Consider the following: I. AlCl3 II. NH3 III. BF3 IV. FeCl3. How many of the above are Lewis acids?

A Lewis acid is an electron-pair acceptor. AlCl3 has an electron-deficient aluminium (only 6 electrons) and readily accepts a lone pair — Lewis acid. BF3 has an electron-deficient boron and is a classic Lewis acid. FeCl3 has an empty d-orbital on iron and acts as a Lewis acid (used as catalyst in halogenation). NH3, on the other hand, has a lone pair on nitrogen which it donates — it is a Lewis BASE, not an acid. Hence three of the four (AlCl3, BF3, FeCl3) are Lewis acids.