For wave functions of atoms expressed by linear combinations of Slater determinants, a general formalism is presented to evaluate the angular expectation values [cos theta(12)] and [cos (θ) over bar (12)] where theta(12) and (θ) over bar (12) are the angles subtended, respectively, by the position and momentum vectors of two electrons. The results are applied to the 102 atoms He through Lr in their ground states within the Hartree-Fock framework. It is found that both [cos theta(12)] and [cos (θ) over bar (12)] are non-positive for all the atoms, implying that the interelectronic angles theta(12) and (θ) over bar (12) are never smaller than 90degrees in an average sense. (C) 2002 Elsevier Science B.V. All rights reserved.