VSEPR and Molecular Geometry

VSEPR and Molecular Geometry

High School

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Learning Objectives

• Explain how VSEPR theory predicts molecular shapes using electron pair repulsion.

• Identify the five parent molecular geometries, from linear to octahedral.

• Determine how lone pairs affect a molecule's parent geometry and final shape.

• Predict a molecule's polarity using its geometry and overall symmetry.

Key Vocabulary

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VSEPR Theory

A model that predicts the 3-D geometry of molecules from electron pairs around central atoms.

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Molecular Geometry

The three-dimensional arrangement of atoms in a molecule, which determines many of its important properties.

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Electron Region

A chemical bond or lone pair of electrons surrounding a central atom in a molecule.

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Lone Pair

A pair of valence electrons not shared with another atom, which strongly repels other electrons.

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Bond Angle

The angle formed between three atoms across at least two bonds, defining molecular shape and geometry.

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Foundations of VSEPR Theory

• VSEPR theory predicts molecular shape based on electron pair repulsion.

• Electrons around a central atom stay as far apart as possible.

• An electron region can be a lone pair or any bonding pair.

• Repulsion strength: lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair.

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Solved Example 1
Predict the molecular geometry and bond angles of ammonia (NH₃) using VSEPR theory.

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Step 1: Analyze and Sketch the Problem

• Goal: To predict the molecular geometry and bond angles of ammonia (NH₃).

• Knowns: The chemical formula is NH₃. Nitrogen (N) is the central atom. N has 5 valence electrons; each Hydrogen (H) has 1 valence electron.

• Unknown: The molecular geometry and bond angles of the NH₃ molecule.

• Formula: Total valence electrons = 5 + 3(1) = 8. The Lewis structure has 3 bonding pairs and 1 lone pair, creating 4 electron regions around the central atom.

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Solved Example 1
Predict the molecular geometry and bond angles of ammonia (NH₃) using VSEPR theory.

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Step 2: Solve for the Unknown

• Electron Geometry: With 4 electron regions, the electron geometry is tetrahedral, which has ideal bond angles of 109.5°.

• Molecular Geometry: The molecule has 3 bonding pairs and 1 lone pair. The lone pair repels the bonding pairs more strongly than they repel each other.

• This repulsion pushes the three hydrogen atoms closer together, resulting in a trigonal pyramidal molecular shape.

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Solved Example 1
Predict the molecular geometry and bond angles of ammonia (NH₃) using VSEPR theory.

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Step 3: Evaluate the Answer

• The lone pair-bonding pair repulsion is greater than the bonding pair-bonding pair repulsion, and this stronger repulsion decreases the angle between the N-H bonds.

• The H-N-H bond angle will be slightly less than the ideal tetrahedral angle of 109.5°. It is experimentally found to be approximately 107°.

Multiple Choice

According to VSEPR theory, what is the primary principle that determines the shape of a molecule?

• The maximization of attraction between bonding pairs.

• The number of atoms in the molecule.

• The minimization of repulsion between valence shell electron pairs.

• The attraction between lone pairs of electrons.

Linear and Trigonal Planar Geometries

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Linear

• Occurs when there are two electron regions around the central atom.

• The atoms are arranged in a straight line with 180° bond angles.

• An example of a linear molecule is Beryllium chloride, BeCl₂.

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Trigonal Planar

• This geometry occurs with three electron regions and no lone pairs.

• The atoms form a flat triangle shape with bond angles of 120°.

• An example of this molecular shape is Boron trifluoride, BF₃.

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Bent

• This shape forms when one of three electron regions is a lone pair.

• The lone pair's repulsion pushes the other bonding pairs much closer together.

• This results in a bent shape with bond angles less than 120°.

Multiple Choice

A molecule has a central atom with 3 electron regions, one of which is a lone pair. What is its molecular shape and approximate bond angle?

• Bent, <120°

• Trigonal planar, 120°

• Linear, 180°

• Tetrahedral, 109.5°

Shapes from Tetrahedral Geometry

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Tetrahedral

• This is the parent shape for four electron regions.

• The central atom is bonded to four other atoms.

• Bond angles are 109.5°, like in methane (CH₄).

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Trigonal Pyramidal

• This shape forms when one electron region is a lone pair.

• The lone pair reduces the bond angles to about 107°.

• An example of this molecular shape is ammonia (NH₃).

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Bent

• This shape occurs when two electron regions are lone pairs.

• Two lone pairs cause even greater repulsion between bonded atoms.

• This compresses the bond angle down to approximately 104°.

Multiple Choice

What is the molecular shape of a molecule with a central atom that has 4 electron regions, two of which are bonding pairs and two are lone pairs?

