electronegativity (Pauling and Mulliken scales)
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When two atoms share a bond, do they share the electrons fairly, or does one hog them? Electronegativity measures the grabbiness of an atom for the shared electrons in a chemical bond. It is not a property of a lone atom in a vacuum (that is ionization energy and electron affinity) but a property of an atom inside a bond — a number that lets you predict, at a glance, whether a bond will be even (covalent), lopsided (polar), or so one-sided it is essentially ionic.
Several scales put numbers on this idea. The Pauling scale, the most widely used, is empirical: Linus Pauling worked out relative values from bond energies, anchoring fluorine near 4.0 (the greediest element) and running down to about 0.7 for caesium (the most generous). The Mulliken scale takes a more physical route, defining electronegativity as roughly the average of an atom's ionization energy and its electron affinity — the idea being that an atom which both holds its own electrons tightly and welcomes more will pull hardest on shared ones. The two scales tell the same story and can be converted into one another, even though their numbers differ. Across a period electronegativity rises (Zeff up, atom smaller); down a group it falls (electrons farther out).
Electronegativity is the workhorse for predicting bond character and reactivity. The difference in electronegativity between two bonded atoms tells you the bond's polarity: a small difference means a nearly nonpolar covalent bond, a moderate one a polar bond with a partial-charge end, and a large difference an ionic bond. It explains why water is bent and polar, why hydrogen fluoride is acidic, and which end of a molecule is electron-rich. Be honest about its nature, though: electronegativity is a useful, somewhat fuzzy concept, not a precisely measurable physical constant; values shift slightly with oxidation state and bonding environment, and the various scales are models that mostly agree rather than one true measurement.
Pauling values: F 3.98, O 3.44, N 3.04, C 2.55, H 2.20, Na 0.93, Cs 0.79. The H-O difference (about 1.24) makes water polar; the Na-Cl difference (about 2.1) makes NaCl ionic.
The bigger the gap, the more lopsided (and ionic) the bond.
Electronegativity is not a directly measured constant — it is a derived, model-dependent number, and different scales (Pauling, Mulliken, Allred-Rochow) give different values that nonetheless rank the elements almost identically. Fluorine is the most electronegative element; the noble gases are usually left off or given special values.