The theory of relativity in physics has led to an increased amount of correlation in physical phenomena and to a keener insight into the fundamental problems of physics. More recently, another kind of relativity theory has made its appearance in chemistry. We owe this theory to the brilliant intuition of G. N. Lewis, by whom it was first proposed in 1923. Unfortunately, it was largely overlooked until about 1940. Since then, numerous papers have been published supporting and developing the theory, which is becoming known as the electronic theory of acids and bases.According to the Lewis theory, acidity and basicity have nothing to do with the presence of any one element or specific group of elements. Nor is any particular substance to be considered an acid or a base in an absolute sense. Acidity and basicity are relative terms indicating certain types of experimental behavior which may be exhibited by the vast majority of chemical compounds. Most of them may behave either as acids or as bases depending upon conditions.