Question

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

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

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: \[1s^2 \ 2s^2 \ 2p^6 \ 3s^2 \ 3p^6 \ 4s^2 \ 3d^{10} \ 4p^4\]

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

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.

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).

Key concepts

Electron Configuration

Electron configuration refers to the arrangement of electrons around the nucleus of an atom. It describes how electrons inhabit energy levels and orbitals, detailing how they fill up from lower to higher energy levels based on the principles of quantum mechanics.

**Key principles guiding electron configuration**

• Aufbau Principle: This principle states that electrons fill lower-energy orbitals before moving to higher-energy ones. This principle helps predict the ground state of an atom’s electron configuration.
• Hund's Rule: Electrons will fill an unoccupied orbital before they pair up in an already occupied orbital. This minimizes electron repulsion and maintains atom stability.
• Pauli Exclusion Principle: No two electrons in an atom can have the same set of quantum numbers; hence, an orbital can hold a maximum of two electrons with opposite spins.

In the specific context of the problem, arsenic (As) and selenium (Se) have similar electron configurations because they are in the same period, both reaching the 4p subshell. Arsenic has an electron configuration of \[1s^2 \ 2s^2 \ 2p^6 \ 3s^2 \ 3p^6 \ 4s^2 \ 3d^{10} \ 4p^3\] and selenium's is \[1s^2 \ 2s^2 \ 2p^6 \ 3s^2 \ 3p^6 \ 4s^2 \ 3d^{10} \ 4p^4\].

The difference in the number of electrons in the 4p subshell affects the atom's overall electronic environment and properties, like ionization energy.

Atomic Structure

Atomic structure refers to the arrangement and composition of an atom's core components: protons, neutrons, and electrons. The nucleus houses protons and neutrons, while electrons orbit in various energy levels or shells around the nucleus.

**Components of an atom and their roles**

• Protons: These positively charged particles, located in the nucleus, determine the atomic number, thus defining the element's identity.
• Neutrons: Neutral particles that contribute to the atom's mass and influence isotopic characteristics.
• Electrons: Negatively charged particles responsible for chemical behavior and interactions. They reside in defined orbitals around the nucleus.

The overall stability and reactivity of an atom depend on its atomic structure. Atoms strive for a balanced, low-energy state by achieving a full outer shell of electrons, a phenomenon closely linked to the ionization energy. The energy levels closest to the nucleus are lower in energy, which makes removing electrons from them more energy-intensive. Hence, understanding the atomic structure helps explain why atoms like As and Se have different ionization energies due to their varied electron configurations.

Effective Nuclear Charge

Effective nuclear charge (Z_eff) is a concept that accounts for the net positive charge experienced by an electron in a multi-electron atom. It is an important determinant of an element's properties, including ionization energy.

**Why effective nuclear charge matters**

• The repulsion between electrons in inner shells (known as shielding) reduces the full nuclear charge experienced by the outer electrons.
• As a result, Z_eff becomes the partial charge felt by the outermost electrons, affecting how tightly they are held by the atom.
• A higher Z_eff typically indicates greater control over the valence electrons, leading to higher ionization energy.

In arsenic (As), the effective nuclear charge is slightly less due to having 4p^3 with a half-filled orbital, which causes added electron-electron repulsion, reducing its Z_eff compared to selenium (Se) with 4p^4, hence experiencing more nuclear pull. Consequently, this subtle difference explains why As has a slightly lower ionization energy than Se.

Periodicity

Periodicity refers to the recurring trends that are observed in the properties of elements across different periods and groups in the periodic table. These trends arise due to similar outer electron configurations and influence properties such as atomic radius, electronegativity, and ionization energy.

**Important periodic trends related to ionization energy**

• Ionization Energy: Tends to increase across a period from left to right due to stronger effective nuclear charge, making electrons more difficult to remove.
• Atomic Radius: Generally decreases across a period, as more protons increase Z_eff, pulling electrons closer.
• Electron Affinity and Electronegativity: Generally increase across a period as the Z_eff holds valence electrons more tightly, favoring electron gain.

The ionization energies of arsenic (As) and selenium (Se) reflect these trends. As has a marginally lower ionization energy compared to Se, due to its electron configuration leading to higher repulsion between 4p electrons, illustrating subtle variations dictated by element positioning within a period.