Back/Chemistry: Atoms First 2e
Section 4.616 Key Terms

Molecular Structure and Polarity

Learning Objectives
  • Predict the structures of small molecules using valence shell electron pair repulsion (VSEPR) theory
  • Explain the concepts of polar covalent bonds and molecular polarity
  • Assess the polarity of a molecule based on its bonding and structure

Core Concepts & Principles

While Lewis structures provide a flat, two-dimensional representation of molecules, real molecules exist in three dimensions. The physical arrangement of atoms in space is defined by bond angles (the angle between two bonds sharing a common atom) and bond distances (the straight-line distance between the nuclei of two bonded atoms).

VSEPR Minimization Principle

Valence shell electron-pair repulsion theory assumes that regions of high electron density (bonding pairs and lone pairs) surrounding a central atom will arrange themselves as far apart as possible in three-dimensional space to minimize electrostatic repulsion.

Electron-Pair Geometry vs. Molecular Structure

It is vital to distinguish between how electrons are arranged and how atoms are arranged:

  • Electron-pair geometry: Describes the spatial location of all regions of electron density around a central atom, including both chemical bonds and lone pairs.
  • Molecular structure: Describes the spatial location and shape defined strictly by the atoms, ignoring lone pairs.

When a central atom has no lone pairs, the electron-pair geometry and molecular structure are identical. When lone pairs are present, they occupy more physical space than bonding pairs and exert stronger repulsive forces (repulsion order: lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair). This causes slight distortions and alters the final molecular shape (e.g., tetrahedral electron geometry with one lone pair results in a trigonal pyramidal structure like NH3\text{NH}_3, or two lone pairs result in a bent structure like H2O\text{H}_2\text{O}).

Specialized Geometries

  • Trigonal Bipyramidal: Features five regions of electron density with two distinct sites: axial positions (90° angles to the equator) and equatorial positions (120° angles around the middle). Lone pairs always occupy the roomier equatorial positions to minimize repulsion.
  • Octahedral: Features six regions of electron density. When two lone pairs are present, they place themselves on opposite sides (180° apart) to form a square planar structure.
Molecular Polarity and Dipole Moments

A polar molecule has a net charge separation resulting in a nonzero dipole moment (μ\mu). Polarity depends on two factors:

  1. The presence of polar covalent bonds with individual bond dipole moments (represented as vectors pointing from partial positive to partial negative).
  2. A 3D molecular structure where individual bond dipoles do not cancel out via vector addition. Symmetrical molecules (like CO2\text{CO}_2 or CCl4\text{CCl}_4) have bond moments that cancel entirely, making them nonpolar.

Problem-Solving Routines & Methods

Predicting Electron-Pair Geometry and Molecular Structure
  1. 1
    Draw the valid Lewis structure for the molecule or polyatomic ion.
  2. 2
    Count the total regions of high electron density around the central atom (single, double, and triple bonds each count as one region; lone pairs count as one region).
  3. 3
    Determine the electron-pair geometry based on total regions (2 = linear, 3 = trigonal planar, 4 = tetrahedral, 5 = trigonal bipyramidal, 6 = octahedral).
  4. 4
    Determine the molecular structure using the number of attached atoms and lone pairs, placing lone pairs in preferred positions (equatorial for trigonal bipyramidal, opposite sides for octahedral) to minimize repulsions.
Pro-Tip: Multiple bonds (double or triple) count as only one region of electron density for geometry, but they occupy more physical space than single bonds.
Bond Dipole Moment Formula
μ=Q⋅r\mu = Q \cdot r

Calculates the magnitude of a bond dipole moment based on charge separation and distance.

Variables & Constants
QQ=magnitude of partial charges;
rr=distance between charges;
μ\mu=dipole moment

Practice & Concept Checks

Concept Check
Why does water (H2O\text{H}_2\text{O}) have a bent molecular structure rather than a linear one, despite having only two bonds?
Concept Check
Carbon dioxide (CO2\text{CO}_2) contains polar covalent bonds, yet the molecule is classified as nonpolar. Explain why.

Key Terms & Vocabulary

bond angleGeometry
The angle between any two bonds that share a common central atom.
Example: The H-O-H bond angle in water is 104.5°.
bond distanceGeometry
The straight-line distance between the nuclei of two bonded atoms.
Example: Measured in picometers (pm) or Ångstroms (Å).
Valence shell electron-pair repulsion theory (VSEPR theory)Theory
A model used to predict 3D molecular structures by assuming valence electron pairs maximize distance to minimize electrostatic repulsion.
Example: Predicts a tetrahedral shape for methane (CH4\text{CH}_4).
linearGeometries
A molecular or electron-pair geometry where two regions of electron density lie 180° apart on opposite sides of a central atom.
Example: CO2\text{CO}_2 or BeF2\text{BeF}_2.
trigonal planarGeometries
A geometry where three regions of electron density lie in a single plane 120° apart around a central atom.
Example: BCl3\text{BCl}_3.
tetrahedralGeometries
A geometry where four regions of electron density point toward the corners of a tetrahedron with 109.5° angles.
Example: CH4\text{CH}_4.
trigonal bipyramidalGeometries
A geometry where five regions of electron density form three equatorial positions (120° apart) and two axial positions (90° apart).
Example: PCl5\text{PCl}_5.
octahedralGeometries
A geometry where six regions of electron density point toward the corners of an octahedron with 90° angles.
Example: SF6\text{SF}_6.
electron-pair geometryGeometry
The spatial arrangement of all regions of high electron density, including both chemical bonds and lone pairs, around a central atom.
Example: Tetrahedral for water's four electron regions.
molecular structureGeometry
The 3D spatial arrangement and shape defined strictly by the positions of the atoms in a molecule, excluding lone pairs.
Example: Bent for water, trigonal pyramidal for ammonia.
axial positionPositions
In a trigonal bipyramidal geometry, one of the two positions located directly vertical along the main rotational axis (90° to the equatorial plane).
Example: Top and bottom positions of PCl5\text{PCl}_5.
equatorial positionPositions
In a trigonal bipyramidal geometry, one of the three positions forming a triangular plane around the equator (120° apart).
Example: Occupied by lone pairs in SF4\text{SF}_4 or ClF3\text{ClF}_3.
bond dipole momentPolarity
The separation of electrical charge within an individual polar covalent bond.
Example: H-F bond dipole.
vectorMath/Physics
A mathematical quantity possessing both magnitude and direction, used to represent bond dipoles and molecular dipoles.
Example: Bond dipole arrows.
polar moleculePolarity
A molecule possessing a net nonzero dipole moment due to polar bonds and an asymmetric 3D structure.
Example: H2O\text{H}_2\text{O} or NH3\text{NH}_3.
dipole momentPolarity
A quantitative measure of the net charge separation and overall polarity of an entire molecule.
Example: μ=0\mu = 0 for CO2\text{CO}_2, μ>0\mu > 0 for H2O\text{H}_2\text{O}.
Previous
4.5 Formal Charges and Resonance
Next
Chapter 4 Key Terms & Vocabulary