Answer:
Cr and O
Explanation:
The metal should be an atom that doesn't lose its electrons too easily. That is, it should have a relatively high electronegativity (for a metal), so the M-X bond can have some covalent character.
Metals with EN ≥ 1.6 can usually form polyatomic ions with oxygen.
Metals in Groups 1 and 2 have EN < 1.3, so they tend to lose their valence electrons completely and form ions like Li⁺, K⁺, and Mg²⁺.
Thus, the elements most likely to form a polyatomic ion are Cr and O.
They form chromite (Cr₂O₄²⁻), chromate (CrO₄²⁻), and dichromate (Cr₂O₇²⁻) ions.
