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Ch.8 - Covalent Compounds: Bonding Theories and Molecular Structure
Chapter 8, Problem 106a

At high temperatures, sulfur vapor is predominantly in the form of S2(g) molecules. (a) Assuming that the molecular orbitals for third-row diatomic molecules are analogous to those for second-row molecules, construct an MO diagram for the valence orbitals of S2(g).

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Identify the valence electrons for sulfur (S). Sulfur is in group 16 of the periodic table and has 6 valence electrons. In the S2 molecule, there are two sulfur atoms, so you start with a total of 12 valence electrons.
Understand the types of molecular orbitals that can form in diatomic molecules. These include sigma (σ), pi (π), sigma star (σ*), and pi star (π*) orbitals. Sigma orbitals are symmetrical along the axis connecting the two nuclei, while pi orbitals are oriented perpendicular to this axis.
Arrange the atomic orbitals (AOs) of sulfur to form molecular orbitals (MOs). The valence orbitals of sulfur are 3s and 3p orbitals. These orbitals combine to form bonding (lower energy) and antibonding (higher energy) molecular orbitals.
Fill the molecular orbitals with the available electrons according to the Aufbau principle (fill lower energy orbitals first), Hund's rule (maximize the number of unpaired electrons in orbitals of equal energy), and the Pauli exclusion principle (each orbital can hold a maximum of two electrons with opposite spins).
Draw the MO diagram with the energy levels, showing the filling of the orbitals from the lowest to the highest energy, and label each orbital with the number of electrons it contains. This diagram will help visualize the electron configuration and bonding characteristics of the S2 molecule.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Molecular Orbital Theory

Molecular Orbital (MO) Theory describes the behavior of electrons in molecules by combining atomic orbitals to form molecular orbitals. These orbitals can be bonding, antibonding, or non-bonding, and they help predict the stability, magnetic properties, and electronic configurations of molecules. Understanding MO theory is essential for constructing MO diagrams, which visually represent the energy levels and occupancy of these orbitals.
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Valence Orbitals

Valence orbitals are the outermost orbitals of an atom that are involved in chemical bonding. In diatomic molecules like S2, the valence orbitals of each sulfur atom combine to form molecular orbitals. For sulfur, the relevant valence orbitals include the 3s and 3p orbitals, which will interact to form the molecular orbitals that dictate the electronic structure of the S2 molecule.
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Diatomic Molecules and Their Orbitals

Diatomic molecules consist of two atoms bonded together, and their molecular orbitals are formed from the combination of the atomic orbitals of the constituent atoms. For sulfur (S2), the molecular orbitals are constructed by considering the symmetry and energy levels of the 3s and 3p orbitals. The arrangement of these orbitals in an MO diagram helps in understanding the bond order, stability, and reactivity of the molecule.
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