Since the advent of quantum mechanics, many models to explain the nature of chemical bonding have been put forward. Various bonding theories developed relatively recently have been discussed. The concepts of the Pauli Exclusion Principle and the liner transformations which are prerequisite to follow the enusing arguments have been described. These are treated in some details because of their importance in showing correlation between different models discussed in these articles. The models treated here have been classified as (a) quantum mechanical and (b) nonquantum mechanical on the basis of the arguments as to their foundations. The models treated under heading (a), Part I, are the Hartree-SCF scheme, the Walsh diagrams; the Linnet double spin set model, and the FSGO model.