Which formula represents the carbonate ion?

Which formula represents the carbonate ion?

• A. CO3^-
• B. CO3^2-
• C. CO2^2-
• D. CO3^2-

Answer: (D)

The main idea is that an ion’s formula must reflect its overall charge. The carbonate ion is a species with three oxygens bonded to a central carbon and carries a total charge of 2−. That means the correct representation must include a 2− charge, giving CO3^2−. The three oxygens contribute a lot of negative charge, and the extra electrons are delocalized over the oxygens, producing resonance and equal bond character among the C–O bonds. A form with a smaller or neutral charge would not describe carbonate, and a neutral CO2 is not an ion. So the carbonate ion is CO3^2−.

The main idea is that an ion’s formula must reflect its overall charge. The carbonate ion is a species with three oxygens bonded to a central carbon and carries a total charge of 2−. That means the correct representation must include a 2− charge, giving CO3^2−. The three oxygens contribute a lot of negative charge, and the extra electrons are delocalized over the oxygens, producing resonance and equal bond character among the C–O bonds. A form with a smaller or neutral charge would not describe carbonate, and a neutral CO2 is not an ion. So the carbonate ion is CO3^2−.