Detailed Summary
Electron Gain Enthalpy (ΔegH) measures the energy change that occurs when an electron is added to a neutral gaseous atom (X), forming a negative ion (X−). The enthalpy change can either be exothermic, where energy is released, indicating a negative ΔegH, or endothermic, where energy is required, resulting in a positive ΔegH.
For elements such as halogens, the values of ΔegH are notably negative because they are eager to achieve stable noble gas configurations. In contrast, noble gases possess high ΔegH values, indicating that adding an electron destabilizes their stable electron configuration by pushing the additional electron to a higher energy level. Generally, the electron gain enthalpy is less systematic compared to ionization enthalpy, but there are trends observable across the periodic table.
Typically, ΔegH becomes more negative across a period due to increasing effective nuclear charge, which facilitates the addition of electrons. Conversely, as one moves down a group, ΔegH tends to become less negative as atomic size increases, leading to a reduced attraction between the nucleus and the added electron. While these trends are general, exceptions exist, particularly between O and F, where added electron-electron repulsion plays a notable role. Understanding electron gain enthalpy provides insights into the chemical behavior of elements, especially in predicting reactivities and stability of ions.