(d) Electron Gain Enthalpy

(d) Electron Gain Enthalpy

Definition and Sign Convention

Electron gain enthalpy ($\Delta_{eg} H$) is the enthalpy change that occurs when an electron is added to a neutral, isolated gaseous atom to form a negative ion.

The sign of the electron gain enthalpy depends on the element.

• Negative $\Delta_{eg} H$: For most elements, energy is released when an electron is added. This is because the added electron is attracted to the positively charged nucleus. A more negative value means a greater tendency to gain an electron. Halogens (Group 17) have highly negative values because they achieve a stable noble gas configuration by gaining one electron.
• Positive $\Delta_{eg} H$: For some elements, energy must be supplied to force an electron onto the atom. This happens when the added electron must enter a new, higher energy level. Noble gases have large positive values because the added electron must go into the next principal quantum level, creating a highly unstable configuration.

The General Trends in Electron Gain Enthalpy

The variation in electron gain enthalpy is less systematic than for ionization enthalpy, but general rules do exist.

Trend 1: Electron gain enthalpy becomes more negative across a period.

As you move from left to right across a period, the atomic size decreases and the effective nuclear charge increases. The nucleus can attract an additional electron more strongly. Therefore, the process of adding an electron becomes more exothermic (more negative $\Delta_{eg} H$).

Trend 2: Electron gain enthalpy becomes less negative down a group.

As you go down a group, the atomic size increases. The added electron is farther from the nucleus and experiences more shielding. The attraction is weaker, so less energy is released (or more energy is required) when the electron is added. The $\Delta_{eg} H$ value becomes less negative.

An Important Anomaly: Oxygen and Fluorine

The general trend down a group has a notable exception. The electron gain enthalpy of oxygen (O) is -141 kJ mol$^{-1}$, but for sulfur (S) it is -200 kJ mol$^{-1}$. Similarly, the value for fluorine (F) is -328 kJ mol$^{-1}$, but for chlorine (Cl) it is -349 kJ mol$^{-1}$.

This means that sulfur has a more negative electron gain enthalpy than oxygen, and chlorine has a more negative value than fluorine. This is the opposite of what the "down a group" trend would predict. …