Premed · Premed · General Chemistry 1
Lecture 12: Molecular Geometry and VSEPR
General Chemistry I
Learning Objectives
By the end of this lecture, students will be able to:
- State the principles of VSEPR (Valence Shell Electron Pair Repulsion) theory
- Determine the electron-domain geometry and molecular geometry of a molecule from its Lewis structure
- Predict bond angles for common molecular geometries
- Classify molecules as polar or nonpolar based on geometry and bond polarity
- Draw and name all common molecular shapes (linear through octahedral)
Lecture Content
I. Introduction to VSEPR Theory
VSEPR stands for Valence Shell Electron Pair Repulsion theory. Its core principle is that electron domains -- bonding pairs and lone pairs -- around a central atom repel one another and arrange themselves to maximize the distance between them, thereby minimizing electrostatic repulsion and determining the three-dimensional shape of the molecule. An electron domain is defined as any of the following around the central atom: a single bond, a double bond, a triple bond, or a lone pair. Crucially, multiple bonds (double or triple) count as just one electron domain for the purpose of determining geometry.
II. Electron-Domain Geometry vs. Molecular Geometry
Electron-domain geometry (also called electron-group geometry) describes the arrangement of all electron domains -- both bonding pairs and lone pairs -- around the central atom. Molecular geometry, by contrast, describes the arrangement of only the atoms, ignoring the lone pairs. While lone pairs influence the shape, they are not "seen" when naming the molecular geometry. It is the molecular geometry that corresponds to what is observed experimentally.
III. The Five Basic Electron-Domain Geometries
| # Electron Domains | Electron-Domain Geometry | Ideal Bond Angle |
|---|---|---|
| 2 | Linear | 180 degrees |
| 3 | Trigonal planar | 120 degrees |
| 4 | Tetrahedral | 109.5 degrees |
| 5 | Trigonal bipyramidal | 90 and 120 degrees |
| 6 | Octahedral | 90 degrees |
IV. Molecular Geometries Derived from Each Electron-Domain Geometry
A. Two Electron Domains (Linear)
With 2 bonding domains and 0 lone pairs, the molecular geometry is linear with bond angles of 180 degrees. Examples include CO2, BeCl2, and HCN.
B. Three Electron Domains (Trigonal Planar)
With 3 bonding domains and 0 lone pairs, the geometry is trigonal planar with 120-degree bond angles, as seen in BF3, SO3, and NO3-. When one bonding domain is replaced by a lone pair (2 bonding, 1 lone pair), the molecular geometry becomes bent or V-shaped, with bond angles compressed to slightly less than 120 degrees. SO2, O3, and NO2- adopt this shape.
C. Four Electron Domains (Tetrahedral)
With 4 bonding domains and 0 lone pairs, the geometry is tetrahedral with 109.5-degree bond angles, as in CH4, NH4+, and CCl4. Three bonding domains and 1 lone pair yield a trigonal pyramidal shape with bond angles of roughly 107 degrees (NH3, PCl3, H3O+). Two bonding domains and 2 lone pairs produce a bent shape with bond angles near 104.5 degrees (H2O, H2S, SCl2).
D. Five Electron Domains (Trigonal Bipyramidal)
With all 5 domains bonding, the geometry is trigonal bipyramidal with bond angles of 90 and 120 degrees (PCl5). Four bonding domains and 1 lone pair give a seesaw (or sawhorse) shape with bond angles near 90 and 120 degrees, as in SF4, where the lone pair preferentially occupies an equatorial position. Three bonding and 2 lone pairs produce a T-shaped geometry with angles near 90 degrees (ClF3). Two bonding and 3 lone pairs yield a linear arrangement at 180 degrees (XeF2).
E. Six Electron Domains (Octahedral)
With 6 bonding domains and 0 lone pairs, the geometry is octahedral with 90-degree bond angles (SF6). Five bonding domains and 1 lone pair give a square pyramidal shape with angles near 90 degrees (BrF5, IF5). Four bonding domains and 2 lone pairs (with the lone pairs placed on opposite sides) produce a square planar geometry with 90-degree angles (XeF4).
