Molecular Geometry
Molecular geometry (or molecular shape) refers to the three-dimensional spatial arrangement of atoms within a molecule, determined by the positions of the atoms — not the lone pairs — around the central atom. The geometry is predicted using VSEPR theory or hybridization models and directly influences physical properties such as polarity, reactivity, phase of matter, colour, magnetism, and biological activity. Common geometries include linear, bent, trigonal planar, trigonal pyramidal, tetrahedral, and octahedral.
Worked Example
Problem
Determine the molecular geometry and bond angles of CO₂ and H₂O. Both have 2 bonding pairs on the central atom, but their geometries differ. Explain why.
Solution
CO₂ Analysis: Step 1: Lewis structure of CO₂: O=C=O. C has 2 double bonds, 0 lone pairs. Step 2: 2 electron domains → linear electron geometry and linear molecular geometry. Step 3: Bond angle = 180°. H₂O Analysis: Step 1: Lewis structure of H₂O: H–O–H. O has 2 single bonds, 2 lone pairs. Step 2: 4 electron domains → tetrahedral electron geometry. Step 3: 2 bonding pairs + 2 lone pairs → bent molecular geometry. Step 4: Lone pairs repel bonding pairs, reducing bond angle from 109.5° to 104.5°.
Answer
CO₂ is linear (180°) with no lone pairs on C; H₂O is bent (104.5°) due to 2 lone pairs on O compressing the bond angle.
Common Molecular Geometries and Their Properties
| Geometry | Example Molecule | Bond Angle | Polarity | Hybridization |
|---|---|---|---|---|
| Linear | CO₂ | 180° | Nonpolar | sp |
| Trigonal planar | BF₃ | 120° | Nonpolar | sp² |
| Bent | H₂O | 104.5° | Polar | sp³ |
| Tetrahedral | CH₄ | 109.5° | Nonpolar | sp³ |
| Trigonal pyramidal | NH₃ | 107° | Polar | sp³ |
| Octahedral | SF₆ | 90° | Nonpolar | sp³d² |
Interactive Tools
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Related Terms
VSEPR Theory
Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used to predict the three-dimensional geometry of molecules based on the principle that electron pairs in the valence shell of a central atom repel each other and arrange themselves as far apart as possible to minimize repulsion. The theory considers both bonding pairs and lone pairs, with lone pairs exerting greater repulsive force than bonding pairs, which distorts ideal bond angles. VSEPR theory was developed by Ronald Gillespie and Ronald Nyholm in 1957 and remains one of the most useful and accessible tools for predicting molecular shape.
Hybridization
Hybridization is a theoretical concept in chemistry describing the mixing of atomic orbitals of similar energy within the same atom to form new hybrid orbitals of equivalent energy and shape, oriented to minimize electron repulsion. Developed by Linus Pauling in 1931, hybridization explains molecular geometry that cannot be accounted for by simple orbital overlap — for example, carbon's four equivalent C–H bonds in methane despite having distinct 2s and 2p orbitals. The type of hybridization (sp, sp², sp³, sp³d, sp³d²) determines bond angles, molecular geometry, and the presence of pi bonds.
Lewis Structure
A Lewis structure (also called a Lewis dot structure or electron dot diagram) is a two-dimensional representation of a molecule that shows the arrangement of atoms, bonding electron pairs (as lines or pairs of dots between atoms), and lone (non-bonding) electron pairs. Developed by Gilbert N. Lewis in 1916, these diagrams are essential tools for predicting molecular geometry, reactivity, and understanding bond types. Lewis structures obey the octet rule — most atoms in a molecule strive to have 8 electrons in their valence shell, with the notable exception of hydrogen (which requires only 2).
From Greek "molekula" (diminutive of "moles", mass) and Greek "geometria" (earth measurement). The term gained scientific precision in the 20th century as X-ray crystallography and spectroscopy allowed direct measurement of atomic positions.