Which of the following is not Lewis acid?

Explanation

The correct answer is Cl-. A Lewis acid is defined as a substance that can accept an electron pair. In this context, Cl- is not a Lewis acid because it already possesses a full octet and is negatively charged, indicating it is more likely to donate an electron pair, thus acting as a Lewis base. On the other hand, HCl can become a Lewis acid when it dissociates, BF3 is electron-deficient and can accept electron pairs, and AlCl3 can also accept electron pairs due to its electron-deficient aluminum center.