Lewis Structures and Molecular Geometry

Lewis Structures and Molecular Geometry

High School

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Learning Objectives

• Explain the Valence Shell Electron Pair Repulsion (VSEPR) theory.

• Draw Lewis structures to represent molecular bonding.

• Predict molecular geometry and bond angles using VSEPR theory.

• Distinguish between different molecular shapes like linear, bent, and tetrahedral.

Key Vocabulary

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Molecular Geometry

The specific 3D arrangement of atoms within a molecule, determined by atomic nuclei positions.

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VSEPR Theory

A model that predicts molecular geometry by minimizing repulsion between pairs of valence electrons.

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Lewis Structure

A diagram showing bonding between atoms in a molecule and any lone pairs of electrons present.

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Lone Pair

A pair of valence electrons that are not shared with another atom in a covalent bond.

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Bond Angle

The angle formed between two adjacent bonds on the same central atom in a molecule.

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What is Molecular Geometry?

• Molecular geometry is the three-dimensional arrangement of atoms in a molecule.

• Valence Shell Electron Pair Repulsion (VSEPR) theory helps predict molecular shape.

• Electron pairs, shared and unshared, repel and get as far apart as possible.

• This repulsion determines the final three-dimensional shape of the molecule.

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Solved Example 1
Using VSEPR theory, determine the molecular geometry, AXE notation, and bond angle for a molecule of phosphorus trichloride (PCl₃).

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Step 1: Analyze and Sketch the Problem

• Goal: Find the molecular geometry, AXE notation, and bond angle of PCl₃.

• Knowns: The molecule is phosphorus trichloride (PCl₃).

• Formula: First, sum the valence electrons. Then, draw the Lewis structure to identify bonding and lone pairs on the central atom. Finally, use VSEPR rules to determine the geometry.

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Solved Example 1
Using VSEPR theory, determine the molecular geometry, AXE notation, and bond angle for a molecule of phosphorus trichloride (PCl₃).

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Step 2: Solve for the Unknown

• Valence Electrons: Phosphorus (Group 15) has 5. Chlorine (Group 17) has 7. Total = 5 + 3(7) = 26 electrons.

• Lewis Structure: P is the central atom. After forming 3 P-Cl bonds and giving each Cl a full octet, one lone pair of electrons remains on the central P atom.

• VSEPR Analysis: The central P atom has 3 bonding pairs and 1 lone pair.

• Conclusion: The AXE notation is AX₃E₁, which corresponds to a trigonal pyramidal geometry with a bond angle of approximately 107°.

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Solved Example 1
Using VSEPR theory, determine the molecular geometry, AXE notation, and bond angle for a molecule of phosphorus trichloride (PCl₃).

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Step 3: Evaluate the Answer

• Check Electrons: The Lewis structure uses 3 bonds (6e^-) + 3 lone pairs on each Cl (18e^-) + 1 lone pair on P (2e^-) = 26 valence electrons. This is correct.

• Check Geometry: According to VSEPR theory, a central atom with 3 bonding domains and 1 lone pair will have a trigonal pyramidal shape. The answer is consistent with the model.

Multiple Choice

What is the fundamental principle of the VSEPR theory?

• The number of protons in the nucleus determines the shape of the molecule.

• Electron pairs in the valence shell attract each other to form the most compact shape.

• Electron pairs in the valence shell repel each other, moving as far apart as possible to determine the molecular shape.

• Only shared electron pairs are considered when determining molecular geometry.

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Steps for Drawing Lewis Structures

• Sum the total valence electrons for all atoms in the molecule.

• Identify the central atom and connect other atoms with single bonds.

• Add lone pairs to outer atoms, placing any extras on the central atom.

• Form double or triple bonds if the central atom lacks a full octet.

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Solved Example 2
Following the 5-step process, what is the Lewis structure for a water molecule (H₂O)? Determine the total valence electrons, central atom, and the arrangement of bonds and lone pairs to create a stable molecule.

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Step 1: Analyze and Sketch the Problem

• Goal: Draw the Lewis structure for a water molecule (H₂O).

