The three-dimensional shape of a molecule dictates its chemical and physical properties, from how it interacts with other molecules to its melting and boiling points. To predict this shape, chemists determine how the molecule’s electrons are organized around its central atom. This organization uses the concept of the electron group, which serves as the foundational step for predicting molecular structure.
Understanding Regions of Electron Density
An electron group is any single, localized region where electrons are concentrated around a central atom in a molecule. This region is treated as a unit, regardless of the number of electrons it contains or the type of bond it forms. Because electrons are negatively charged, they repel one another, forcing them to maximize the distance between their respective groups.
This repulsion leads the electron groups to adopt the arrangement that minimizes mutual strain, which defines the molecule’s spatial structure. Electron groups fall into two categories: bonding pairs and lone pairs. Bonding pairs are electrons shared between the central atom and a peripheral atom. Lone pairs are unshared, non-bonding electrons resting solely on the central atom. Both types occupy distinct regions of space and contribute equally to the electron group count.
Practical Rules for Counting Electron Groups
To determine the number of electron groups, one examines the molecule’s structure and applies counting rules to the central atom. Every lone pair of electrons on the central atom counts as a single electron group because it occupies one distinct area of space.
A single bond, a double bond, or a triple bond connecting the central atom to a peripheral atom all count as only one electron group. For example, in methane (\(\text{CH}_4\)), the carbon atom has four single bonds, equating to four electron groups. In carbon dioxide (\(\text{CO}_2\)), the central carbon atom has two double bonds, which are counted as only two electron groups. The total count is the sum of the lone pairs and the number of atoms bonded to the central atom, where multiple bonds to the same atom are considered a single connection.
How Electron Group Number Determines Spatial Arrangement
The total number of electron groups determines the spatial arrangement of those groups, known as the electron geometry. This geometry is based on the principle that electron groups will spread out as far apart as possible to minimize repulsion. A count of two electron groups results in a linear arrangement, where the groups are positioned \(180^\circ\) apart.
When the count increases to three electron groups, the arrangement is trigonal planar, separated by angles of \(120^\circ\). A four-group count results in a three-dimensional tetrahedral arrangement, placing the groups at the corners of a tetrahedron with bond angles near \(109.5^\circ\). Larger molecules can have five groups, leading to a trigonal bipyramidal arrangement, or six groups, resulting in an octahedral arrangement, all maximizing spatial distance.
Molecular Shape Versus Electron Group Arrangement
While the electron group number establishes the electron geometry, the actual visible shape of the molecule, known as the molecular geometry, differs when lone pairs are present. Electron geometry describes the spatial arrangement of all electron groups, including bonding and lone pairs. Molecular shape, however, describes only the arrangement of the atoms themselves.
Lone pairs occupy space and contribute to the repulsive forces that set the electron geometry, but they are not included when naming the final molecular shape. For instance, the water molecule (\(\text{H}_2\text{O}\)) has four electron groups (two bonding pairs and two lone pairs), giving it a tetrahedral electron geometry. Since the lone pairs are not atoms, the molecular shape is described by the positions of the hydrogen and oxygen atoms, resulting in a bent shape. Lone pairs exert a greater repulsive force than bonding pairs, slightly distorting the final bond angles.