The characteristic value t_i=m_i(n_i-1)+h_i of an atom is defined by principal quantum number. The topological index ofprincipal quantum number ⁿQ is formed by t_i: ⁿQ=∑(t_i·t_j·t_k…)^0.5, in which ⁰Q and ¹Q are defined as ⁰Q=∑(t_i)^0.5, ¹Q=∑(t_i·t_j)^0.5. A general and empirical formula that is the correlation between physical and chemical properties with ⁰Q, ¹Q of alkyl benzenes is proposed as follows: p=a+b ⁰Q+c ¹Q, where p stands for physical and chemical properties of alkyl benzene. ⁰Q and ¹Q have good correlation with physical and chemical propertiesof alkyl benzenes such as combustion heat, parachor, boiling point, heat of vaporization, standard free energy of formation, standard entropy, molar volume, water solubility, partition coefficient of n-octanol/water. The results showthat all the correlation coefficients are larger than 0.99.

Key words: principal quantum number, topological index, alkyl benzene, physical and chemical properties, correlation