The coincidence of the classical and quantum cross-sections of elastic scattering for charged particles allows one to use the classical approach for ionization of atomic particles by electron impact. Comparison of experimental data for electron-impact ionization of atoms, ions and molecules with identical valence electrons gives an universal function for such process, considered as a result of interaction of an incident electron with independent valence electrons of an atomic particle. The method is applied to the ionization cross-sections of vibrationally and electronically excited nitrogen molecules. (c) 2005 Elsevier B.V. All rights reserved.