Which of the following is not a Lewis acid?

• A. BF₃
• B. AlCl₃
• C. FeCl₃
• D. PH₃

To determine which of the options is not a Lewis acid, let's first define what a Lewis acid is. A Lewis acid is a substance that can accept an electron pair from a Lewis base during a chemical reaction.

Analyzing the Options

• BF₃ (Boron Trifluoride): This is a well-known Lewis acid because it can accept electron pairs due to the electron deficiency of boron.
• AlCl₃ (Aluminum Chloride): This compound also acts as a Lewis acid, as aluminum can accept electron pairs.
• FeCl₃ (Iron(III) Chloride): Iron(III) chloride is another Lewis acid, capable of accepting electrons.
• PH₃ (Phosphine): Unlike the others, phosphine does not have a tendency to accept electron pairs and is not classified as a Lewis acid.

Final Answer

Therefore, the correct answer is PH₃, as it is not a Lewis acid.