Ammonia (\(\text{NH}_3\)) is a common compound that is far from flat or two-dimensional. The way its atoms are arranged in space dictates nearly all of its chemical and physical properties, from its ability to dissolve in water to its characteristic pungent odor. Understanding the three-dimensional shape of molecules is fundamental to predicting their behavior. The specific geometry of ammonia allows it to function as a household cleaner, a fertilizer, and an important component in the atmosphere. This structural understanding reveals how electrons are shared and distributed around the central nitrogen atom.
Counting the Electron Domains
The theoretical basis for determining the shape of any molecule is the Valence Shell Electron Pair Repulsion (VSEPR) theory. This principle states that groups of valence electrons around a central atom will arrange themselves in space to minimize repulsion. Electron groups, or domains, can be a bond to another atom (a bonding domain) or a pair of electrons not involved in a bond (a lone pair domain).
To apply this to \(\text{NH}_3\), the central atom is nitrogen. Nitrogen brings five valence electrons, and the three hydrogen atoms each contribute one, resulting in a total of eight valence electrons. Nitrogen forms three single bonds with the hydrogen atoms, using six electrons as bonding pairs.
The remaining two valence electrons form one non-bonding lone pair on the central nitrogen atom. Therefore, the nitrogen atom is surrounded by three bonding domains and one lone pair domain, totaling four electron domains. According to VSEPR theory, these four domains orient themselves as far apart as possible in a tetrahedral electron geometry.
The electron geometry describes the spatial arrangement of all electron domains. The four electron domains point toward the corners of a tetrahedron, which serves as the foundation for determining the molecule’s actual atomic structure.
The Three-Dimensional Shape: Trigonal Pyramidal
The actual three-dimensional shape of the \(\text{NH}_3\) molecule, which considers only the positions of the atoms, is called trigonal pyramidal. This structure arises directly from the tetrahedral electron geometry because the molecular shape is defined by the location of the nuclei, not the lone pair. The nitrogen atom sits at the apex of the pyramid, while the three hydrogen atoms form the triangular base.
The presence of the lone pair causes the molecule to adopt this pyramidal shape instead of remaining tetrahedral. Lone pairs occupy a slightly larger volume of space compared to bonding pairs. Their greater repulsive force pushes the three hydrogen atoms closer together, distorting the bond angles away from the ideal tetrahedral angle of \(109.5^\circ\).
As a result of the lone pair’s influence, the H-N-H bond angle in ammonia is reduced to approximately \(107^\circ\). The molecule can be visualized like a tripod, with the nitrogen at the center and the three hydrogen atoms as the legs.
Why Geometry Matters: Molecular Polarity
The asymmetrical, trigonal pyramidal shape of ammonia has a direct consequence on its chemical behavior, making it a highly polar molecule. Molecular polarity is determined by the overall distribution of electrical charge, summarized by a net dipole moment. In the \(\text{NH}_3\) molecule, the nitrogen atom is significantly more electronegative than the hydrogen atoms.
This difference in electronegativity means that the electrons in each N-H bond are pulled closer to the nitrogen atom, creating individual bond dipoles. Because the molecule is not symmetrical, the three individual bond dipoles, along with the dipole created by the lone pair, do not cancel each other out. Instead, they add up vectorially to create a strong net dipole moment, which has an experimentally determined value of about 1.4 D.
This resulting charge separation means the nitrogen end of the molecule carries a partial negative charge, and the base formed by the hydrogen atoms carries a partial positive charge. The polarity of ammonia explains its high solubility in water and its ability to act as a proton acceptor, or a base.