In this reaction, CaO is a Lewis base, and CO2 is a Lewis acid. A Lewis base is defined as a species that donates an electron pair, while a Lewis acid accepts an electron pair.
CaO consists of calcium and oxide ions. The oxide ion (O2-) has a strong tendency to donate its electron pairs, making it behave like a Lewis base. In contrast, CO2, which can accept an electron pair due to its electron-deficient carbon atom, acts as a Lewis acid.
When CaO interacts with CO2, the oxide ion from CaO donates its electron pair to the carbon atom in CO2, resulting in the formation of CaCO3. This illustrates the acid-base reaction in terms of Lewis definitions.