The polyatomic ion that forms a neutral compound when combined with a group 1A monatomic ion in a 1:2 ratio is carbonate (CO₃²⁻).

Here's why:

* Group 1A metals have a +1 charge (e.g., Li⁺, Na⁺, K⁺).

* Carbonate has a -2 charge (CO₃²⁻).

To achieve neutrality, you need two +1 charges (from two group 1A metal ions) to balance out the -2 charge of the carbonate ion. This results in a 1:2 ratio between the metal ion and the carbonate ion.

Examples:

* Lithium carbonate (Li₂CO₃)

* Sodium carbonate (Na₂CO₃)

* Potassium carbonate (K₂CO₃)