"Introductory courses in organic chemistry usually rely primarily on the valence bond description of molecular structure. Valence bond theory was the first structural theory applied to the empirical information about organic chemistry. During the second half of the nineteenth century, correct structural formulas were deduced for a wide variety of organic compounds. The concept of “valence” was recognized. That is, carbon almost always formed four bonds, nitrogen three, oxygen two, and the halogens one."