Ionization And Electron Gain Enthalpies

Ionization Enthalpy

Ionization Enthalpy ($\Delta_i H$) is a quantitative measure of the tendency of an element to lose an electron. It is defined as the energy required to remove an electron from an isolated gaseous atom in its ground state.

$\text{X}(g) \rightarrow \text{X}^+(g) + e^-$

Worked Example: Predicting IE values

Problem. The first ionization enthalpy ($\Delta_i H$) values of the third period elements Na, Mg, and Si are respectively $496$, $737$, and $786 \text{ kJ mol}^{-1}$.

Predict whether the first $\Delta_i H$ value for Al will be more close to $575$ or $760 \text{ kJ mol}^{-1}$.

Electron Gain Enthalpy

Electron Gain Enthalpy ($\Delta_{eg} H$) measures the enthalpy change when an electron is added to a neutral gaseous atom to form a negative ion.

$\text{X}(g) + e^- \rightarrow \text{X}^-(g)$

Unlike ionization enthalpy, which is always positive, adding an electron can be either endothermic (requires energy, positive value) or exothermic (releases energy, negative value).

Worked Example: The Fluorine/Chlorine Anomaly

Problem. Which of the following elements will have the most negative electron gain enthalpy and which will have the least negative?

Elements: Phosphorus (P), Sulfur (S), Chlorine (Cl), Fluorine (F).