Electron gain enthalpy and electronegativity

Electron gain enthalpy ΔegH is the enthalpy change when a neutral gaseous atom takes an electron: X(g) + e⁻ → X⁻(g). It can be negative (energy released) or positive (energy needed).

Halogens have strongly negative ΔegH because one electron completes a noble gas configuration. Noble gases have large positive values because the electron must enter a new, higher shell. The most negative values are at the upper right, just before the noble gases.

Trend, less regular than ionization enthalpy: ΔegH usually becomes more negative across a period (smaller atom, higher effective nuclear charge) and less negative down a group (larger atom).

Exception: O and F are less negative than S and Cl. The electron entering the compact n = 2 shell is strongly repelled; in n = 3 there is more room. So Cl (−349) is more negative than F (−328), and of P, S, Cl and F, Cl is most negative and P least.

NCERT values (kJ mol⁻¹): H −73; Li −60, Na −53, K −48, Rb −47, Cs −46; O −141, S −200, Se −195, Te −190, Po −174; F −328, Cl −349, Br −325, I −295, At −270; He +48, Ne +116, Ar +96, Kr +96, Xe +77, Rn +68.

Many books use electron affinity Aₑ, the negative of ΔegH, so a release of energy gives a positive Aₑ. Defined at absolute zero, it relates as ΔegH = −Aₑ − (5/2)RT.

Electronegativity is a qualitative measure of how strongly an atom in a compound pulls shared electrons to itself. It cannot be measured directly and changes with the bonding partner. Scales: Pauling (most used), Mulliken-Jaffe, Allred-Rochow; Pauling (1922) fixed fluorine at 4.0.

Pauling values: Li 1.0, Be 1.5, B 2.0, C 2.5, N 3.0, O 3.5, F 4.0; Na 0.9, Mg 1.2, Al 1.5, Si 1.8, P 2.1, S 2.5, Cl 3.0; down group 1: Li 1.0, Na 0.9, K 0.8, Rb 0.8, Cs 0.7; down group 17: F 4.0, Cl 3.0, Br 2.8, I 2.5, At 2.2.

Electronegativity rises across a period and falls down a group, following the inverse of atomic radius, like ionization enthalpy. It goes with non-metallic character and against metallic character.