A Trick Nomenclature Question Involving Polyatomic Ions

1) Here's the question:

Can a binary compound contain a polyatomic ion?

2) Here's the answer:

Yes.

3) Why is the ChemTeam calling it a trick question?

Because binary ionic compounds that contain a polyatomic ion make up such a small percentage of all binary ionic compounds that their existence tends to be ignored. On a test, a sneaky teacher may ask about binary ionic compounds containing a polyatomic, even if there was no discussion in class (or examples in the textbook).

4) Here are some ionic compounds that contain a polyatomic ion:

Na₂SO₄, LiClO₄, Ba(NO₃)₂, K₂CO₃, NH₄Cl

In every case, there are more than two different elements in the compound. None of these compounds are binary ionic compounds. They are ionic compounds, just not binary ionic compounds.

5) Here are some binary ionic compounds that do not contain a polyatomic ion:

NaCl, BaO, KBr, Fe₂O₃, MgCl₂

6) There are several polyatomic ions that contain only one element. Here are four:

peroxide (O₂²¯)
azide (N₃¯)
superoxide (O₂¯)
mercury(I) (Hg₂²⁺)

The existence of these polyatomic ions allows for a Yes answer to the question asked at the beginning.

7) Here are some binary ionic compounds that contain a polyatomic ion:

Na₂O₂ (sodium peroxide)
CaO₂ (calcium peroxide)

NaN₃ (sodium azide)

KO₂ (potassium superoxide)

Hg₂Cl₂ (mercury(I) chloride)
Hg₂O₂ (mercury(I) peroxide)

HgO₂ (mercury(II) peroxide)

The above seven compounds are all binary ionic compounds because each formula contains only two different elements.

8) And one last example:

Hg₂(O₂)₂ (mercury(I) superoxide)

I have no idea if this last compound exists. I searched and found lots of information about mercury and superoxides, but nothing about mercury(I) superoxide.

9) Here is a website discussion that gets it right about binary ionic compounds that contain a polyatomic ion:

What is a Binary Ionic Compound?