The ionization enthalpy refers to the amount of energy required to remove an electron from an atom or ion in the gaseous state. In the case of thallium (Tl) and indium (In), the ionization enthalpy of thallium is greater than that of indium. This can be attributed to the following factors:
Effective nuclear charge: The ionization enthalpy is influenced by the effective nuclear charge experienced by the outermost electrons of an atom. Thallium has a higher atomic number than indium, indicating a greater number of protons in its nucleus. The increased positive charge in the nucleus of thallium exerts a stronger pull on its valence electrons, making it more difficult to remove an electron, resulting in a higher ionization enthalpy.
Shielding effect: The shielding effect occurs when inner electrons partially shield the outermost electrons from the full attraction of the nucleus. In thallium, due to its larger atomic size and the presence of filled inner electron shells, the shielding effect is more pronounced compared to indium. As a result, the outermost electron in thallium experiences a weaker effective nuclear charge, requiring more energy to remove it, leading to a higher ionization enthalpy.
Electron configuration: Thallium and indium have different electron configurations. Thallium's electron configuration is [Xe] 4f^14 5d^10 6s^2 6p^1, while indium's electron configuration is [Kr] 4d^10 5s^2 5p^1. Thallium has a filled 4f and 5d subshell before the 6s and 6p orbitals, providing additional stability to its electron configuration. This increased stability makes it energetically unfavorable to remove an electron from thallium, resulting in a higher ionization enthalpy.
Overall, the combination of a higher effective nuclear charge, weaker shielding effect, and electron configuration stability in thallium compared to indium contributes to the greater ionization enthalpy of thallium.