Answer:
Calcium carbonate (CaCO3), essentially, is an ionic compound having the bivalent calcium and carbonate ions.
But the carbonate anion is a polyatomic species. The carbon atom is bonded to all the three oxygen atoms by covalent bonds - two carbon-oxygen single bonds and one carbon-oxygen double bond. Owing to resonance, the three carbon-oxygen bonds are also of equal length in the carbonate anion.
Any salt having a polyatomic ion invariably contains both ionic and covalent bonds.
