Question

Which among the following is a Lewis acid?

• A. ${\mathrm{NH}}_3$
• B. ${\mathrm{NH}}_4^{+}$
• C. ${\mathrm{BF}}_3$
• D. ${\mathrm{H}}_2\mathrm{O}$

Answer 1

The correct option is C

${\mathrm{BF}}_3$

Explanation for correct option

(C)${\mathrm{BF}}_3$

Lewis acid-base concept

• According to Lewis's acid-base concept, acid is a species that takes an electron pair.
• Lewis acids can be electron-deficient entities.
• A Lewis base is any substance that can donate electron pair.
• In the case of ${\mathrm{BF}}_3$, the Boron atom has $3$ electrons in its outermost shell and forms 3 single covalent bonds with fluorine and attains 6 electrons. So, it can accept an electron pair.
• So, ${\mathrm{BF}}_3$ is a lewis acid.

Explanation for incorrect option

(A)

• ${\mathrm{NH}}_3$ is a lewis base. Because one lone pair is present in ammonia. So, it can donate its lone pair.

(B)

• ${\mathrm{NH}}_4^{+}$ is not a lewis acid, all atoms are in the stable configuration in this species. So, it can not accept an electron pair.

(D)

• ${\mathrm{H}}_2\mathrm{O}$ is also a lewis base. Because two lone pair is present in the water. So, it can donate its lone pair.
• Hence, options (A), (B), and (D) are incorrect.

Hence, option (C) is correct. ${\mathrm{BF}}_3$ is a lewis acid.