• Knowns: Hydrogen (H) has 1 valence electron. Oxygen (O) has 6 valence electrons.

• Unknown: The final arrangement of electrons as bonds and lone pairs in the molecule.

• Formula: We will use the 5-step process for drawing Lewis Structures.

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Solved Example 2
Following the 5-step process, what is the Lewis structure for a water molecule (H₂O)? Determine the total valence electrons, central atom, and the arrangement of bonds and lone pairs to create a stable molecule.

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Step 2: Solve for the Unknown

• Step 1: Total valence electrons = (2 H atoms × 1 e^-) + (1 O atom × 6 e^-) = 8 valence electrons.

• Step 2: Oxygen is the central atom as hydrogen can only form one bond.

• Step 3: Draw single bonds from the oxygen to each hydrogen (H-O-H). This uses 4 electrons (2 per bond).

• Step 4: Place the remaining 4 electrons (8 - 4 = 4) on the central oxygen atom as two lone pairs.

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Solved Example 2
Following the 5-step process, what is the Lewis structure for a water molecule (H₂O)? Determine the total valence electrons, central atom, and the arrangement of bonds and lone pairs to create a stable molecule.

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Step 3: Evaluate the Answer

• Check for full octets. Each hydrogen has a full duet (2 shared electrons). The oxygen has a full octet (4 shared electrons + 4 lone pair electrons).

• The structure is stable.

Multiple Choice

When drawing a Lewis structure, what is the correct step after you have placed bonds from the central atom to the outer atoms?

• Draw double or triple bonds to all outer atoms.

• Immediately check if all atoms have a full octet.

• Place all remaining electrons on the central atom.

• Fill in lone pairs on the outer atoms before placing any remaining electrons on the central atom.

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Shapes: Linear vs. Bent

Linear (AX₂)

• This shape occurs when a central atom is bonded to two other atoms and has no lone pairs.

• The repulsion between the two bonded pairs is minimized when they are on opposite sides of the atom.

• This results in a 180° bond angle, creating a straight line, as seen in CO₂.

Bent (AX₂E or AX₂E₂)

• This shape occurs when a central atom is bonded to two atoms and has one or two lone pairs.

• Lone pairs exert greater repulsion than bonding pairs, pushing the bonds on the atom closer together.

• This results in a bond angle less than 180°, such as in H₂O which is about 104.5°.

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Solved Example 3
Using VSEPR theory, predict the molecular geometry and bond angle for Beryllium Dichloride (BeCl₂). Start by determining the total number of valence electrons.

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Step 1: Analyze and Sketch the Problem

• Goal: Predict the molecular geometry and bond angle of BeCl₂.

• Knowns: The molecule is Beryllium Dichloride (BeCl₂).

• Unknown: The molecular geometry and bond angle.

• Formula: We will use the VSEPR theory steps: first, find the total valence electrons; second, draw the Lewis structure; and third, determine the geometry based on electron pairs.

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Solved Example 3
Using VSEPR theory, predict the molecular geometry and bond angle for Beryllium Dichloride (BeCl₂). Start by determining the total number of valence electrons.

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Step 2: Solve for the Unknown

• First, calculate the total valence electrons: Be (1 atom × 2 e^-) + Cl (2 atoms × 7 e^-) = 2 + 14 = 16 valence electrons.

• Next, draw the Lewis Structure. Be is the central atom. Connect the two Cl atoms with single bonds (Cl-Be-Cl). This uses 4 electrons.

• Distribute the remaining 12 electrons to the outer Cl atoms (6 each) to satisfy their octets.

• Finally, analyze the central atom. Be has 2 bonded pairs and 0 lone pairs. According to VSEPR theory, this AX₂ arrangement minimizes repulsion.

• This results in a linear geometry with a bond angle of 180°.

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Solved Example 3
Using VSEPR theory, predict the molecular geometry and bond angle for Beryllium Dichloride (BeCl₂). Start by determining the total number of valence electrons.

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Step 3: Evaluate the Answer

• The Lewis structure correctly uses all 16 valence electrons.

