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Chapter 7: Problem 119

The first ionization energies of As and Se are 0.947 and 0.941 MJ/mol, respectively. Rationalize these values in terms of electron configurations.

Short Answer

Expert verified
The first ionization energies of As and Se are 0.947 MJ/mol and 0.941 MJ/mol, respectively. This can be rationalized based on their electron configurations: As: \[1s^2 \ 2s^2 \ 2p^6 \ 3s^2 \ 3p^6 \ 4s^2 \ 3d^{10} \ 4p^3\] Se: \[1s^2 \ 2s^2 \ 2p^6 \ 3s^2 \ 3p^6 \ 4s^2 \ 3d^{10} \ 4p^4\] The outermost electrons are in the 4p subshell for both elements. As has a half-filled 4p subshell (3 electrons) leading to more electron-electron repulsion and slightly less effective nuclear charge than Se, which has a more than half-filled 4p subshell (4 electrons). Thus, it is easier to remove an electron from As than Se, hence its slightly lower ionization energy.

Step by step solution

01

Determine the electron configurations of As and Se

To determine the electron configurations of As and Se, we will first find their atomic numbers from the periodic table. As has the atomic number 33, while Se has the atomic number 34. Using this, we can find their electron configurations, keeping in mind the Aufbau principle and Hund's rule. As electron configuration: \[1s^2 \ 2s^2 \ 2p^6 \ 3s^2 \ 3p^6 \ 4s^2 \ 3d^{10} \ 4p^3\] Se electron configuration: \[1s^2 \ 2s^2 \ 2p^6 \ 3s^2 \ 3p^6 \ 4s^2 \ 3d^{10} \ 4p^4\]
02

Understand the energy levels of these electron configurations

The ionization energy mainly depends on the energy level of the outermost electron of an atom. It's easier to remove an electron from a higher energy level and more difficult to remove one from a lower energy level. In this case, the outermost electrons are located in the 4p subshell for both As and Se. As: 4p has 3 electrons Se: 4p has 4 electrons
03

Analyze the shielding and effective nuclear charge

Shielding refers to the weakening of an electric field due to the presence of other electrons between the outermost electrons and the nucleus. The effective nuclear charge is the net positive charge experienced by an electron in an atom. It is directly proportional to the ionization energy. In As, the 4p^3 electrons are half-filled, leading to slightly more electron-electron repulsion and slightly less effective nuclear charge. In Se, the 4p^4 electrons are more than half-filled, leading to less electron-electron repulsion and slightly higher effective nuclear charge.
04

Rationalize the ionization energies values

The first ionization energies of As and Se are 0.947 MJ/mol and 0.941 MJ/mol, respectively. As has a half-filled 4p subshell, which offers slightly more shielding and less effective nuclear charge compared to Se's 4p subshell with 4 electrons. Due to this, As has a higher electron-electron repulsion and slightly less effective nuclear charge, making it easier to remove an electron from As as compared to Se. That's why As has a slightly lower ionization energy (0.947 MJ/mol) than Se (0.941 MJ/mol).

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