Electron Gain Enthalpy and Electronegativity — Core Principles

Core Principles

Electron gain enthalpy ($\Delta_{eg}H$) is the energy change when an electron is added to a neutral gaseous atom. A negative value indicates energy release (exothermic), signifying a strong attraction for the electron, while a positive value indicates energy absorption (endothermic), meaning the atom resists electron addition.

Factors influencing EGE include effective nuclear charge (higher $Z_{eff}$ means more negative EGE), atomic size (larger size means less negative EGE), and electronic configuration (stable configurations like noble gases or half-filled orbitals lead to positive EGE).

EGE generally becomes more negative across a period and less negative down a group, with notable exceptions like fluorine having a less negative EGE than chlorine due to electron-electron repulsion in its small 2p subshell.

The second electron gain enthalpy is always positive due to repulsion.

Electronegativity (EN) is a relative measure of an atom's ability to attract shared electrons in a chemical bond. It increases across a period and decreases down a group, mirroring the trends of effective nuclear charge and atomic size.

Factors like hybridization (more 's' character increases EN) and oxidation state (higher positive oxidation state increases EN) also play a role. Electronegativity differences determine bond polarity and character.

While both properties relate to electron attraction, EGE is an absolute energy value for isolated atoms, whereas EN is a relative measure within a bond.

Important Differences

vs Electronegativity

Aspect	This Topic	Electronegativity
Definition	Electron Gain Enthalpy ($\Delta_{eg}H$): Enthalpy change when an electron is added to a neutral gaseous atom.	Electronegativity (EN): Tendency of an atom to attract shared electrons in a bond.
Nature	Absolute energy value for an isolated atom.	Relative measure, dimensionless, for an atom in a chemical bond.
Units	kJ/mol or eV.	No units (relative scale, e.g., Pauling scale).
Sign	Can be negative (exothermic, energy released) or positive (endothermic, energy absorbed).	Always positive (a measure of attraction strength).
Measurement	Measured experimentally by adding an electron to a gaseous atom.	Calculated from various properties like bond energies (Pauling), IE and EGE (Mulliken), or effective nuclear charge (Allred-Rochow).
Application	Indicates the ease of anion formation and stability of anion.	Predicts bond polarity, bond character (ionic/covalent), and reactivity in compounds.

Electron gain enthalpy quantifies the energy change when an isolated atom *gains* an electron, reflecting its intrinsic electron-accepting tendency and the stability of the resulting anion. It's an absolute energy value with a sign indicating energy release or absorption. Electronegativity, conversely, describes an atom's relative ability to *attract shared electrons* within a chemical bond, influencing bond polarity. It's a dimensionless, relative scale. While both relate to electron attraction, EGE is about complete electron transfer to an isolated atom, whereas EN is about electron sharing within a molecule.