Question

Which of the following sets has Lewis acid behaviour for all the components?

• A. BF$_3$
• B. BF$_3$, SiF$_4$, PF$_5$
• C. SiF$_4$, PF$_5$
• D. BF$_3$, PF$_5$
• E. (A) and (B)

Hint:

Molecules containing central atoms from the 3rd period or below (like Si, P, S) often act as Lewis acids even if they have a full octet, because they can use their empty d-orbitals to accept more electrons.

Answer 1

The Correct Option is B

Concept: A Lewis acid is a chemical species that can accept an electron pair. This behavior occurs in molecules with electron-deficient atoms or atoms that can expand their octet due to available d-orbitals.

Step 1: {Examine BF$_3$.}
Boron in BF$_3$ has only 6 electrons in its valence shell (incomplete octet). It is a classic electron-deficient molecule that readily accepts an electron pair.

Step 2: {Examine SiF$_4$ and PF$_5$.}
Silicon and Phosphorus have empty d-orbitals in their valence shells. This allows them to act as Lewis acids by accepting electron pairs from Lewis bases to form coordinate bonds, expanding their octet (e.g., forming $[SiF_6]^{2-}$ or $[PF_6]^-$).

Step 3: {Identify the set where all components act as Lewis acids.}
All three components in set (B)—BF$_3$ (incomplete octet), SiF$_4$ (octet expansion), and PF$_5$ (octet expansion)—can accept electron pairs, satisfying the Lewis acid definition.