Question

Which pair of elements is most likely to be part of a polyatomic ion?
Responses

Mg and O
uppercase M lowercase g and O

Li and Br

uppercase L lowercase i and uppercase B lowercase r

Cr and O
uppercase C lowercase r and O

K and F

K and F

Answer 1

Cr and O

Reason: Transition metals plus oxygen commonly form polyatomic oxyanions (e.g., chromate CrO4^2– and dichromate Cr2O7^2–). The other pairs form simple binary ionic compounds (e.g., MgO, LiBr, KF).