• The VSEPR model for a central atom with two bonded electron groups and zero lone pairs (AX₂) consistently predicts a linear shape and a 180° bond angle.

Multiple Choice

What key feature distinguishes a bent molecule from a linear molecule, given both have a central atom bonded to two other atoms?

• The presence of one or more lone pairs on the central atom.

• The total number of valence electrons in the molecule.

• The type of atoms involved in the bonds.

• The presence of double bonds.

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Shapes: Trigonal Planar vs. Trigonal Pyramidal

Trigonal Planar

• ​A central atom is bonded to three other atoms and has no lone pairs of electrons.

• ​​This creates a flat, triangular shape with bond angles of exactly 120°.

• ​A common example is sulfur trioxide, which has the chemical formula SO₃.

Trigonal Pyramidal

• ​A central atom is bonded to three other atoms and has one lone pair of electrons.

• ​​The lone pair creates a pyramid shape with bond angles less than 120°.

• ​A common example is ammonia, which has the chemical formula NH₃.

Multiple Choice

A molecule with the formula AX₃ has a trigonal pyramidal shape instead of trigonal planar. Why?

• The central atom has one lone pair, which repels the bonding pairs and creates a pyramid shape.

• The central atom has no lone pairs, forcing a flat shape.

• The bond angles are exactly 120°.

• The molecule contains double bonds.

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Shape: Tetrahedral

• This shape occurs when a central atom is bonded to four other atoms.

• The central atom in this shape has no lone pairs of electrons (AX₄).

• To minimize repulsion, the four bonded atoms have bond angles of 109.5°.

• Examples include Methane (CH₄) and Carbon Tetrabromide (CBr₄).

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Solved Example 5
Predict the molecular geometry and bond angles for Silane (SiH₄) by first drawing its Lewis structure and determining the arrangement of electron pairs around the central atom.

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Step 1: Analyze and Sketch the Problem

• Goal: Determine the molecular geometry and bond angles for Silane (SiH₄).

• Knowns: The molecular formula is SiH₄.

• Unknown: The molecular geometry and bond angles.

• Formula: Use the VSEPR theory rules by first calculating the total valence electrons and drawing the Lewis structure.

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Solved Example 5
Predict the molecular geometry and bond angles for Silane (SiH₄) by first drawing its Lewis structure and determining the arrangement of electron pairs around the central atom.

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Step 2: Solve for the Unknown

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Solved Example 5
Predict the molecular geometry and bond angles for Silane (SiH₄) by first drawing its Lewis structure and determining the arrangement of electron pairs around the central atom.

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Step 3: Evaluate the Answer

• The Lewis structure uses all 8 valence electrons, giving the central silicon atom a stable octet and each hydrogen atom a stable duet.

• The tetrahedral geometry is correct for a central atom with four bonding pairs and no lone pairs, as this arrangement minimizes electron repulsion.

Multiple Choice

Which of the following conditions correctly describes a molecule with a tetrahedral geometry?

• A central atom with 4 bonded pairs and 0 lone pairs, with 109.5° bond angles.

• A central atom with 2 bonded pairs and 2 lone pairs, with 104.5° bond angles.

• A central atom with 3 bonded pairs and 1 lone pair, with 107° bond angles.

• A central atom with 4 bonded pairs and 1 lone pair, with <109.5° bond angles.

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Advanced Shapes: Trigonal Bipyramidal & Octahedral

Trigonal Bipyramidal (AX₅)

• A central atom is bonded to five other atoms, creating this specific molecular shape.

• This shape is characterized by having two different bond angles: 90° and 120°.

• An example of this molecular geometry is Phosphorus Pentafluoride (PF₅).

Octahedral (AX₆)

• This shape occurs when a central atom is bonded to six other atoms.

• In this structure, all atomic positions are equivalent, and all bond angles are 90°.

• A common example of this shape is Sulfur Hexafluoride (SF₆).

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Solved Example 6
What is the molecular geometry and the corresponding bond angle for Sulfur Hexafluoride (SF₆)? Sulfur has 6 valence electrons, and Fluorine has 7.

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Step 1: Analyze and Sketch the Problem

• Goal: Determine the molecular geometry and bond angles of SF₆.

