In the present study, the non-dissociative gas adsorption with different chemisorption geometries on homogeneous solid surface has been studied. By considering the different chemisorption geometries for an adsorbate, an isotherm has been derived by statistical thermodynamics. Statistical thermodynamics study of adsorption leads to the concept of a constant parameter, , which shows the ratio of the number of occupied sites to the number of adsorbed molecules. The isotherm has been analyzed by some of the experimental data of adsorption at solid/gas interface. Comparisons between the obtained isotherm and some famous isotherms like Langmuir, Freundlich, Langmuir-Freundlich, Toth, Temkin, and Frumkin have been provided. The results showed the new equation has a good conformity with experimental data.