Ammonia, or \(\text{NH}_3\), is a common molecule used in chemistry to illustrate how atoms bond and arrange themselves in three-dimensional space. Determining the number of electron domains around the central nitrogen atom is the first step in applying the Valence Shell Electron Pair Repulsion (VSEPR) theory to predict its geometry. VSEPR theory allows scientists to predict the three-dimensional arrangement of a molecule by analyzing the repulsive forces between electron groups.
Defining Electron Domains
An electron domain represents a distinct region of space around a central atom where electrons are concentrated. This concept is fundamental to the VSEPR model, which posits that electron groups arrange themselves as far apart as possible to minimize electrostatic repulsion. Domains can consist of electrons involved in bonding or electrons that exist as lone pairs on the central atom.
The rules for counting these regions are straightforward. Whether a bond is single, double, or triple, it counts as only one electron domain. Any non-bonding pair of electrons on the central atom is also counted as a single, separate domain. The total number of these domains dictates the molecule’s fundamental electron geometry.
Mapping Valence Electrons in \(\text{NH}_3\)
To determine the electron domains in \(\text{NH}_3\), one must first construct the Lewis structure. Nitrogen contributes five valence electrons, and the three hydrogen atoms contribute one each, resulting in a total of eight valence electrons.
Nitrogen is the central atom, with the three hydrogen atoms positioned around it. Drawing single covalent bonds between nitrogen and the three hydrogen atoms accounts for six of the eight total valence electrons.
The remaining two electrons are placed onto the nitrogen atom as a non-bonding lone pair. This placement satisfies the octet rule for nitrogen and the duet rule for hydrogen.
By counting the regions of electron density, the total number of electron domains is revealed. The molecule has three distinct bonding domains (the three N-H bonds) plus one non-bonding domain (the lone pair). Summing these groups shows that the central nitrogen atom in \(\text{NH}_3\) is surrounded by a total of four electron domains.
Electron Geometry vs. Molecular Shape
The presence of four electron domains sets the molecule’s electron geometry, which describes the spatial arrangement of all electron groups. A molecule with four electron domains always adopts a tetrahedral electron geometry, where the four regions point toward the corners of a tetrahedron for maximum separation. In a perfectly symmetrical tetrahedral structure, the bond angles would be \(109.5^{\circ}\).
The molecular shape, however, considers only the positions of the atoms, disregarding the lone pair. The non-bonding lone pair on the nitrogen atom exerts a stronger repulsive force than the bonding pairs.
This enhanced lone pair repulsion pushes the three N-H bonding pairs closer together, distorting the structure from a perfect tetrahedron. As a result, the actual \(\text{H-N-H}\) bond angle is compressed to approximately \(107^{\circ}\). Therefore, while the electron geometry is tetrahedral, the observable molecular shape of \(\text{NH}_3\) is trigonal pyramidal.