- Magnesium (Mg): +2

- Carbon (C): +4

- Oxygen (O): -2

To calculate the overall oxidation number of the compound, we add up the oxidation numbers of each atom, taking into account the number of atoms of each type:

(1 x +2) + (1 x +4) + (3 x -2) = 0

Therefore, the overall oxidation number of MgCO3 is 0, which is expected since the compound is neutral.