Final answer:
Molecular shapes are based on the number of electron density regions around a central atom with geometries such as linear, trigonal planar, tetrahedral, and trigonal bipyramidal. VSEPR theory is used to predict these shapes, where electron groups are arranged to minimize repulsion, but molecular geometry can be altered by the presence of lone pairs.
Step-by-step explanation:
The shape that describes the arrangement of the atoms in a molecule can be one of several geometries, depending on the number of electron density regions around the central atom. These geometries include linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral. For example, a molecule with three electron groups around the central atom will adopt a trigonal planar shape. This orientation allows the electron groups to be as far apart as possible, which minimizes repulsions between them, according to VSEPR (Valence Shell Electron Pair Repulsion) theory. Compounds such as BF3 are examples of this arrangement. On the other hand, a molecule with four electron groups is typically tetrahedral. In cases where there are multiple bonds or lone pairs, these influence the molecular geometry but are not counted separately in determining the electron group geometry. For instance, a double or triple bond is considered as a single electron group. When lone pairs are present, they occupy more space than bonded electron pairs, leading to distortions in the ideal angles between atoms. For example, the molecular geometry can be described as bent, or V-shaped when there are lone pairs, even if the electron group geometry is trigonal planar. This is due to lone pairs taking up more space and causing the angles between bonded atoms to be less than the ideal. An example where the molecular geometry is bent would be a molecule with two atoms and one lone pair, like H2O.