• Knowns: The molecule is SF₆. Sulfur (S), the central atom, has 6 valence electrons. Fluorine (F) has 7 valence electrons.

• Unknown: The molecular geometry and bond angles for SF₆.

• Formula: VSEPR theory will be used to predict the shape based on electron pair arrangement.

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Solved Example 6
What is the molecular geometry and the corresponding bond angle for Sulfur Hexafluoride (SF₆)? Sulfur has 6 valence electrons, and Fluorine has 7.

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Step 2: Solve for the Unknown

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Solved Example 6
What is the molecular geometry and the corresponding bond angle for Sulfur Hexafluoride (SF₆)? Sulfur has 6 valence electrons, and Fluorine has 7.

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Step 3: Evaluate the Answer

• All 48 valence electrons are used. The 6 electron domains around the central sulfur atom repel each other equally, arranging themselves into an octahedral shape.

• The resulting 90° bond angles confirm the maximum possible separation for this arrangement. The answer is correct.

Multiple Choice

A molecule has a central atom bonded to six other atoms. What is its molecular geometry and bond angle?

• Tetrahedral, 109.5°

• Trigonal Bipyramidal, 90° and 120°

• Octahedral, 90°

• Linear, 180°

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Common Misconceptions

Misconception	Correction
Lewis structures show the 3D shape of a molecule.	They only show electron placement, not the actual 3D shape.
All molecules with three atoms are linear.	Lone pairs on the central atom can create a bent shape (e.g., H2O).
Electron geometry and molecular geometry are the same.	Electron geometry includes all electron pairs; molecular geometry shows only atom arrangement.

Multiple Choice

Why does the H₂O molecule have a bent shape with a 104.5° bond angle, while CO₂ has a linear shape with a 180° bond angle?

• H₂O has single bonds, while CO₂ has double bonds, which always results in a linear shape.

• CO₂ has lone pairs on its central atom, while H₂O does not.

• Oxygen is more electronegative than carbon, which forces the bonds into a bent shape.

• The central oxygen in H₂O has two lone pairs that repel the bonding pairs, whereas the central carbon in CO₂ has no lone pairs.

Multiple Choice

According to VSEPR theory, what is the main reason for the difference in bond angles between BF₃ (trigonal planar) and NH₃ (trigonal pyramidal)?

• The presence of Fluorine in BF₃ creates larger bond angles than the Hydrogen in NH₃.

• Trigonal pyramidal is a flat molecule, while trigonal planar is 3D.

• Both have the same bond angles, but different shapes.

• Trigonal planar has 120° angles due to three bonding pairs and no lone pairs, while the lone pair in trigonal pyramidal repels bonds, reducing the angle to ~107°.

Multiple Choice

A molecule has the formula AX₃. Analysis shows its central atom has one lone pair of electrons. Predict its molecular geometry, approximate bond angle, and provide a known example.

• Tetrahedral, 109.5°, CH₄

• Trigonal Pyramidal, ~107°, NH₃

• Trigonal Planar, 120°, BF₃

• Bent, <120°, O₃

Multiple Choice

Sulfur can form both sulfur dichloride (SCl₂) and sulfur hexafluoride (SF₆). Analyze and contrast the expected molecular geometry and bond angles for these two compounds, explaining your reasoning using VSEPR theory.

• SCl₂ is trigonal pyramidal (~107°) and SF₆ is trigonal bipyramidal (90° and 120°).

• SCl₂ has a bent shape (~105°) due to lone pairs on the central sulfur, while SF₆ is octahedral (90°) with no lone pairs on the sulfur.

• SCl₂ is linear (180°) and SF₆ is trigonal planar (120°).

• Both are tetrahedral because sulfur is the central atom.

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Summary

• Molecular geometry describes the 3D arrangement of atoms, determined by VSEPR theory.

• Lewis structures help identify the number of bonding and lone pairs of electrons.

• The number of bonding and lone pairs determines the final shape of the molecule.

• Lone pairs have greater repulsion than bonding pairs, resulting in smaller bond angles.

Poll

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

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