• Bent

• Linear

• Trigonal pyramidal

• Tetrahedral

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Trigonal Bipyramidal Geometry

• Trigonal bipyramidal is the parent shape for five electron regions, like PCl₅.

• With one lone pair occupying an equatorial position, the shape becomes seesaw (SF₄).

• The molecular shape is T-shaped when two lone pairs occupy equatorial positions.

• With three lone pairs in equatorial positions, the molecule becomes linear in shape.

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Solved Example 3
Predict the molecular geometry, shape, and bond angles for sulfur tetrafluoride (SF₄).

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Step 1: Analyze and Sketch the Problem

• Goal: To find the VSEPR characteristics for SF₄.

• Knowns: Central atom: S (6 valence electrons). Surrounding atoms: 4 F (7 valence electrons each).

• Unknown: The total valence electrons and the number of electron regions around the central atom.

• Formula: Total valence electrons = 6 + 4(7) = 34. This means there are 5 electron regions (4 bonding, 1 lone pair).

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Solved Example 3
Predict the molecular geometry, shape, and bond angles for sulfur tetrafluoride (SF₄).

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Step 2: Solve for the Unknown

• Parent Geometry: With 5 electron regions, the parent geometry is trigonal bipyramidal.

• Molecular Shape: The presence of 1 lone pair on the central atom results in a seesaw shape.

• Lone Pair Position: The lone pair occupies an equatorial position to minimize electron-electron repulsion.

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Solved Example 3
Predict the molecular geometry, shape, and bond angles for sulfur tetrafluoride (SF₄).

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Step 3: Evaluate the Answer

• The F-S-F bond angles between the axial and equatorial fluorine atoms are slightly less than 90° due to repulsion from the lone pair.

• The F-S-F bond angle between the two equatorial fluorine atoms is compressed to less than 120°.

Multiple Choice

A molecule with a trigonal bipyramidal parent geometry has two lone pairs on its central atom. What will its final molecular shape be?

• Linear

• Trigonal pyramidal

• T-shaped

• Seesaw

Octahedral Geometry: 6 Electron Regions

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Octahedral

• This is the parent shape for six electron regions.

• A central atom is bonded to six other surrounding atoms.

• All bond angles in this geometry are either 90° or 180°.

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Square Pyramidal

• This shape occurs when one lone pair of electrons is present.

• The five atoms form a pyramid with a square-shaped base.

• An example of this molecular shape is IF₅.

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Square Planar

• This shape is formed when two lone pairs of electrons exist.

• Lone pairs are opposite to each other to minimize repulsion.

• An example molecule is Xenon tetrafluoride (XeF₄).

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Solved Example 4
Given a central atom with 6 electron regions, predict the molecular geometry if 2 of these regions are lone pairs. Provide an example of a molecule with this geometry.

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Step 1: Analyze and Sketch the Problem

• Goal: Determine the molecular geometry for a central atom with 6 electron regions and 2 lone pairs.

• Knowns: Total electron regions = 6; Lone pairs = 2.

• Unknown: Molecular Geometry.

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Solved Example 4
Given a central atom with 6 electron regions, predict the molecular geometry if 2 of these regions are lone pairs. Provide an example of a molecule with this geometry.

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Step 2: Solve for the Unknown

• The parent geometry for 6 electron regions is octahedral.

• With 2 lone pairs, the lone pairs will arrange themselves on opposite sides of the central atom (180° apart) to minimize repulsion.

• This arrangement results in the remaining 4 bonding atoms being in the same plane as the central atom.

• The resulting molecular geometry is square planar.

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Solved Example 4
Given a central atom with 6 electron regions, predict the molecular geometry if 2 of these regions are lone pairs. Provide an example of a molecule with this geometry.

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Step 3: Evaluate the Answer

• An example is Xenon tetrafluoride (XeF₄). Xenon has 8 valence electrons, and each of the 4 Fluorine atoms contributes 1 electron for a single bond. This leaves 2 lone pairs on the central Xenon atom.

• The geometry is correctly identified as square planar, consistent with VSEPR theory for AX₄E₂ systems.

Multiple Choice

Xenon tetrafluoride (XeF₄) has an octahedral parent geometry with two lone pairs on the central xenon atom. What is its molecular shape?

• Square pyramidal

• Square planar

• T-shaped

• Octahedral

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Molecular Shape and Polarity

Nonpolar Molecules

• Symmetrical molecules are typically nonpolar, with an even distribution of electrical charge.

• Even if individual bonds are polar, they cancel each other out due to symmetry.

• This results in a molecule with no overall positive or negative end.

Polar Molecules

• Asymmetrical molecules are polar because their polar bonds do not cancel each other out.

• This asymmetry can be caused by lone pairs of electrons on the central atom.

