According to the concept, an acid is a substance which can accept a pair of e^- while base is a substance which can donate a pair of e^-:

Na⁺, Ag⁺                                             CN^-, OH^-

Lewis acid                                            Lewis Base

Types of Lewis base:-

• All the negatively charged in species, for example: Cl^-.
• Neutral species having as least one lone pair of electron. For example: NH₃, H₂O.

Types of Lewis acid:-

1. Molecule in which the central atom has incomplete octet, BH₃, FeCl₃.
2. Single cation act as Lewis acid for e.g.: Ag⁺, Na⁺, K⁺, Ca₂⁺.
3. Molecule in which the central atom has empty d-orbital due to the presence of empty d-orbital the central atom in these molecule can expand their octet by accepting electron from other substances.

For Example:  SiCl₄ + Cl₂ ⟶ SiCl₆^2- ,   LiF₄.

Limitation of Lewis concept:-

1. It doesn’t explain the behavior of well known protonic acid like HCl, H₂SO₄.
2. It doesn’t explain the relative strength of acid & base.
3. The catalytic activity of many acid is due to H⁺ ions. But Lewis acid mainly not contain proton, hence, may not be used as a catalyst.