Answer. According to Lewis concept an acid is a substance (molecule or ion) which can accept a pair of electrons. For example, BF3, H+
According to this concept, the following species can act as Lewis acids:
Molecules in which the central atom has incomplete octet. For example, in BF3, AlCl3, FeCl3, the central atom has only six electrons around it; therefore, these can accept an electron pair.
Simple cations can act as Lewis acids. All cations act as Lewis acids since they are deficient in electrons. However, cations such as Na+, K+, Ca2+ ions, etc., have a very little tendency to accept electrons. While the cations like H+, Ag+ ions, etc., have a greater electron accepting tendency therefore, act as Lewis acids