The process of determining a molecule’s three-dimensional (3D) shape begins with a flat, two-dimensional (2D) diagram known as the Lewis structure. This structure uses dots and lines to illustrate the arrangement of valence electrons around the atoms in a molecule. Predicting the molecular geometry is important because a molecule’s shape directly influences its properties, such as polarity, reactivity, and biological function. The shape dictates how molecules interact, affecting physical characteristics like boiling point and solubility. This guide explains how to translate the electron distribution shown in a Lewis structure into a predictable spatial arrangement of atoms.
The Foundation: Drawing the Lewis Structure
The first step in predicting molecular shape is correctly constructing the Lewis structure, which maps out the shared and unshared valence electrons. This process begins by calculating the total number of valence electrons contributed by every atom. For polyatomic ions, add one electron for each negative charge and subtract one for each positive charge.
The central atom is typically the least electronegative element, except for hydrogen, which is always terminal. Connect surrounding atoms to the central atom using single lines, representing single covalent bonds (two shared electrons). These initial bonds use up some of the total valence electrons.
Remaining electrons are distributed around terminal atoms to satisfy the octet rule (eight electrons in the valence shell). Hydrogen is the exception, needing only two electrons. After terminal atoms have complete octets, any leftover electrons are placed as lone pairs on the central atom.
If the central atom still lacks an octet, non-bonding electrons from an outer atom must be moved to form a double or triple bond. This continues until all applicable atoms achieve a full octet.
Quantifying Electron Density: Identifying Domains
The completed Lewis structure is translated into a 3D model by quantifying the regions of electron density around the central atom, known as electron domains. The total number of these domains is the most important factor for predicting geometry.
A lone pair of non-bonding electrons constitutes one electron domain. A single, double, or triple bond connecting the central atom to a terminal atom is also counted as only one electron domain. This is because the electrons in multiple bonds occupy the same general space between the two nuclei.
For instance, in carbon dioxide (\(\text{CO}_2\)), the central carbon atom has two double bonds, resulting in two electron domains. In methane (\(\text{CH}_4\)), the central carbon atom forms four single bonds, equating to four electron domains.
Predicting Spatial Arrangement: VSEPR Theory
The theory used to predict the spatial arrangement of electron domains is Valence Shell Electron Pair Repulsion (VSEPR) theory. This model is founded on the principle that electron domains, being regions of negative charge, repel each other and arrange themselves to maximize the distance between them.
The total number of electron domains determines the Electron Geometry, which describes the arrangement of all electron groups (bonding and lone pairs). For example, three domains form a trigonal planar geometry, while four domains form a tetrahedral geometry.
The Molecular Geometry describes the arrangement of only the atoms in the molecule, ignoring lone pairs. Lone pairs exert a greater repulsive force than bonding pairs because they are held closer to the central atom and are not shared. This stronger repulsion compresses the bond angles away from the idealized electron geometry.
Common Geometries and Molecular Examples
Two Electron Domains
For molecules with two electron domains, such as carbon dioxide (\(\text{CO}_2\)), both domains are bonding pairs. This results in a linear electron geometry and a linear molecular geometry with a 180° bond angle.
Three Electron Domains
When three electron domains are present, the electron geometry is always trigonal planar, with ideal 120° angles. If all three domains are bonding pairs, as in boron trifluoride (\(\text{BF}_3\)), the molecular geometry is also trigonal planar. If one domain is a lone pair, such as in sulfur dioxide (\(\text{SO}_2\)), the molecular shape becomes bent, or V-shaped, because the lone pair takes up space but is not included in the description of the atom arrangement.
Four Electron Domains
Four electron domains establish a tetrahedral electron geometry, with an ideal bond angle of 109.5°. Methane (\(\text{CH}_4\)) has four bonding pairs, resulting in a tetrahedral molecular geometry.
Ammonia (\(\text{NH}_3\)) has three bonding pairs and one lone pair, which compresses the bond angles to approximately 107° and results in a trigonal pyramidal molecular geometry. Water (\(\text{H}_2\text{O}\)) has two bonding pairs and two lone pairs, resulting in a bent molecular geometry and a further compressed bond angle of about 104.5°.