Question

Which of the following is a Lewis acid?
A. \[{{H}_{2}}O\]
B. \[C{{l}^{-}}\]
C. \[B{{F}_{3}}\]
D. \[N{{H}_{3}}\]

Answer 1

Hint: The ions which carry positive charges and the molecules or the chemical compounds which have the central atom deficiency of electrons, accept electrons.
The correct answer is (C).

Complete answer:
In all the above-mentioned species \[B{{F}_{3}}\] is a Lewis acid. This is because of the following analysis:
\[{{H}_{2}}O\] his oxygen as its central atom. Oxygen in water has two bonds and two lone pairs. Due to the presence of The Lone pairs, it is electron rich and cannot be a Lewis acid.
Similarly, \[C{{l}^{-}}\] has gained one electron and does not need any more electrons to reach stability so it is also not a Lewis acid. Now if we consider \[B{{F}_{3}}\] the central atom Boron has only 3 electrons in the outermost shell. Fluorine by forming chemical bonds increases the number up to 6. But Boron still requires 2 more electrons to attain fully filled stability.
It has one p- orbital empty and as a result becomes an electron acceptor and hence acts as a Lewis acid. In the last option \[N{{H}_{3}}\] nitrogen again possesses a lone pair of electrons making the central atom electron rich and acts as a Lewis base by donating these lone pairs to two electron deficient species.
Lewis acids are the species which accept electrons, that is they are electron deficient. Lewis acids can be the species carrying positive charges or with empty orbitals of the outermost shells.

Note:
\[B{{F}_{3}}\] is a Lewis acid but it cannot be called a Bronsted acid because it does not donate any Proton ever.