VSEPR Theory Cheatsheet

Cheatsheet Content

### Introduction to VSEPR Theory - **Valence Shell Electron Pair Repulsion (VSEPR) Theory** is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. - It is based on the idea that valence shell electron pairs (both bonding and non-bonding) repel each other and will therefore adopt an arrangement that minimizes this repulsion. - This minimization of repulsion leads to specific geometric arrangements that dictate the molecular shape. - **Key principle:** Electron domains (lone pairs and bonds) around a central atom will arrange themselves as far apart as possible. - **Electron Domain:** A region in space where electrons are likely to be found. This includes single bonds, double bonds, triple bonds, and lone pairs. Each counts as one electron domain. ### Electron Domain Geometry vs. Molecular Geometry - **Electron Domain Geometry:** The arrangement of all electron domains (bonding and non-bonding) around the central atom. This determines the basic spatial arrangement. - **Molecular Geometry:** The arrangement of only the atoms in a molecule. Lone pairs influence the molecular geometry but are not part of the molecular geometry description itself. - **Example:** A molecule with 4 electron domains will always have a tetrahedral electron domain geometry. However, if one of those domains is a lone pair, the molecular geometry will be trigonal pyramidal. #### Steps to Determine Geometry 1. **Draw the Lewis Structure:** Determine the central atom and draw all bonds and lone pairs. 2. **Count Electron Domains:** Count the total number of electron domains around the central atom (each lone pair, single bond, double bond, or triple bond counts as one domain). 3. **Determine Electron Domain Geometry:** Based on the number of electron domains, predict the electron domain geometry. 4. **Determine Molecular Geometry:** Based on the number of bonding domains and lone pairs, predict the molecular geometry. 5. **Predict Bond Angles:** Estimate ideal bond angles, noting that lone pairs generally compress bond angles. ### Common Electron Domain Geometries #### 1. Linear (2 Electron Domains) - **Description:** Two electron domains positioned 180° apart. - **Ideal Bond Angle:** 180° - **Examples:** $\text{BeCl}_2$, $\text{CO}_2$ (central C has two double bonds, each counts as one domain) - **Molecular Geometries:** Only Linear #### 2. Trigonal Planar (3 Electron Domains) - **Description:** Three electron domains arranged in a flat triangle. - **Ideal Bond Angle:** 120° - **Examples:** $\text{BF}_3$, $\text{NO}_3^-$ - **Molecular Geometries:** - **Trigonal Planar:** 3 bonding domains, 0 lone pairs ($\text{BF}_3$) - **Bent:** 2 bonding domains, 1 lone pair ($\text{SO}_2$) #### 3. Tetrahedral (4 Electron Domains) - **Description:** Four electron domains pointing to the corners of a tetrahedron. - **Ideal Bond Angle:** 109.5° - **Examples:** $\text{CH}_4$, $\text{NH}_3$, $\text{H}_2\text{O}$ - **Molecular Geometries:** - **Tetrahedral:** 4 bonding domains, 0 lone pairs ($\text{CH}_4$) - **Trigonal Pyramidal:** 3 bonding domains, 1 lone pair ($\text{NH}_3$) - **Bent:** 2 bonding domains, 2 lone pairs ($\text{H}_2\text{O}$) ### Expanded Electron Domain Geometries (Central Atom from Period 3 or Below) #### 4. Trigonal Bipyramidal (5 Electron Domains) - **Description:** Five electron domains. Three are in an equatorial plane (120° apart), and two are axial (90° to the equatorial plane). Lone pairs prefer equatorial positions to minimize repulsion. - **Ideal Bond Angles:** 90°, 120° - **Examples:** $\text{PCl}_5$, $\text{SF}_4$, $\text{ClF}_3$, $\text{I}_3^-$ - **Molecular Geometries:** - **Trigonal Bipyramidal:** 5 bonding domains, 0 lone pairs ($\text{PCl}_5$) - **Seesaw (or Distorted Tetrahedral):** 4 bonding domains, 1 lone pair ($\text{SF}_4$) - **T-shaped:** 3 bonding domains, 2 lone pairs ($\text{ClF}_3$) - **Linear:** 2 bonding domains, 3 lone pairs ($\text{I}_3^-$) #### 5. Octahedral (6 Electron Domains) - **Description:** Six electron domains pointing to the corners of an octahedron. All positions are equivalent. - **Ideal Bond Angles:** 90° - **Examples:** $\text{SF}_6$, $\text{BrF}_5$, $\text{XeF}_4$ - **Molecular Geometries:** - **Octahedral:** 6 bonding domains, 0 lone pairs ($\text{SF}_6$) - **Square Pyramidal:** 5 bonding domains, 1 lone pair ($\text{BrF}_5$) - **Square Planar:** 4 bonding domains, 2 lone pairs ($\text{XeF}_4$) ### VSEPR Geometry Summary Table | Electron Domains | Bonding Domains | Lone Pairs | Electron Domain Geometry | Molecular Geometry | Ideal Bond Angles | Example | |------------------|-----------------|------------|--------------------------|--------------------|-------------------|---------| | 2 | 2 | 0 | Linear | Linear | 180° | $\text{CO}_2$ | | 3 | 3 | 0 | Trigonal Planar | Trigonal Planar | 120° | $\text{BF}_3$ | | 3 | 2 | 1 | Trigonal Planar | Bent | ### Effect of Lone Pairs on Molecular Geometry and Bond Angles - **Lone pair-lone pair repulsion > Lone pair-bonding pair repulsion > Bonding pair-bonding pair repulsion.