Monotonicity of the electron momentum density for atomic closed shells in a bare Coulomb field

For the bare Coulomb potential energy -Ze²/r, it is shown that the total electron momentum density {Π}₀(p) for an arbitrary number of closed shells is a monotonically decreasing function of p. Since it is known that {Π}(p) is nonmonotonic for all atomic ground states with completely closed shells except helium, the implication of this result is that the observed nonmonotonicity is due to terms of higher order than Π₀(p) in Z^-1 perturbation theory. Numerical calculations are presented and analyzed for the ten-electron isoelectronic series.