Why is the Ionization Enthalpy of Indium Lower than Aluminum?

When comparing the ionization enthalpy of indium (In) and aluminum (Al), several factors come into play, primarily related to atomic structure and electron configurations. The key aspects that contribute to a lower ionization enthalpy for indium include increased atomic size, enhanced shielding effect, and a lower effective nuclear charge.

Understanding the Factors

Atomic Size: Indium is positioned below aluminum in Group 13 of the periodic table. As moving down a group increases the atomic size due to the addition of electron shells, indium has larger outermost electrons compared to aluminum. These outer electrons are farther from the nucleus, resulting in a weaker attraction between the nucleus and the outer electrons. This makes it easier to remove an electron from indium than from aluminum.

Shielding Effect

Shielding Effect: Indium has more inner electron shells than aluminum, leading to a greater shielding effect. The inner electrons repel the outer electrons effectively reducing the nuclear charge felt by the outermost electron. This further decreases the ionization energy for indium compared to aluminum.

Electron Configuration

Electron Configuration: The electron configuration of aluminum is [Ne] 3s^2 3p^1, whereas indium is [Kr] 4s^2 4p^1. The presence of the 4s orbital in indium contributes to the increased distance between the outermost electron and the nucleus, leading to a lower ionization enthalpy.

Effective Nuclear Charge (Z_eff)

Effective Nuclear Charge: The effective nuclear charge experienced by the outermost electrons in indium is lower than that in aluminum due to the increased shielding from inner electrons. A lower effective nuclear charge means the outermost electron is held less tightly, leading to a lower ionization enthalpy.

Overall Explanation

In summary, the combination of increased atomic size, greater shielding, and lower effective nuclear charge in indium results in a lower ionization enthalpy compared to aluminum. Atomic size predominates, and this effect is not negated by the increasing effective nuclear charge in indium, which remains lower than that of aluminum.

The general trend of ionization enthalpy across the group is that it decreases down the group. However, for indium, the increase in atomic radii due to the additional electron shell outweighs the increase in effective nuclear charge due to the shielding effect. Therefore, the ionization enthalpy of indium remains lower than that of aluminum.

Indium's lower ionization enthalpy can thus be attributed to its larger atomic size and the enhanced shielding effect, which collectively result in a weaker attraction between the nucleus and the outer electrons.