Why are two atoms of Group 1A and one atom of Group 2A needed to react with carbonate?

Answer 1

As CO3-- has a valence of two , we need two atoms of groupA (valence1) or 1 valence of group B (valence2) . The atoms of group1 have a charge =+1 , of group B +2 , so they equilibrate the charges -2 of carbonate