In the 1920s, Gilbert Lewis proposed a new set of definitions for acids and bases.
According to the Lewis definition an acid is an electron pair acceptor.
Concept #1: Understanding Lewis Acids.
When hydrogen is connected to an electronegative element such as P, O, N, S or halogens then it gains a partially positive charge, which makes hydrogen act as a Lewis acid.
Since these metal ions accept negative electron pairs then some Lewis Acids may be positively charged.
If the central element has less than 8 valence electrons around it then it is more likely to accept an electron pair to follow the octet rule.
According to the Lewis definition a base is an electron pair donor.
Concept #2: Understanding Lewis Bases.
Compounds that have lone pair(s) on a central element can act as Lewis bases.
Compounds that possess a negative charge can act as Lewis Bases.
Example #1: Identify each of the compounds in the following chemical equation.
Practice: Identify the Lewis acids and bases in the following reactions.
a) H+ + OH– ⇌ H2O
b) Cl– + BCl3 ⇋ BCl4–
c) SO3 + H2O ⇌ H2SO4
Practice: Identify each of the following compounds as either a Lewis acid, a Lewis base or neither.
a) ZnCl2 b) CN –
c) NH4+ d) Co3+