According to Lewis, an acid is an electron pair acceptor and a base is an electron-pair donor. So, an acid must have at least one vacant orbital in its valence shell where it can accommodate an electron pair.
Thus, H+ which is recognized as an acid by all the concepts has its 1s orbital vacant and can accept one pair of electrons and attain the He-structure.
Similarly, OH–, NH3 are based because both of them have one or more lone pair of electrons which they can donate to acid and hence they are Lewis bases.
The process of neutralisation is thus nothing but the formation of a coordinate bond between the acid and the base in which the acid is the electron pair acceptor and the base is the electron-pair donor.
Thus, all Bronsted-lowry acids and bases are Lewis acids and Lewis bases but all the Lewis acids and bases are not Bronsted-lowry acids and bases.
Types of Lewis Acid:
1. Compound whose central atom has an incomplete octet. In other words, acids of this type are electron deficient molecules such as BF3, BCl3, AlCl3, FeCl3, etc.
2. Compounds in which the central atom has available d-orbital and may acquire more than an octet of valence electrons.
3. All simple cation such as Na+, Ag+, Al+3, Fe+3. They can combine with electron pairs.
4. Molecules with multiple bonds between atoms of dissimilar electronegativities.
5. Elements with an electron sextet e.g. oxygen and sulphur atoms contain six electrons in their valence shell and therefore, act as Lewis acids.
Types of Lewis Base:
1. All simple negative ions such as Cl–, F–, OH–, etc.
2. Molecules with one or two unshared pairs of electrons, such as – H2O, NH3, Pyridine, etc.
3. Multiply bonded compounds which from co-ordinate compounds with transition metals, such as – CO, NO, C2H4, etc.
Limitations of Lewis Theory:
Despite its more fundamental and comprehensive character, the lewis concept suffers from a defect of quite a serious nature. It can’t be applied to explain and illustrate the relative strengths of acids and bases. The acid-base phenomenon can be explained by using Bronsted theory or the protonic theory as most of the common acids are proton donors and most of the bases are proton acceptors.