Valence Shell Electron Pair RepulsionEdit
Valence Shell Electron Pair Repulsion (VSEPR) is a practical framework in chemistry for predicting the shapes of molecules by considering how electron pairs around a central atom repel one another. The theory rests on the idea that electron domains—whether bonding pairs or lone pairs—arrange themselves to minimize repulsion in the valence shell, giving rise to characteristic electronic and molecular geometries. In teaching and applied chemistry, VSEPR serves as a bridge between valence electron counts and the observable three-dimensional arrangements of atoms in space. The model distinguishes between the electronic geometry, which describes the arrangement of all electron pairs, and the molecular geometry, which reflects the arrangement of atoms only. It is closely connected to ideas about the valence shell, electron density, and how atoms bond in diverse environments. See, for example, the discussions of Molecular geometry and Steric number as part of the same framework.
VSEPR rests on a few core conventions. First, the total number of electron domains around the central atom—each lone pair, single bond, double bond, or triple bond counts as one domain—defines the steric number and largely determines the electronic geometry. Second, lone pairs occupy more space than bonding pairs because of their greater electron-electron repulsion, leading to smaller bond angles when lone pairs are present. This difference is a central reason why molecules with the same central atom and same number of electron domains can exhibit different shapes depending on how many lone pairs they carry. For the terminology, see Lone pair and Bonding pair and the broader idea of Electron pair repulsion.
Core concepts
- Electron domains and repulsion: Each domain around a central atom creates repulsion against neighboring domains. The strongest repulsions typically come from lone pairs, which push bonding pairs inward and adjust bond angles accordingly.
- Electronic geometry vs molecular geometry: The arrangement of all electron domains defines the electronic geometry; the arrangement of the atoms defines the molecular geometry. See Electronic geometry and Molecular geometry for related concepts.
- Steric number: The sum of bonding and lone-pair domains around the central atom. The steric number largely fixes the basic geometry (e.g., 2 → linear, 3 → trigonal planar, 4 → tetrahedral), with additional refinements when lone pairs are present.
- Handling multiple bonds: In VSEPR, multiple bonds (double or triple bonds) are counted as a single domain for the purposes of predicting geometry, though they contribute more electron density than a single bond.
Common geometries and examples
- Linear (two electron domains, 180°): e.g., CO2 in its simplest form displays a straight arrangement of two external atoms around the central atom.
- Trigonal planar (three electron domains, 120°): e.g., BF3 commonly adopts a planar arrangement with 120° angles between substituents.
- Tetrahedral (four electron domains, ~109.5°): e.g., CH4 shows a classic tetrahedral geometry with four equivalent C–H bonds.
- Trigonal bipyramidal (five electron domains, 90° and 120°): e.g., PF5 features two distinct positions (axial and equatorial) to accommodate five domains.
- Octahedral (six electron domains, 90°): e.g., SF6 arranges six ligands around the central atom in an octahedral framework.
- Distorted shapes due to lone pairs: Molecules with lone pairs often deviate from idealized angles. For instance, H2O has two bonding pairs and two lone pairs, giving a bent shape with a bond angle reduced from the ideal tetrahedral value.
A number of common derivative geometries arise when lone pairs are present, such as bent, trigonal pyramidal, and see-saw shapes. These predictions are widely used in inorganic and organic chemistry to rationalize reactivity, polarity, and spectroscopy. See Molecular geometry and VSEPR theory for related discussions and variations on how geometry is described.
Applications and limitations
VSEPR is especially effective as an introductory and intuitive tool for predicting the shapes of many main-group molecules, where valence electron counts are well defined and bonding is reasonably localized. It underpins quick estimations of bond angles and has proven valuable in explaining molecular polarity, reactivity trends, and spectroscopy in a broad range of compounds. The approach is frequently taught before more advanced treatments because it translates electron-domain thinking into three-dimensional molecular pictures that students can visualize.
However, VSEPR has limitations. It is a heuristic model rather than a full quantum-mechanical description. For molecules involving transition metals or extensive π-bonding and delocalized electrons, the simple domain-repulsion picture can fail to capture the true geometry, electronic structure, or bonding characteristics. In such cases, more sophisticated frameworks—such as Molecular orbital theory or Valence bond theory—provide more reliable pictures, and computational methods (for example, density functional theory) can quantify electron density and forces with higher fidelity. The ongoing dialogue among chemists acknowledges these boundaries: VSEPR remains a practical first-pass tool, while advanced theories refine or revise predictions in complex systems. See also discussions of Hypervalent molecule behavior and how expanded octets are treated within extended modeling approaches.
History and development
The concept emerged in the mid-20th century as chemists sought a simple, predictive rule set linking valence electrons to observable shapes. Although the exact historical credits vary, the practical formulation is associated with work by researchers such as Ronald J. Gillespie and colleagues building on earlier observations by others. The enduring value of VSEPR lies in its blend of conceptual clarity and empirical success, which keeps it in active use for teaching and for quick qualitative reasoning about molecular structure. See VSEPR theory for a broader bibliographic and historical context.