Molecular Geometry Study Pack
Kibin's free study pack on Molecular Geometry includes a 6-section study guide, 25 quiz questions, 30 flashcards, and 5 open-ended Explain review questions. Sign up free to track your progress toward mastery, plus upload your own notes and recordings to create personalized study packs organized by course.
Last updated May 27, 2026
Molecular Geometry Study Guide
Master VSEPR theory and its role in predicting molecular shapes — from linear and tetrahedral to trigonal pyramidal and bent — while exploring how lone pairs compress bond angles and why symmetric geometry in molecules like CO₂ cancels bond dipoles entirely.
Key Takeaways
- •VSEPR theory predicts molecular geometry by stating that electron domains around a central atom arrange themselves as far apart as possible to minimize electrostatic repulsion.
- •Electron domain geometry accounts for all electron pairs (bonding and lone pairs), while molecular geometry describes only the positions of atoms, so these two geometries differ whenever lone pairs are present.
- •Lone pairs exert greater repulsive force than bonding pairs, compressing bond angles below their ideal values — for example, water's H–O–H angle is 104.5° rather than the tetrahedral ideal of 109.5°.
- •Common molecular geometries include linear, trigonal planar, tetrahedral, trigonal pyramidal, bent, trigonal bipyramidal, and octahedral, each associated with specific electron domain counts and lone pair arrangements.
- •Molecular polarity depends on both individual bond dipoles and the three-dimensional shape of the molecule; a molecule with polar bonds can be nonpolar overall if its geometry causes the bond dipoles to cancel symmetrically, as in CO₂ and CCl₄.
- •Electronegativity differences between bonded atoms determine bond polarity, and the vector sum of all bond dipoles gives the molecule's net dipole moment.
The Logic Behind VSEPR Theory
Valence Shell Electron Pair Repulsion (VSEPR) theory provides a systematic method for predicting the three-dimensional shape of a molecule based on the premise that electron domains around a central atom will orient themselves to be as far apart as possible.
What an Electron Domain Is
- •An electron domain is any region of electron density around a central atom — this includes single bonds, double bonds, triple bonds, and lone pairs, each counting as one domain regardless of bond order.
- •A double bond between carbon and oxygen, for example, still counts as a single electron domain because both bonding electron pairs occupy the same general region of space between the two atoms.
The Core VSEPR Principle
- •Because electrons carry negative charge, adjacent electron domains repel one another; the geometry that minimizes this repulsion is the one a molecule actually adopts.
- •This principle applies to the central atom of a molecule or polyatomic ion — peripheral atoms with only one bonded neighbor do not require VSEPR analysis.
Building a VSEPR Analysis
- •Start by drawing the Lewis structure of the molecule to identify the number of bonding pairs and lone pairs on the central atom.
- •Count all electron domains to determine the electron domain geometry, then identify which domains are lone pairs to arrive at the molecular geometry.
Electron Domain Geometries and Their Ideal Bond Angles
The number of electron domains around a central atom dictates the electron domain geometry and sets the ideal bond angles that the molecule approaches when no lone pairs are present.
Two Electron Domains: Linear Arrangement
- •Two domains position themselves 180° apart, producing a linear electron domain geometry.
- •BeCl₂, with two bonding pairs and no lone pairs, exemplifies this geometry with a Cl–Be–Cl angle of exactly 180°.
Three Electron Domains: Trigonal Planar Arrangement
- •Three domains spread into the same plane at 120° angles, giving a trigonal planar electron domain geometry.
- •BF₃ adopts this geometry because its three bonding pairs distribute symmetrically around boron.
Four Electron Domains: Tetrahedral Arrangement
- •Four domains orient toward the corners of a tetrahedron, establishing bond angles of approximately 109.5°.
- •Methane (CH₄) is the textbook example, with four equivalent C–H bonding pairs and no lone pairs.
Five and Six Electron Domains
- •Five domains adopt a trigonal bipyramidal arrangement with two distinct positions: axial (90° from the equatorial plane) and equatorial (120° from each other).
- •Six domains adopt an octahedral arrangement with 90° angles between all adjacent domains, placing every domain an equal distance from its neighbors.
How Lone Pairs Alter Molecular Geometry
When one or more of the electron domains on a central atom is a lone pair rather than a bonding pair, the observable molecular geometry — defined by atom positions only — diverges from the electron domain geometry, and bond angles compress below their ideal values.
Why Lone Pairs Cause Compression
- •Lone pairs are held only by one nucleus rather than two, so they spread out and occupy more space than bonding pairs, exerting stronger repulsion on neighboring domains.
- •This additional repulsion pushes the bonding pairs closer together, reducing the angle between them.
Tetrahedral Domain Geometries with Lone Pairs
- •Ammonia (NH₃) has four electron domains — three bonding pairs and one lone pair — giving it a tetrahedral domain geometry but a trigonal pyramidal molecular geometry with an H–N–H angle of about 107°.
- •Water (H₂O) has four electron domains — two bonding pairs and two lone pairs — giving it a tetrahedral domain geometry but a bent molecular geometry with an H–O–H angle of about 104.5°; the two lone pairs compress the angle even further than in ammonia.
Trigonal Bipyramidal Domains with Lone Pairs
- •When lone pairs occupy positions in a trigonal bipyramidal arrangement, they preferentially occupy equatorial positions because these positions have only two 90° repulsions rather than three.
- •SF₄, with one lone pair in the equatorial plane, takes on a see-saw shape; ClF₃, with two equatorial lone pairs, is T-shaped; XeF₂, with three equatorial lone pairs, is linear.
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About this Study Pack
Created by Kibin to help students review key concepts, prepare for exams, and study more effectively. This Study Pack was checked for accuracy and curriculum alignment using authoritative educational sources. See sources below.
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What is the fundamental premise of VSEPR theory?
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Electron Domains
Explain what an electron domain is in your own words. Why does a double bond count as only one electron domain, and why does this concept matter for predicting molecular shape?
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