becoming essentially zero for r aTF. In this way, aTF becomes an indicator of
atomic size. The Thomas–Fermi model prescribes aTF proportional to Z
1/3; the
weak variation with Z suggests that all atoms are essentially the same size, regardless
of how many electrons they may have. Because the Thomas–Fermi potential is not
Coulombic, the one-electron energies that result from the use of Equation 9.21 vary
within a given shell; that is, they depend on the principal (n) and orbital () quantum
numbers. The study of these energies and their associated wavefunctions
requires numerical methods, or further approximation. The Thomas–Fermi
approximation improves with larger values of Z and so is especially well suited to
describe the outer electronic structure of the heavier elements.
In another approach, called the quantum-defect method, nuclear shielding is
described by