Electronegativity

Electronegativity is the tendency of an atom to attract electrons toward itself, especially when participating in a chemical bond. Usually represented by the symbol $\chi$, it is a central concept in chemistry for describing unequal electron sharing. Electronegativity is not a single directly measurable property: several scales define it through different experimental or theoretical quantities. The widely used Pauling scale assigns dimensionless relative values, whereas the Mulliken definition relates it to energies of electron removal and attachment. (openstax.org)

Physical meaning and related quantities

In a covalent bond, atoms share electrons, but sharing need not be equal. When one atom is more electronegative, the bonding electron density is displaced toward it. This produces bond polarity: the more electronegative atom generally carries a partial negative charge, $\delta^-$, and its partner a partial positive charge, $\delta^+$. These symbols describe an unequal distribution of charge rather than complete electron transfer. (openstax.org)

Electronegativity differs from ionization energy, the energy required to remove an electron from an atom or other specified species, and from electron affinity, which concerns electron attachment to an isolated species. Electron affinity is associated with a measurable energy change, while electronegativity describes electron attraction in a bonding context. A highly electronegative atom often resists electron removal strongly, but the quantities are not interchangeable. In particular, an electronegativity value should not be interpreted as the energy released when an atom acquires an electron. (openstax.org)

Principal scales

Pauling scale

Linus Pauling developed a relative scale based on bond dissociation energies. Its underlying idea is that a bond between unlike atoms may be stronger than expected from the corresponding bonds between like atoms; the additional stabilization is associated with unequal electron sharing. In the arithmetic-mean formulation recorded by IUPAC,

where the bond energies are expressed per bond. A reference value fixes the scale’s otherwise arbitrary origin, and chemical information determines which atom receives the larger value. (old.goldbook.iupac.org)

Fluorine has a Pauling electronegativity of 3.98, commonly rounded to 4.0 in introductory tables, and is the most electronegative element on this scale. These numbers are relative indices, not charges, probabilities, or percentages. (periodic-table.rsc.org)

Mulliken definition

The Mulliken definition expresses electronegativity as

where $I$ is ionization energy and $A$ is electron affinity, conventionally positive for energetically favorable electron attachment. It combines resistance to electron loss with affinity for electron gain. Unlike Pauling values, unrescaled Mulliken values have units of energy, commonly electronvolts. Values from different scales therefore require a specified conversion before numerical comparison. (old.iupac.org)

Periodic trends

Electronegativity generally increases from left to right across a period of the periodic table and decreases down a group. Consequently, many nonmetals near the upper right attract bonding electrons strongly, whereas metals toward the lower left have relatively low values. These are broad trends rather than a rule that every successive element must follow. (openstax.org)

The trends can be understood through nuclear attraction and electron configuration. Across a period, increasing nuclear charge strengthens attraction to outer electrons, while added electrons enter the same principal shell. Down a group, outer electrons occupy more extended shells and experience greater shielding by inner electrons. The resulting changes in atomic size and effective attraction help explain the general electronegativity pattern. (en.wikipedia.org)

Many introductory Pauling tables omit noble gases because their usual chemical behavior provides relatively few conventional bonding data. A missing entry does not mean zero electronegativity. It indicates that the particular scale or table does not assign a value; electron attraction and chemical inertness are distinct concepts. (openstax.org)

Bond type and molecular polarity

The difference between the electronegativities of two bonded atoms provides a rough guide to electron sharing. Small differences usually indicate relatively nonpolar covalent bonds; larger differences indicate polar covalent bonds or substantial ionic bonding character. Hydrogen–hydrogen, hydrogen–chlorine, and sodium–chlorine illustrate increasing inequality of electron sharing. Numerical boundaries used in textbooks are approximate: covalent and ionic character form a continuum rather than categories separated by a universal cutoff. (openstax.org)

Bond polarity must also be distinguished from the polarity of an entire molecule. Molecular shape determines whether individual bond dipoles reinforce or cancel one another. In carbon dioxide, the two polar carbon–oxygen bonds have opposing dipole contributions that cancel in the linear molecule. In water, the bent geometry prevents cancellation, producing a permanent electric dipole. Electronegativity differences alone therefore cannot establish molecular polarity without structural information. (openstax.org)

Chemical applications and limits

Electronegativity supports electron bookkeeping in assigning oxidation states. In the ionic approximation, electrons in a bond between unlike atoms are assigned to the more electronegative partner, while bonds between identical atoms are divided equally. IUPAC’s practical formulation uses Allen electronegativities and includes qualifications for certain reversibly bonded ligands. Oxidation states are formal assignments, not measurements of the actual partial charges within a compound. (goldbook.iupac.org)

Electronegativity tables are especially useful for qualitative comparisons, but they do not independently specify bond strength, molecular shape, or the complete electron distribution. Detailed bonding descriptions require structural evidence and electronic models, including molecular orbital theory. Different scales emphasize different aspects of electron attraction, so meaningful numerical comparisons must identify the scale being used. (openstax.org)