Which of the following polyatomic ions will form an ionic compound with two sodium ions? co32− hco31− no21− no31−

Sodium has a noble gas configuration, plus a single high energy 3s1 electron. This electron is more stable when associated with more electronegative atoms and polyatomic ions. Therefore, sodium loses a single electron (losing a negative charge, granting the sodium atom a +1 charge). Positive and negative charges attract each other in order to balance out the charge (reach a low energy state), so two +1 sodium atoms need to associate with either two separate -1 charges, or a single -2 charge. The only polyatomic listed here that satisfies the -2 (formally written 2-) needed to attract both sodium atoms is the Carbonate ion, CO3(2-).