Ammonia (\(\text{NH}_3\)) is a chemical compound of immense practical importance. To understand how this molecule functions and interacts with other substances, it is necessary to examine its precise physical structure. The arrangement of atoms in three-dimensional space dictates a molecule’s properties, including its reactivity and polarity.
Locating the Valence Electrons
Determining molecular shape begins by mapping the arrangement of electrons that participate in bonding, known as valence electrons. The ammonia molecule contains one nitrogen atom and three hydrogen atoms. The total number of valence electrons is calculated by summing the contributions from each atom. Nitrogen (Group 15) contributes five valence electrons, and the three hydrogen atoms contribute one each, resulting in a total of eight valence electrons for \(\text{NH}_3\).
The central nitrogen atom forms a single covalent bond with each of the three hydrogen atoms. These three bonds use six of the eight total valence electrons. The remaining pair of two electrons forms a non-bonding or lone pair on the central nitrogen atom. The nitrogen atom is thus surrounded by four distinct regions of electron density, known as electron domains. These four domains consist of three bonding pairs and one lone pair.
How Electron Repulsion Determines Geometry
The three-dimensional arrangement of these four electron domains is predicted by the Valence Shell Electron Pair Repulsion (VSEPR) theory. This principle states that all electron domains, whether bonding pairs or lone pairs, are negatively charged and repel one another to achieve maximum separation in space. Since there are four electron domains centered on the nitrogen atom, the optimal arrangement to minimize repulsion is a tetrahedral geometry.
This tetrahedral arrangement is the electron geometry, placing the four domains at the corners of a tetrahedron. However, the molecule’s molecular geometry is defined only by the positions of the atoms. The non-bonding lone pair occupies more space than a bonding pair because it is held exclusively by the central nitrogen atom, unlike a bonding pair which is shared between two nuclei. This greater spatial requirement leads to a stronger repulsive force from the lone pair against the three bonding pairs. This increased repulsion pushes the three hydrogen atoms downward, distorting the simple tetrahedral shape.
Defining the Trigonal Pyramidal Shape
The powerful lone pair repulsion results in the final molecular geometry being Trigonal Pyramidal. The three hydrogen atoms form the base of a triangle, and the central nitrogen atom sits at the apex, giving the molecule a pyramid-like structure.
The lone pair’s stronger push causes the H-N-H bond angle to shrink from the ideal tetrahedral angle of \(109.5^\circ\) to approximately \(107.3^\circ\). This deviation from perfect symmetry has a significant consequence for the molecule’s chemical behavior. The lone pair and the three polar \(\text{N-H}\) bonds contribute to an uneven distribution of electric charge.
The nitrogen atom has a higher attraction for electrons, pulling electrons toward it and creating a concentration of negative charge at the nitrogen end. This asymmetry, combined with the non-symmetrical trigonal pyramidal shape, results in a significant net dipole moment. This overall charge separation means that ammonia is a highly polar molecule, which accounts for its ability to dissolve in water and function as a weak base.