<image>A comprehensive VSEPR geometry chart organized in a grid. Columns represent the number of lone pairs (0, 1, 2, 3). Rows represent the number of electron domains (2, 3, 4, 5, 6). Each cell contains: a 3D ball-and-stick model of the molecular geometry, the name of the molecular shape, the bond angles, and an example molecule. Bonding pairs are shown as solid wedge/dash bonds, lone pairs as electron cloud lobes. Cells that do not correspond to common geometries are grayed out. Key geometries highlighted: linear (2 domains), trigonal planar (3 domains, 0 LP), bent (3 domains 1 LP or 4 domains 2 LP), tetrahedral (4 domains 0 LP), trigonal pyramidal (4 domains 1 LP), trigonal bipyramidal (5 domains 0 LP), seesaw (5 domains 1 LP), T-shaped (5 domains 2 LP), octahedral (6 domains 0 LP), square pyramidal (6 domains 1 LP), square planar (6 domains 2 LP).</image>
V. Effect of Lone Pairs on Bond Angles
Lone pairs are held closer to the central atom than bonding pairs and spread out more in space, occupying greater angular room. The order of repulsion strength is: lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair. As a result, lone pairs compress the bond angles between bonding pairs. This effect is clearly visible in the sequence CH4 (109.5 degrees, no lone pairs), NH3 (107 degrees, one lone pair), and H2O (104.5 degrees, two lone pairs). Multiple bonds also occupy more space than single bonds and can widen adjacent bond angles.
VI. Lone Pair Placement in Trigonal Bipyramidal Geometry
In a trigonal bipyramidal arrangement, there are two types of positions: three equatorial positions in the triangular plane and two axial positions above and below it. Lone pairs preferentially occupy equatorial positions because an equatorial lone pair has only 2 nearest-neighbor 90-degree interactions, while an axial lone pair would experience 3. This placement rule determines the shapes of the seesaw, T-shaped, and linear molecules in the five-domain family.
VII. Molecular Polarity
A molecule is polar if it possesses a net dipole moment (mu is not equal to zero) and nonpolar if the individual bond dipoles cancel due to symmetry (mu = 0). Two conditions must be met for a molecule to be polar: it must contain polar bonds (a difference in electronegativity between bonded atoms), and its geometry must be such that the bond dipoles do not cancel. Water (bent), ammonia (trigonal pyramidal), HCl, and CHCl3 are all polar molecules. On the other hand, CO2 (linear, with opposing C=O dipoles), BF3 (trigonal planar), CCl4 (tetrahedral), and SF6 (octahedral) are nonpolar despite containing polar bonds, because their symmetric geometries cause the dipoles to cancel perfectly. To assess polarity, draw the molecule in three dimensions, assign bond dipole arrows, and add them as vectors; if the resultant vector is not zero, the molecule is polar.
<image>A four-panel comparison of molecular polarity. Panel A (CO2 -- nonpolar): linear geometry with two equal C=O bond dipole arrows pointing outward in opposite directions; vector sum shown as zero. Panel B (H2O -- polar): bent geometry with two O-H bond dipole arrows pointing toward O; the resultant net dipole moment arrow is shown pointing from between the H atoms toward O. Panel C (CCl4 -- nonpolar): tetrahedral geometry with four equal C-Cl bond dipole arrows pointing toward the Cl atoms; they cancel due to symmetry. Panel D (CHCl3 -- polar): distorted tetrahedral with three C-Cl dipoles and one smaller C-H dipole; the dipoles do not cancel, resulting in a net dipole moment arrow pointing toward the Cl side. Each panel shows the vector addition explicitly with arrows.</image>
VIII. Determining Geometry -- Step-by-Step Summary
To determine the geometry of any molecule, follow these steps. First, draw the Lewis structure. Second, count the total number of electron domains around the central atom. Third, identify the electron-domain geometry from the table. Fourth, count the number of lone pairs on the central atom. Fifth, name the molecular geometry based on the arrangement of atoms only. Sixth, estimate bond angles, adjusting for compression due to lone pairs. Seventh, assess polarity by considering both bond dipoles and molecular symmetry.
IX. Geometry of Molecules with Multiple Central Atoms
For molecules with more than one central atom, such as CH3OH or CH3NH2, VSEPR is applied independently to each central atom. Each atom has its own local geometry, and the overall molecular shape is described by combining these local geometries. In acetic acid (CH3COOH), for example, the carbon in the CH3 group has a tetrahedral arrangement (4 bonding domains), while the carbon in the COOH group has a trigonal planar arrangement (3 electron domains, counting the C=O double bond as one domain).
