Consider the molecular orbitals of the P2 molecule. Assume that the MOs of diatomics from the third row of the periodic table are analogous to those from the second row. (a) Which valence atomic orbitals of P are used to construct the MOs of P2?

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Identify the valence atomic orbitals of phosphorus (P). Phosphorus, being in the third period of the periodic table, has valence electrons in the 3s and 3p orbitals.

Understand the concept of molecular orbitals (MOs). MOs are formed by the linear combination of atomic orbitals (LCAO) from each atom in the molecule.

Recognize that the molecular orbitals in diatomic molecules like P2 are formed by the combination of similar orbitals from each phosphorus atom. This includes combinations of s orbitals with s orbitals, p orbitals with p orbitals, and possibly mixing between s and p orbitals.

Apply the analogy between second row and third row diatomic molecules. Since the molecular orbitals of O2 (a second row diatomic molecule) involve 2s and 2p orbitals, by analogy, the MOs of P2 will involve the 3s and 3p orbitals of each phosphorus atom.

Conclude that the valence atomic orbitals of phosphorus used to construct the MOs of P2 are the 3s and 3p orbitals.

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Key Concepts

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

Molecular Orbitals (MOs)

Molecular orbitals are formed by the linear combination of atomic orbitals (LCAO) when atoms bond together. In diatomic molecules, these MOs can be classified as bonding or antibonding, depending on whether they stabilize or destabilize the molecule. Understanding how atomic orbitals combine to form MOs is crucial for predicting the electronic structure and properties of molecules.

Valence Atomic Orbitals

Valence atomic orbitals are the outermost orbitals of an atom that participate in chemical bonding. For phosphorus (P), the relevant valence orbitals include the 3s and 3p orbitals. Identifying which of these orbitals contribute to the formation of MOs in a molecule like P2 is essential for understanding its bonding characteristics and molecular geometry.

Periodic Trends in Orbital Energy

The energy levels and types of atomic orbitals can vary across periods in the periodic table. For elements in the third row, such as phosphorus, the energy and arrangement of orbitals are similar to those in the second row but with some differences due to increased nuclear charge and electron shielding. Recognizing these trends helps in predicting how atomic orbitals from different elements will interact when forming MOs.

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The iodine bromide molecule, IBr, is an interhalogen compound. Assume that the molecular orbitals of IBr are analogous to the homonuclear diatomic molecule F2. (c) One of the valence MOs of IBr is sketched here. Why are the atomic orbital contributions to this MO different in size?