To describe the non-equilibrium behavior of the electronic avalanches and to simulate the development of streamers, the Monte Carlo method at microscopic scale has been used. The physical parameters of the molecules that compose the studied gas: atomic radius, excitation and ionization energies and electron affinities, were used to obtain the parameters of electronic avalanches development and growth: drift velocity, diffusion, mean energy, ionization and attachment coefficients. The main difference between the conventional simulation techniques and the proposed method is that the former is based on the mean free path or the mean free flight time, while the latter is based on the molecule physical parameters.