** - Lone pairs occupy more space than bonding pairs because they are attracted to only one nucleus, whereas bonding pairs are attracted to two nuclei (less constrained). - This increased repulsion from lone pairs causes a compression of bond angles between bonding pairs. - The molecular geometry is described by the positions of the atoms, not the lone pairs, but the lone pairs strongly influence the shape. #### Examples of Lone Pair Effects - **$\text{CH}_4$ (Methane):** 4 bonding pairs, 0 lone pairs. Tetrahedral, 109.5° bond angles. - **$\text{NH}_3$ (Ammonia):** 3 bonding pairs, 1 lone pair. Electron domain: Tetrahedral. Molecular geometry: Trigonal pyramidal. Bond angles: ~107° (compressed from 109.5°). - **$\text{H}_2\text{O}$ (Water):** 2 bonding pairs, 2 lone pairs. Electron domain: Tetrahedral. Molecular geometry: Bent. Bond angles: ~104.5° (further compressed from 109.5°). - In $\text{SF}_4$ (Seesaw), the lone pair occupies an equatorial position to minimize 90° repulsions. ### Multiple Bonds in VSEPR - Double and triple bonds are treated as a **single electron domain** for the purpose of determining electron domain geometry. - While they count as one domain, multiple bonds contain a higher density of electrons than single bonds. - This higher electron density can slightly increase their repulsion with other bonding pairs, similar to how lone pairs exert greater repulsion, but usually less pronounced than lone pairs. This can lead to slight deviations from ideal bond angles. #### Example: Carbon Dioxide ($\text{CO}_2$) - Lewis structure: $\text{O=C=O}$ - Central atom: Carbon - Electron domains around C: Two (one for each double bond) - Electron domain geometry: Linear - Molecular geometry: Linear - Bond angle: 180° #### Example: Formaldehyde ($\text{H}_2\text{CO}$) - Lewis structure: ``` H | C=O | H ``` - Central atom: Carbon - Electron domains around C: Three (two for C-H single bonds, one for C=O double bond) - Electron domain geometry: Trigonal Planar - Molecular geometry: Trigonal Planar - Ideal bond angles: ~120° (the C=O double bond, being bulkier, might slightly compress the H-C-H angle to be slightly less than 120°, and the O=C-H angles slightly more). ### Molecular Polarity and VSEPR - Molecular polarity depends on two factors: 1. **Polarity of individual bonds:** Determined by the difference in electronegativity between the bonded atoms. 2. **Molecular geometry:** Determines if bond dipoles cancel out or sum up to create a net dipole moment. #### Determining Molecular Polarity 1. **Draw the Lewis Structure and determine VSEPR geometry.** 2. **Identify polar bonds:** If there's a significant electronegativity difference between bonded atoms, the bond is polar. Draw bond dipoles (arrows pointing towards the more electronegative atom). 3. **Determine if bond dipoles cancel:** - If the molecule is symmetrical and all terminal atoms are identical, the bond dipoles often cancel, resulting in a **nonpolar molecule**. - If the molecule is asymmetrical (due to different terminal atoms or the presence of lone pairs on the central atom), the bond dipoles usually do not cancel, resulting in a **polar molecule** (net dipole moment). #### Examples - **Nonpolar Molecules:** - **$\text{CO}_2$ (Linear):** $\text{O=C=O}$. Both C=O bonds are polar, but they point in opposite directions and cancel out. - **$\text{CH}_4$ (Tetrahedral):** C-H bonds are slightly polar, but the tetrahedral symmetry ensures they cancel. - **$\text{CCl}_4$ (Tetrahedral):** C-Cl bonds are polar, but the tetrahedral symmetry ensures they cancel. - **$\text{BF}_3$ (Trigonal Planar):** B-F bonds are polar, but the trigonal planar symmetry ensures they cancel. - **Polar Molecules:** - **$\text{H}_2\text{O}$ (Bent):** O-H bonds are polar. The bent geometry (due to lone pairs on O) means the dipoles do not cancel, resulting in a net dipole moment. - **$\text{NH}_3$ (Trigonal Pyramidal):** N-H bonds are polar. The trigonal pyramidal geometry (due to a lone pair on N) means the dipoles do not cancel. - **$\text{CHCl}_3$ (Tetrahedral derivative):** C-Cl bonds are more polar than C-H. The asymmetry created by having different terminal atoms (H vs. Cl) means the dipoles do not cancel. - **$\text{SO}_2$ (Bent):** S-O bonds are polar. The bent geometry (due to a lone pair on S) means the dipoles do not cancel. ### Exceptions and Limitations of VSEPR Theory - **Transition Metal Compounds:** VSEPR theory generally does not apply well to compounds involving transition metals, as d-orbitals can be involved in bonding, leading to more complex geometries not easily predicted by simple electron domain repulsion. - **Heavy Main Group Elements:** For some heavy main group elements (e.g., $\text{SeH}_2$, $\text{TeH}_2$), bond angles can be closer to 90° than predicted by VSEPR. This is attributed to reduced lone pair repulsion and increased p-orbital bonding character. - **Molecules with Delocalized Electrons:** While multiple bonds are treated as single domains, complex resonance structures or extensive delocalization can sometimes lead to geometries that are not perfectly ideal. - **Relativistic Effects:** For very heavy elements, relativistic effects can influence orbital sizes and energies, leading to deviations from VSEPR predictions. - **Steric Effects:** For very bulky ligands, steric hindrance can play a significant role in determining molecular geometry, sometimes overriding electron pair repulsion principles. - **Solid State Structures:** VSEPR is primarily for isolated molecules in the gas phase. In the solid state, intermolecular forces and crystal packing can influence molecular geometry.