Final answer:
The shape of a molecule is based on the number of electron groups around the central atom and is predicted using VSEPR theory, where electron groups include bonding pairs and lone pairs. These groups are arranged to minimize repulsion, resulting in specific molecular shapes.
Step-by-step explanation:
The shape of a molecule can be determined by examining the number of electron groups around a central atom. Molecules form specific geometries to minimize electron repulsion, which can be predicted using the VSEPR (Valence Shell Electron Pair Repulsion) theory. Generally, electron groups include bonding pairs of electrons, as found in single, double, or triple bonds, and non-bonding pairs of electrons, also known as lone pairs. The number of bonding groups, together with the number of lone pairs, dictate the molecular structure that will form.
For example, if there are two electron groups around the central atom and no lone pairs, the molecule will take on a linear geometry. With three electron groups and no lone pairs, the molecule will have a trigonal planar shape. If additional lone pairs are present, they too will affect the geometry, as seen in water (H2O), where two hydrogen atoms (bonding groups) and two lone pairs on the oxygen result in a bent molecular shape.
This concept is crucial in understanding how chemical reactions occur and how molecular shapes influence the physical and chemical properties of substances. Tables and figures in chemistry textbooks, such as Table 4.8.1, Table 9.7.1, Table 4.11.1 and Figures 6.8.6, 6.7.6, 6.8.2, and 6.7.2, summarize these molecular shapes based on the number of electron groups and lone pairs.