• It also occurs if different atoms are bonded to the central atom, creating imbalance.

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Solved Example 5
Determine if Carbon Dioxide (CO₂) is a polar or nonpolar molecule based on its molecular geometry.

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Step 1: Analyze and Sketch the Problem

• Goal: Determine the polarity of a CO₂ molecule.

• Knowns: The molecule is CO₂, and carbon is the central atom.

• Unknown: The molecule's geometry and overall polarity.

• Formula: Use Lewis structure, VSEPR theory, and molecular symmetry.

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Solved Example 5
Determine if Carbon Dioxide (CO₂) is a polar or nonpolar molecule based on its molecular geometry.

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Step 2: Solve for the Unknown

• Lewis Structure: O=C=O. The central carbon atom has two double bonds and no lone pairs.

• VSEPR Geometry: With two electron regions (the two double bonds) and no lone pairs on the central atom, the molecule has a linear geometry. The bond angle is 180°.

• Symmetry and Polarity: Although each C=O bond is polar (due to the electronegativity difference between carbon and oxygen), the molecule is symmetrical. The two bond dipoles are equal in magnitude and point in opposite directions, so they cancel each other out.

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Solved Example 5
Determine if Carbon Dioxide (CO₂) is a polar or nonpolar molecule based on its molecular geometry.

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Step 3: Evaluate the Answer

• The linear and symmetrical shape of the CO₂ molecule causes the individual bond polarities to cancel out.

• Therefore, the Carbon Dioxide (CO₂) molecule is nonpolar.

Multiple Choice

When is a molecule considered polar?

• When it contains only nonpolar bonds.

• When it is asymmetrical, often due to the presence of lone pairs on the central atom.

• When it has a linear or square planar shape.

• When it is symmetrical, and the effects of polar bonds cancel out.

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Common Misconceptions

Misconception	Correction
Lone pairs don't affect the shape of a molecule.	Lone pairs have the strongest repulsion, influencing molecular shape and bond angles.
The shape is determined by the number of atoms.	Shape is determined by all electron regions arranging to minimize repulsion.
A molecule containing polar bonds must be polar.	Symmetrical molecules like CCl4 can be nonpolar if their dipoles cancel out.

Multiple Choice

How does the repulsion hierarchy (LP-LP > LP-BP > BP-BP) explain why the bond angle in ammonia (NH₃) is 107° while in methane (CH₄) it is 109.5°?

• Methane has more atoms, which spreads them out further.

• Nitrogen is less electronegative than carbon, causing the bonds to compress.

• Ammonia is a polar molecule, which always reduces bond angles.

• The lone pair on nitrogen in NH₃ repels the bonding pairs more strongly than the bonding pairs in CH₄ repel each other.

Multiple Choice

A molecule is described as having a central atom with 5 electron domains. What are the possible molecular shapes this molecule can have, and what factor determines which shape it will adopt?

• Linear or trigonal planar, depending on the size of the atoms.

• Octahedral or square pyramidal, depending on the atoms' electronegativity.

• Trigonal bipyramidal, seesaw, T-shaped, or linear, depending on the number of lone pairs.

• Tetrahedral, trigonal pyramidal, or bent, depending on the number of double bonds.

Multiple Choice

A molecule with the formula AX₃ has a trigonal planar molecular geometry. What can you infer about the polarity of the A-X bonds and the overall polarity of the molecule?

• The bonds must be polar, and the molecule is polar.

• The molecule is polar because it has three atoms, which always creates an asymmetrical shape.

• The bonds must be nonpolar, and the molecule is nonpolar.

• The bonds can be polar, but the molecule is nonpolar due to its symmetrical shape.

Multiple Choice

If a central atom has 6 electron regions, predict the change in molecular shape and polarity as you replace one bonding pair with a lone pair, and then a second bonding pair with another lone pair.

• It goes from octahedral (polar) to T-shaped (polar), then to linear (nonpolar).

• It goes from trigonal bipyramidal (nonpolar) to seesaw (polar), then to T-shaped (polar).

• It goes from octahedral (nonpolar) to square pyramidal (polar), then to square planar (nonpolar).

• It goes from tetrahedral (nonpolar) to trigonal pyramidal (polar), then to bent (polar).

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Summary

• VSEPR theory predicts molecular shape by minimizing electron repulsion.

• The five parent geometries are Linear, Trigonal Planar, Tetrahedral, Trigonal Bipyramidal, and Octahedral.

• Lone pairs repel more than bonding pairs, affecting the final molecular shape.

• A molecule's shape determines its polarity; symmetrical shapes are typically nonpolar.

Poll

On a scale of 1-4, how confident are you in predicting molecular geometry using VSEPR theory?

• 1

• 2

• 3

• 4