Write orbital diagrams (boxes with arrows in them) to represent the electron configurations—without hybridization—for all the atoms in PH₃. Circle the electrons involved in bonding. Draw a three-dimensional sketch of the molecule and show orbital overlap. What bond angle do you expect from the unhybridized orbitals? How well does valence bond theory agree with the experimentally measured bond angle of 93.3°?

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1. Identify the electron configurations for phosphorus (P) and hydrogen (H). Phosphorus has an atomic number of 15, so its electron configuration is 1s² 2s² 2p⁶ 3s² 3p³. Hydrogen has an atomic number of 1, so its electron configuration is 1s¹.
2. Draw the orbital diagrams for phosphorus and hydrogen. For phosphorus, represent the 3s and 3p orbitals with boxes and fill them with arrows to indicate electrons: 3s (↑↓), 3p (↑) (↑) (↑). For hydrogen, draw a single box for the 1s orbital with one arrow: 1s (↑).
3. Circle the electrons involved in bonding. In PH₃, each hydrogen atom forms a bond with one of the unpaired electrons in the 3p orbitals of phosphorus. Circle one electron in each of the three 3p orbitals of phosphorus and the single electron in the 1s orbital of each hydrogen.
4. Draw a three-dimensional sketch of the PH₃ molecule. Show the phosphorus atom at the center with three hydrogen atoms bonded to it. Indicate the overlap between the 3p orbitals of phosphorus and the 1s orbitals of hydrogen, forming three P-H sigma bonds.
5. Discuss the expected bond angle from unhybridized orbitals. In PH₃, the unhybridized 3p orbitals of phosphorus are oriented at 90° to each other. However, due to the presence of a lone pair on phosphorus, the bond angle is slightly less than 90°. Compare this with the experimentally measured bond angle of 93.3° and discuss how valence bond theory accounts for this deviation.

Key Concepts

Orbital Diagrams

Orbital diagrams visually represent the distribution of electrons in an atom's orbitals. Each box represents an orbital, and arrows indicate the presence of electrons, with their direction denoting spin. These diagrams help in understanding electron configurations and the pairing of electrons, which is crucial for predicting bonding behavior in molecules.

Valence Bond Theory

Valence Bond Theory (VBT) explains how atoms bond by overlapping their atomic orbitals to form covalent bonds. It emphasizes the role of unhybridized orbitals in determining molecular geometry and bond angles. In the case of PH₃, VBT can be used to predict the bond angle based on the arrangement of the orbitals involved in bonding.

Bond Angles and Molecular Geometry

Bond angles are the angles between adjacent bonds in a molecule, which are influenced by the arrangement of electron pairs around a central atom. In PH₃, the expected bond angle from unhybridized orbitals is affected by the presence of lone pairs, leading to a deviation from the ideal tetrahedral angle. Understanding these angles is essential for comparing theoretical predictions with experimental measurements.