Introduction

The electron gain enthalpy and ionization energy are both measures of the energy changes that occur when an atom gains or loses an electron. However, there are fundamental differences between these two concepts that explain why electron gain enthalpy can be exothermic or endothermic, while ionization energy is always endothermic.

Ionization Energy

Key Point: Ionization energy is always endothermic.

Ionization energy is the energy required to remove an electron from an atom in its gaseous state. It represents the amount of energy needed to overcome the attractive forces between the positively charged nucleus and the negatively charged electron.

The ionization energy is always endothermic because energy must be supplied to overcome the strong electrostatic forces holding the electron in its orbit around the nucleus. As the electron is removed, the electron-nucleus attraction decreases and energy is required to overcome this attraction.

Electron Gain Enthalpy

Key Point: Electron gain enthalpy can be exothermic or endothermic.

Electron gain enthalpy is the energy change that occurs when an atom in its gaseous state gains an electron to form a negative ion. It represents the amount of energy released or absorbed in this process.

The electron gain enthalpy can be exothermic or endothermic depending on the atom and the electron being gained. This is because the energy change is influenced by both the electron-electron repulsion and the electron-nucleus attraction.

Factors Influencing Electron Gain Enthalpy

Key Point: Electron gain enthalpy is influenced by the electron-electron repulsion and the electron-nucleus attraction.

1. Electron-Electron Repulsion: When an atom gains an electron, it adds to the existing electron cloud, resulting in increased electron-electron repulsion. This repulsion requires energy to overcome, making the process endothermic.

2. Electron-Nucleus Attraction: The strength of the electron-nucleus attraction also plays a role in determining the exothermic or endothermic nature of electron gain enthalpy. If the atom already has a high effective nuclear charge (i.e., a strong attraction between the nucleus and electrons), the addition of an electron becomes more difficult, resulting in an endothermic process. Conversely, if the atom has a low effective nuclear charge, the addition of an electron becomes easier, leading to an exothermic process.

Conclusion

In summary, ionization energy is always endothermic because energy must be supplied to remove an electron from an atom. On the other hand, electron gain enthalpy can be exothermic or endothermic depending on the electron-electron repulsion and the electron-nucleus attraction. These factors influence the energy change that occurs when an atom gains an electron to form a negative ion.