Question

How do you identify lewis acids and bases?

Answer 1

Hint: Lewis acids are those species which can accept a pair of electrons to form covalent bonds and lewis bases are just opposite to lewis acid as lewis bases are those species which can donate a pair of electrons to form covalent bonds.

Complete step by step answer:
Lewis acids are those species that contain an empty orbital which make it able to accept a pair of electrons from a Lewis base to form a covalent bond that is called Lewis adduct.
Lewis bases are those species that have a lone pair of electrons to donate to lewis acid to form a covalent bond that is called Lewis adduct.
In order to identify lewis acid and lewis base we have two consider two conditions:
1. If the central atom has any unbonded pair of electrons then it will donate its pair of electrons to acid and act as a lewis base.
2. If the valence shell of the central atom is filled then it will accept the pair of electrons of the lewis base and act as a lewis acid.
Example of lewis base:
$N{H_3}$ : As in the case of ammonia, nitrogen has three unbonded electrons on it so it will easily donate it and act as a lewis base.
Example of lewis acid:
$AlB{r_3}$ : As in the case of aluminium bromide, central atom aluminium does not have any unbonded electrons on it because one of each electron is bonded to each bromine atom. So that the valence shell of aluminium is unfilled so it accepts the electron from outside. So, it will act as lewis acid.

Note:
A typical example of a Lewis acid in action is in the Friedel–Crafts alkylation reaction. The key step is the acceptance by $AlC{l_3}$ of a chloride ion lone-pair, forming $AlCl_4^ - $ and creating the strongly acidic, that is, electrophilic, carbonium ion.