Final answer:
The VSEPR model is used to predict molecular geometries by considering the arrangement of electron pairs around the central atom. Examples like PF5 and H3O+ demonstrate different molecular geometries based on the number and arrangement of bonding and nonbonding electron pairs.
Step-by-step explanation:
The VSEPR (Valence Shell Electron Pair Repulsion) model is commonly used to predict molecular geometries. In this model, the electron pairs surrounding the central atom in a molecule or ion repel each other and arrange themselves in a way to minimize this repulsion. By examining the number of bonding and nonbonding electron pairs, we can determine the molecular geometry.
For example, let's consider PF5 (phosphorus pentafluoride). Phosphorus has 5 valence electrons, and each fluorine contributes 1 valence electron. The Lewis structure of PF5 shows that there are 5 bonding electron pairs and no lone pairs around the central phosphorus atom. This gives a trigonal bipyramidal electron pair geometry and a corresponding molecular geometry of trigonal bipyramidal.
Another example is H3O+ (hydronium ion). The Lewis structure shows that there are 3 bonding electron pairs and 1 lone pair around the central oxygen atom. This gives a tetrahedral electron pair geometry and a corresponding molecular geometry of trigonal pyramidal.