Question

write formula of carbonate of element a, having valency equal to 3??

Answer 1

Aluminium Carbonate
Al2(CO3)3

Answer 2

Answer: $A_2(CO_3)_3$

Explanation: The element A has a valency of 3 and it combines with poyatomic carbonate ion $(CO_3)^{2-}$ which has a valency of 2-. Thus the element A must bear a positive charge to combine with negatively charged carbonate ion.

To form a neutral compound and to balance the charges, 2 atoms of A must combine with 3 molecules of carbonate ion to form $A_2(CO_3)_3$.