Question

Which of these pairs of elements is most likely to be part of a polyatomic ion?

Cr and O
K and F
Li and Br
Mg and O

Answer 1

Cr and O I believe. I feel like I've seen it before as a polyatomic ion

Answer 2

Cr and O are most likely to be part of a polyatomic ion.

Cr belongs to the transition metal series in the periodic table and has vacant d-orbitals. As a result it can exist in more than one oxidation state, the most common being +6, +3 and +2. Oxygen exists in -2 oxidation state. Hence, Cr and O can combine to form different polyatomic ions like: CrO₄²⁻ and Cr₂O₇²⁻ (Cr in +6 state)

K, Li and Mg exist in only one oxidation state +1 (K and Li) and +2(Mg). F and Br exhibit an oxidation state of -1. Therefore, they are not likely to form polyatomic ions.

Ans (a) Cr and O