Lewis Concept of Acid and Base Lewis Acid : According to Lewis Concept, an acid is clearly defined as the substance which can accept a pair of non-bonding electrons. They are clearly known as an electrophile. They consist of vacant orbitals in their molecular structures. Ag + , Cu ++ and Fe +++ are the few examples of acid according to Lewis concept. There are various elements found which can act as Lewis acids. From the definition of Lewis acid, it is clearly known that all cations are Lewis acids since they are able to accept the pair of an electron (Cu 2+ , Fe 2+ , Fe 3+ ). Also, an ion, molecule, or an atom which is with an incomplete octet of electrons can act as a Lewis acid. Lewis Base : According to Lewis Concept, the base is defined as the substance which can donate the pair of non-bonding electrons. They are clearly known as a nucleophile. They consist of a lone-pair of electrons in their molecular structures. Cl - and OH - is an example of the base