Classification of Elements and Periodicity in Properties - Electron Gain Enthalpy

Electron Gain Enthalpy ($\\Delta_{eg}H$) is the enthalpy change that occurs when an electron is added to an isolated gaseous atom to form a negative ion: $X(g) + e^- \rightarrow X^-(g)$.

Depending on the element, the process can be exothermic (negative $\\Delta_{eg}H$) or endothermic (positive $\\Delta_{eg}H$). Most elements release energy when they gain an electron.

Across a period (left to right), $\\Delta_{eg}H$ generally becomes more negative because the atomic radius decreases and the effective nuclear charge ($Z_{eff}$) increases, leading to a stronger attraction for the incoming electron.

Down a group, $\\Delta_{eg}H$ generally becomes less negative because the atomic size increases, and the incoming electron is farther from the nucleus, experiencing less attraction.

Group 17 (Halogens) have the most negative electron gain enthalpies because they only need one electron to achieve a stable noble gas configuration.

Noble gases (Group 18) have large positive electron gain enthalpies because the added electron has to enter a higher principal quantum level, creating a highly unstable configuration.

The $\\Delta_{eg}H$ of $Cl$ is more negative than that of $F$. This is because the $2p$ subshell in $F$ is very compact, leading to significant inter-electronic repulsion when an electron is added, whereas the $3p$ subshell in $Cl$ is larger and accommodates the electron more easily.

Successive electron gain enthalpies: Adding a second electron to a uninegative ion (e.g., $O^-(g) + e^- \rightarrow O^{2-}(g)$) is always endothermic ($\\Delta_{eg}H > 0$) due to the strong electrostatic repulsion between the negative ion and the incoming electron.