a covalent compound as well. My gut instinct tells me "no" but it feels like it's going to turn into a game of definitions and such.
Any insight would be greatly appreciated. Thanks!

I guess you need to distinguish bonds from compounds. A molecule can be nonpolar but contain polar bonds (carbon dioxide). And a polyatomic ion like sulfate can form ionic compounds despite containing covalent bonds.

Would it be useful to include electrical neutrality in a definition of compounds?

I guess so, but it's no wonder students are confused when you look at the way things are sloppily defined around the internet. Consider the Wikipedia page for chemical compounds, which makes this distinction: "Molecular compounds are held together by covalent bonds; ionic compounds are held together by ionic bonds...." That's certainly confusing, since the presence of ionic bonds is only the minimum requirement for an ionic compound.  Sodium chloride may satisfy this simple exclusionary definition, but sodium phosphate is held together by both ionic bonds and covalent bonds, so could, in principle, satisfy both of these definitions. The separate Wikipedia page dedicated to ionic compounds is better: "a chemical compound composed of ions held together by electrostatic forces termed ionic bonding". It is better in that it indicates that the ions (be they mono- or polyatomic) are held together by ionic bonds, even if the constituent ions may themselves be held together by covalent bonds, although this is left unwritten in any explicit form.


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