In 1923, G.N. Lewis proposed a general definition for acids and bases which is based upon donation and acceptance of electron pair. According to Lewis concept, an acid is a substance which can accept a pair of electron whereas a base is a substance which can donate a pair of electron i.e, an acid is an electron pair acceptor this means that acid is a substance which has vacant orbitals to accommodate one or more electron pairs. On the other hand a base is an electron pair donor. It means that any substance with unshared pair of electron can behave as Lewis base. Therefore, acid-base or neutralization reaction which involves formation of co-ordinate bond between electron pair donor and electron acceptor for example reaction between BF3 and NH3 takes place as follows-
In above reaction on NH3 donates electron pair to BF3 to form a co-ordinate Bond, therefore NH3 behaves as Lewis base while BF3 behaves as Lewis acid.
1. Molecules having central atom with incomplete octet of electrons. BCl3, BF3 and AlCl3 are acids of this type. The central atom boron and aluminium has only 6 electrons.
2. All the cations are short of electrons and behave as Lewis acid, large size cations like Na⁺, K+, Ca2+ etc behave as weak Lewis acid. This is because these have little tendency to accept the electrons. On other hand small size cations such as H+, Ag+ ion have a strong tendency to accept the pair of electrons and strong acids.
3. In addition highly charged cations have higher tendency to accept electron pairs relative to lower charge cations of the given metal. For example Sn4+ is stronger acid than Sn2+ some of typical examples of Lewis acid-base reactions are given below.
Molecules in which central atom have available d orbitals and make acquire more than 8 electrons compound such as AlCl3, SiF4, SnCl₄, and SO3 are Lewis acids where the central atom can expand its valence shell.4. Molecules with multiple bond between atoms of dissimilar electronegativities.
Similarly, SO3 acts as a Lewis acid towards water.
1. All negatively charged anionic species are Lewis base because they are rich of electrons and have strong tendency to donate electron pairs, for example Cl−,OH⁻,NH2.–
2. Neutral molecules with one or more than one unshared pair of electrons. For example NH3, H2O acts as electron donor.
3. Alkenes and alkynes with multiple bond also form co-ordinate bond with metal ion in organometallic compounds.
for example-4. Lewis acid-base concept is widely used because of its simplicity in understanding many acid base reactions inspite of its wider application than Arrhenius and Bronsted-Lowry concept, it has several disadvantages which are given below.
Lewis concept is too general for reactions having a co-ordinate bond to be acid-base reactions but this is not always true. According to Lewis concept some of transition metals that form compounds with CO such as [Ni(CO)4], Fe(CO)5 etc are acid-base reactions which is not correct.
5. Formation of a co-ordinate bond between an acid and a base is basis of lewis concept. However, well known acids like HCl, HNO₃ and H2SO4 do not form any co-ordinate Bond with base such as NaOH or KOH therefore, according to this concept HCl, HNO₃ and H2SO4 should not be regarded as acids.
6. Lewis concept does not give any uniform scale of acid and base strength.