Problem 115 - Chemistry An Atoms First Approach

Question

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

Step-by-Step Solution

Answer

The first ionization energies of As and Se can be rationalized by examining their electron configurations. As (atomic number 33) has an electron configuration of \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^3\), while Se (atomic number 34) has an electron configuration of \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^4\). In both cases, the outermost electron is in the 4p orbital; however, Se has more electron repulsion in the 4p orbital due to having four electrons there. This increased repulsion makes it slightly easier to remove an electron from Se, resulting in a lower first ionization energy (0.941 MJ/mol) compared to As (0.947 MJ/mol).

Step 1: Determine the electron configurations

To begin, we must determine the electron configurations of As and Se. Use the periodic table to find their atomic numbers and write the electron configurations accordingly. As (Atomic number: 33): \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^3\) Se (Atomic number: 34): \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^4\)

Step 2: Relate electron configurations to first ionization energies

First ionization energies are related to the difficulty of removing one electron from the atom. Electrons closer to the nucleus or with less shielding are more difficult to remove, causing higher ionization energies. We should analyze the shielding and effective nuclear charge for the outermost electrons of As and Se. - As: The outermost electron is in the 4p orbital, and it has 3 electrons there. It experiences shielding from the 1s, 2s, 2p, 3s, 3p, 4s, and 3d orbitals. - Se: The outermost electron is also in the 4p orbital, and it has 4 electrons there. It experiences shielding from the same orbitals as As.

Step 3: Compare the stability of the outermost electrons

In As, the outermost electron occupies one of the three degenerate 4p orbitals, so it experiences less repulsion from other electrons than in Se. In Se, the outermost electron is in a 4p orbital with an additional electron, which means it experiences more repulsion from other electrons. This electron repulsion increases the shielding effect and makes it slightly easier to remove an electron from Se compared to As. As a result, we should expect Se to have a slightly lower first ionization energy than As due to its electron configuration and the increased electron repulsion it experiences.

Step 4: Compare the given ionization energies

According to the given ionization energies, As has a first ionization energy of 0.947 MJ/mol, while Se has a first ionization energy of 0.941 MJ/mol. This confirms our analysis, as Se has a lower first ionization energy than As, which can be rationalized by considering their electron configurations and the repulsion their outermost electrons experience.

Key Concepts

Electron Configuration

Electron configuration is the distribution of electrons of an atom in the various atomic orbitals. It tells us how electrons fill the available energy levels and sublevels in an atom. To find this configuration, we use the periodic table, arranging electrons in order of increasing energy levels.

For instance:

• Arsenic (As) with an atomic number of 33 has the configuration: \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^3\).
• Selenium (Se) with an atomic number of 34 has the configuration: \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^4\).

These configurations help us understand the position of electrons around the nucleus, which in turn impacts their chemical properties and ionization energies.
Notice that As and Se both have their outermost electrons in the 4p sublevel, influencing their ionization energy levels.

Periodic Table

The periodic table is a systematic arrangement of elements, organized by increasing atomic number. It provides essential information about elements, aiding in understanding their properties and behaviors, including ionization energy.

Located in the 4th period:

• Arsenic (As) is situated among the metalloids, and its group helps explain certain characteristics like semimetallic properties.
• Selenium (Se) is found among the chalcogens, a group known for forming compounds such as acids and various oxides.

Elements in the same period show gradual changes in properties. For As and Se, being in the same period means they have similar electron configurations and underlying structure, impacting how their ionization energies compare. Their position provides insight into their electron structure and the energy required to remove those outer electrons.

Shielding Effect

The shielding effect, or electron shielding, occurs when inner electrons block the outer electrons from the full attractive force of the nucleus. It affects how strongly an atom holds its outer electrons. With more inner electron layers, the outer electrons are less tightly bound, affecting properties such as ionization energy.

For As and Se:

• Both have electrons in orbitals such as 1s to 3d that contribute to shielding.
• Their outermost electrons in the 4p sublevel experience different levels of effective nuclear charge because of electron repulsion and shielding.

While the shielding is similar for both, the presence of an extra electron in Se compared to As increases electron-electron repulsion in similar orbitals, slightly reducing its ionization energy.

Effective Nuclear Charge

Effective nuclear charge is the net positive charge experienced by an electron in a multi-electron atom. It considers both the total positive charge of the nucleus and the amount of negative charge from shielding electrons.

For As and Se:

• Both have a full set of inner electrons that partially shield the outermost 4p electrons.
• Due to this partial shielding, the outer electrons in As and Se feel less than the full charge of the nucleus.

The effective nuclear charge in Se is slightly offset by increased electron-electron repulsion due to an additional electron. This makes removing an outer electron a bit easier in Se than in As, correlating with lower ionization energy. Understanding these intricate balances helps explain the slight difference in their ionization energies.