Question

Which of the following is NOT a lewis acid?
A. $B2H6$
B. $BeCl2$
C. $AlH3$
D. $NH3$

Answer 1

Hint: Lewis acids are lone pair acceptors or we can say electron deficient compounds. Lewis bases are lone pair donor or electron rich compounds.

Complete solution:
Let us first discuss the structure and type of bonding in these compounds.
$B_2{H}_6$ This compound is known as diborane .It is an electron deficient compound.In its structure, the four terminal hydrogen atoms and the two boron atoms lie in one plane. Above and below this plane, there are two bridging hydrogen atoms.

Since boron is electron deficient and this compound get stability using back bonding.So it accept a lone pair very easily and that is why it is a lewis acid
$BeC{l}_2$ Beryllium chloride is an electron deficient compound because octet of Be is not complete and it can easily accept a lone pair. So it is a lewis acid.

$AlC{l}_3$ It is an electron deficient compound molecule that has a tendency to accept a pair of electrons to achieve stable electronic configuration and thus behaves as lewis acids. It achieves stability by forming a dimer.

$N{H}_3$ It has pyramidal structure and has lone pairs on nitrogen atoms so it is an electron rich compound.It acts as a lewis base and can easily donate a lone pair of electrons.

Our correct option: D

Note:
Try to draw the structure of compound and lone pair on the compound. If compounds have lone pairs then it is not a lewis acid. For a compound to be a Lewis acid it should be capable of accepting electrons.