Final answer:
The molecular geometry and bond angles of a molecule are determined using VSEPR theory and information about the structure of the molecule. Bismuth triiodide has a trigonal planar molecular geometry, while Phosphine has a trigonal pyramidal shape due to the presence of a lone pair.
Step-by-step explanation:
To determine the electron geometry, molecular geometry, and bond angles of a molecule, one should first represent the molecule with its chemical and structural formulas. With these representations, you can use the VSEPR (Valence Shell Electron Pair Repulsion) theory to determine the number of regions of electron density. Single bonds, multiple bonds, radicals, and lone pairs each count as one region of electron density. The electron group geometry is based on the total number of electron groups, while the molecular geometry is based on the number of bonding electron groups specifically.
By this method, for Bi3 (Bismuth triiodide), the molecular geometry is trigonal planar with bond angles of approximately 120 degrees. For PH3 (Phosphine), the electron group geometry is tetrahedral with one lone pair, resulting in a molecular geometry that is trigonal pyramidal with bond angles of less than 109.5 degrees.
Depending on the complexity of the molecule and the regions of electron density, other geometries may include linear, bent, octahedral, and others, each with their respective idealized bond angles as summarized in tables provided in several chemistry texts.