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A one parameter expression for the single ionization cross-section of atoms by electron impact is presented. Using this expression, the agreement obtained with available experimental data for 45 elements (from ionization threshold up to 200 eV ionizing electron energy) is shown to be comparable with (or better than) that achieved using other multi-parameter approaches. We suggest that the single parameter used here is associated with the effective reduction in the number of equivalent electrons within a given shell accessible for ionization by electron impact. We attribute this reduction effect to intra-shell shadowing of part of the shell electrons by other electrons of the same shell. For atoms with 2s^2p^i (i = 3,4,5,6) outer shell configurations we suggest also an intershell shadowing effect. Finally, we discuss the possibility of a meaningful contribution of inner shell ionization to the single ionization cross-section due to various post-collision interactions. These interactions may be responsible for decreasing the formation probability of doubly charged ions., Comment: 22 pages, 1 table, 6 figures. (1). The approach in the 1st version resulted in the exclusion of some elements. In new version we have used an approach which describes the cross section for all elements without exclusions. (2). Section 3 from the 1st version is omitted