Carbonate rocks contain the ion as part of their chemical composition.

Hi there,

The answer should be: Carbonate rocks contain the CO23- ion as part of their chemical composition.

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.