Electron domains are key to understanding the three-dimensional structures of molecules. They represent regions around a central atom where electrons are concentrated. These regions can be electrons involved in chemical bonds or unshared pairs, known as lone pairs. This concept provides insight into how atoms arrange themselves in space, influencing a molecule’s properties and interactions.
What Counts as an Electron Domain?
Counting electron domains around a central atom is simple. An electron domain is any bond (single, double, or triple) or any lone pair of electrons. A double or triple bond, despite having more shared electrons, occupies only one spatial region around the central atom, just like a single bond or a lone pair.
In a water molecule (H₂O), the central oxygen atom bonds to two hydrogen atoms and has two lone pairs. Each oxygen-hydrogen bond counts as one domain, and each lone pair counts as one domain. Thus, the oxygen atom in water has a total of four electron domains.
Similarly, in carbon dioxide (CO₂), the central carbon atom forms double bonds with two oxygen atoms. Each carbon-oxygen double bond counts as a single electron domain. This gives the carbon atom in carbon dioxide a total of two electron domains.
How Electron Domains Determine Shape
The arrangement of electron domains around a central atom determines a molecule’s three-dimensional shape. This is because electron domains, being negatively charged, repel each other. They orient themselves to be as far apart as possible in space, minimizing these repulsive forces.
This concept is the basis of Valence Shell Electron Pair Repulsion (VSEPR) theory. This theory states that the most stable arrangement for a given number of electron domains minimizes repulsion between them. The specific geometric arrangement adopted by these electron domains around the central atom is known as the electron domain geometry.
The presence of lone pairs also influences molecular shape. Lone pairs exert a greater repulsive force than bonding pairs because they are held by only one nucleus and are more spread out. This stronger repulsion can compress the angles between bonding domains, leading to deviations from idealized geometries. The overall molecular shape, or molecular geometry, describes the positions of the atoms themselves, which can differ from the electron domain geometry when lone pairs are present.
Predicting Molecular Shapes: Examples
Applying the principles of electron domains and repulsion allows for the prediction of molecular shapes. The total number of electron domains around a central atom determines the electron domain geometry, and the number of bonding pairs versus lone pairs defines the molecular geometry.
A molecule with two electron domains, such as carbon dioxide (CO₂), has a linear electron domain geometry. Since both domains are bonding pairs, CO₂ also exhibits a linear molecular geometry, with oxygen atoms 180 degrees apart around the central carbon atom.
When there are three electron domains, a trigonal planar electron domain geometry results, with angles of approximately 120 degrees. If all three domains are bonding, as in boron trifluoride (BF₃), the molecular geometry is also trigonal planar. The boron atom sits at the center, with three fluorine atoms arranged in a flat triangle around it.
Molecules with four electron domains adopt a tetrahedral electron domain geometry, with angles close to 109.5 degrees. Methane (CH₄) is an example where all four domains are bonding pairs, resulting in a tetrahedral molecular geometry. The carbon atom is at the center, and the four hydrogen atoms are at the corners of a tetrahedron.
If a molecule with four electron domains has one lone pair and three bonding pairs, like ammonia (NH₃), the electron domain geometry remains tetrahedral. The molecular geometry becomes trigonal pyramidal. The lone pair occupies space but is not seen as part of the molecular shape, causing the three hydrogen atoms to form a pyramid with nitrogen at its apex.
For water (H₂O), there are four electron domains around the central oxygen atom: two bonding pairs and two lone pairs. This arrangement leads to a tetrahedral electron domain geometry. The two lone pairs, exerting stronger repulsive forces, cause the two O-H bonds to bend, resulting in a bent or angular molecular geometry. The bond angle in water is approximately 104.5 degrees, slightly less than the ideal tetrahedral angle.