When any two electrons are considered simultaneously, the radial momentum density function I(p) in many-electron systems is shown to be rigorously separated into low-momentum I-<(p) and high-momentum I->(p) density functions. Accordingly, radial momentum properties such as the Compton peak height J(0), the electronic kinetic energy T, and the mass-velocity correction energy epsilon(mv) are the sum of low- and high-momentum contributions. In relation to the shell structure of atoms, different characteristics of local maxima in the momentum densities I-<(p), I->(p), and I(p) are reported for the 102 atoms He through Lr in their ground states. (c) 2005 Elsevier B.V. All rights reserved.