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A scale proportional to the average spectroscopic one-electron binding energy of a free ground-state atom’s valence electrons. For an s,p atom, $\chi =C(n_s{\epsilon }_s+n_p{\epsilon }_p)/(n_s+n_p)$, with positive binding energies and a scale constant $C$. It distinguishes s and p binding rather than assigning both one Slater charge.^[ref]

The large drop from carbon to silicon is the main trend; the smaller reversals at Ge and Pb reflect changes in shielding and valence-electron binding. Allen electronegativity is based on the mean binding energy of valence electrons in free ground-state atoms, rather than an arbitrary charge-transfer count. Moving to larger principal shells generally makes the valence electrons more diffuse and more shielded, reducing their attraction to the nucleus. carbon's compact second-shell valence orbitals therefore give particularly high electronegativity.

The intervening filled 3d shell shields imperfectly, increasing effective nuclear attraction in germanium relative to a simple smooth extrapolation from silicon. This helps explain Ge being slightly above Si rather than below it. Increased size then lowers Sn relative to Ge. The filled inner d and f shells and relativistic stabilization of lead's 6s electrons modify the next step; contraction and stronger binding of the 6s contribution help Pb lie slightly above Sn. The small reversals should not obscure the much larger C-to-Si change. The spectroscopic definition includes both occupied s and p valence contributions, so the relativistic s effect can influence the Allen average even though not every electron participates equally in every bond.^[ref]

Allen electronegativity is proportional to the average binding energy of the free ground-state atom's valence electrons. For an s,p main-group atom, use positive, spectroscopically inferred one-electron binding energies ${\epsilon }_s,{\epsilon }_p$ and their occupancies $n_s,n_p$:Here $C>0$ fixes the conventional scale; the one-electron energies are obtained from appropriate atomic ionization/term energies, not merely from the first ionization energy. For group 15 the weighted average is $(2{\epsilon }_s+3{\epsilon }_p)/5$.^[ref]

The approximate connection to effective nuclear charge is ${\epsilon }_{n\ell }\sim R_{\infty }[Z_{\mathrm{eff}}(n\ell )/n_{\mathrm{eff}}{]}^2$. At a fixed shell, stronger effective nuclear attraction generally raises the valence binding energies and therefore Allen electronegativity. Down a group, increasing shell size and changes in penetration offset that attraction. There is no universal relation ${\chi }_{\mathrm{Allen}}\propto Z_{\mathrm{eff}}$ across different shells, nor can one substitute a core-electron $Z_{\mathrm{eff}}$ for the valence average.