Abstract: Chemical graph theory is one of the branches in mathematical chemistry that uses graph theory as an index for the chemical occurrence. A topological index is a numerical value that describes a chemical structure for many physical and chemical properties. All organic compounds can be considered as hydrocarbons or compounds derived from hydrocarbons. Double bonds as well as atom kinds, i.e., C, N, O, etc. cannot be recognized using simple topological indices which are defined for only unidirectionally connected graphs. Many topological indices have been developed taking into consideration the state of hybridization for each atom included within the molecule. This study is mainly dedicated to collect most of the data obtained for the chemical and mathematical H indices. In this study, general formulas are established and then proved for a particular topological index of some trees which is the harmonic index for alkanes, alkenes and alkynes ISO-alkanes (2-methyl alkanes) and ISO-alkanes (2, 2-methyl alkanes).