2.6: Molecular Shapes
The shape of a molecule contributes to its function and its interactions with other molecules. Over the years, many different models have been developed for the visual representation of molecules.
Two-Dimensional Representations of Molecules
Lewis structures were developed by Gilbert Newton Lewis, who first published these structures in his paper “The Atom and the Molecule” in 1916. Lewis structures use the chemical symbols for elements. Lines linking elements represent covalent bonds, and pairs of dots represent pairs of electrons that do not participate in a bond.
The bond line structure is a simpler way to visualize organic (carbon-based) molecules than the Lewis structure. In bond line structures, carbon and hydrogen atoms are understood to exist anywhere that a line terminates or bends at an angle, rather than being explicitly drawn. Bond line structures are especially useful for modeling larger molecules that contain large amounts of carbon and hydrogen or very long hydrocarbon chains, such as those commonly encountered in organic chemistry and biochemistry.
Three-Dimensional Representations of Molecules
2D models are useful for understanding basic molecular structures. However, to predict how molecules will interact with one another and with other substances, it is important to understand how molecules exist in three-dimensional space. Ball and stick models show 3D relationships between atoms within a molecule.
Space-filling models take the concept of ball and stick models further, providing a more accurate 3D view of molecules by depicting atoms in a way that maintains the ratio of atomic radii. Rather than using lines between atoms to represent bonds, the proximity of spheres to one another indicates bond strength. Atoms with stronger bonds, such as double bonds, are represented by spheres that overlap more than spheres representing more weakly-bonded atoms. Labeling atoms with their chemical symbols is generally not necessary, because space-filling and ball and stick models both use standardized colors to represent atoms of different elements. Red represents oxygen, black is carbon, and white is hydrogen. Other elements that commonly form covalent compounds, including nitrogen, sulfur, phosphorus, chlorine, fluorine, and bromine, are indicated by specific colors as well.