Molecular geometry describes the three-dimensional arrangement of atoms within a molecule, which dictates how the molecule interacts with its environment and other substances. This spatial configuration influences properties like its reactivity, polarity, melting point, and biological activity. Chemists rely on the Valence Shell Electron Pair Repulsion (VSEPR) theory to predict these shapes.
VSEPR theory states that the electron pairs in the valence shell of a central atom repel each other, behaving as negatively charged clouds. To minimize this electrostatic repulsion and achieve the lowest energy state, these electron domains arrange themselves in space as far apart as possible. Predicting the positions of these electron domains allows chemists to determine the resulting three-dimensional structure of the molecule.
Visualizing Electron Arrangement
Predicting the three-dimensional shape of a molecule begins with mapping the two-dimensional arrangement of its electrons using a Lewis structure. This structure accounts for all valence electrons, showing those involved in bonding and those existing as non-bonding lone pairs. Identifying the central atom, typically the least electronegative element, provides the anchor point for the structure.
To construct the Lewis structure, valence electrons from all atoms are counted and distributed to satisfy the octet rule for each atom (eight electrons), except for hydrogen (two electrons). The final structure shows the placement of bonding electrons (single, double, or triple bonds) and any remaining lone pairs on the central atom. This map explicitly shows all the electron groups that determine the molecule’s geometry.
Determining Electron Domains
The VSEPR model requires calculating the steric number, which is the total count of electron domains surrounding the central atom. An electron domain is defined as any region of electron density, including a lone pair or a bond. A single bond, a double bond, or a triple bond all count as only one domain because the electrons are localized in the same region between the two atoms.
A lone pair of electrons also counts as a single, distinct electron domain. The total number of domains, typically ranging from two to six, dictates the electron domain geometry. This geometry is the arrangement of all electron groups (bonding and non-bonding) and establishes the basic symmetrical structure, such as linear for two domains or tetrahedral for four domains.
Translating Domains into Molecular Shape
The final step involves distinguishing between the electron domain geometry and the molecular geometry, which is the arrangement of only the atoms in space. If the central atom has no lone pairs, the two geometries are identical, as seen in methane ($CH_4$) with its tetrahedral arrangement of four bonding domains.
Lone pairs introduce distortion because they are held closer to the central atom’s nucleus, occupying more space than bonding pairs. This increased spatial requirement means lone pairs exert a stronger repulsive force on adjacent electron domains. This greater repulsion pushes bonding pairs closer together, compressing the angle between the bonded atoms.
For example, ammonia ($NH_3$) has four electron domains (three bonds, one lone pair), resulting in a tetrahedral electron domain geometry. The lone pair repulsion, however, results in a trigonal pyramidal molecular shape with a bond angle smaller than the ideal $109.5^{\circ}$. Water ($H_2O$), with two lone pairs, experiences even greater compression, resulting in a bent molecular geometry.
Key Geometrical Structures
The VSEPR model establishes a set of predictable electron domain geometries based on the total number of domains:
- Two electron domains result in a Linear geometry, where the atoms are positioned $180^{\circ}$ apart.
- Three electron domains result in a Trigonal Planar geometry, where the ideal bond angle is $120^{\circ}$.
- Four electron domains result in a Tetrahedral arrangement, with bond angles of $109.5^{\circ}$.
- Five electron domains form a Trigonal Bipyramidal shape, featuring three equatorial positions at $120^{\circ}$ and two axial positions at $90^{\circ}$ to the equatorial plane.
- Six electron domains result in an Octahedral geometry, where all six positions are equivalent and separated by $90^{\circ}$ angles.