The bond connectivity topological index S_i based on chemical bonds is defined and the molecular index S is formed by S_i according to the properties of the top atoms and their bonds on the study. To band A-B, S_i = (1 + ΔI)/R·[((Z_1 - h_1)m_2)/n_1 + ((Z_2 - h_2)m_1)/n_2], S = ∑ from i=1 to m of (S_i)~(1/2) or ∑ from i=1 to m of 1/(S_i)~(1/2) etc. I stands for electronegativity of A or B, ΔI ≥ 0; Z_1 and Z_2 are the number of valence electrons of A or B; n_1 and n_2 are the numbers of highest main quantum of A or B; h_1 and h_2 are the numbers of hydrogens which are connected with A or B; R is the bond length of A-B. S_i can be applied in not only saturated hydrocarbons but also unsaturated hydrocarbons and organic systems which contain some heteroatoms. By calculating S_i and S, this paper studies standard formative enthalpies of saturated hydrocarbons, as well as CH_mX_n (X = F, Cl, Br, I; m + n = 1～4), distribution coefficients of halogeno- benzene, some unsturated hydrocarbons and the compounds including oxygen. All the correlation coefficients are larger than 0.96.

Key words: organic system, CHEMICAL BONDS, CORRELATIONS