Which of the following polyatomic ions will form an ionic compound with two sodium ions?

CO32-

HCO31-

NO21-

NO31-

B. HCO31-

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-).