= Chemical environment dependence of electronegativity
A tabulated <electronegativity> is useful because it compresses many comparisons, but one number per <chemical element> suppresses chemical context. Changing the bonded partners, <bond length> or <electron configuration> changes the conditions under which a <chemical atom> attracts <electron density>. The <bond-dissociation energy> can change for reasons beyond <bond polarization>, so attributing every extra unit of <energy> solely to an <electronegativity> difference is an approximation.<Energy Electronegativity and Chemical Bonding by Stepan S. Batsanov>[§2.2, opening paragraph: bond distance and atomic valence affect bond energy]{ref}
An <oxidation state> labels a formal allocation of <Electrons>; it is not a measured <partial atomic charge>. State-sensitive <electronegativity> models may give different values for different <oxidation states> or <electron configurations>, while a simple <chemical element> table intentionally provides a common comparison convention. Likewise, replacing an isolated <chemical atom> by a site in an <ionic crystal> introduces a surrounding <electric field> and many neighbours. Applying the same tabulated <electronegativity> without adjustment becomes an additional assumption.<Energy Electronegativity and Chemical Bonding by Stepan S. Batsanov>[§2.1, Table 1 and valence footnotes; §2.2, Table 2; §2.3, opening paragraph on cooperative effects in solids]{ref}
For the ground-state <Mulliken scale>, the state dependence is visible directly in the formula. Replacing $X$ by $X^+$ replaces $E(X^+)-E(X)$ with $E(X^{2+})-E(X^+)$ as the removal cost, and replaces $E(X)-E(X^-)$ with $E(X^+)-E(X)$ as the attachment release. These are different <ionization energies> and <electron affinities>. This isolated-<ion> arithmetic does not claim that a bonded <chemical atom> with the same formal <oxidation state> is an isolated <ion>. In a <molecule>, the actual <electron density> and <dipole evidence for bond polarization> supply complementary information about that environment.
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