Lewis Dot Structures

Lewis Dot Structures

High School

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Learning Objectives

• Define Lewis Dot Structures, valence electrons, and the octet rule.

• Draw Lewis Dot Structures for elements, ionic compounds, and covalent molecules.

• Identify and explain the common exceptions to the octet rule.

• Predict bond polarity and determine molecular geometry using the VSEPR model.

Key Vocabulary

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Valence Electrons

The outermost electrons of an atom that participate in chemical reactions and the formation of bonds.

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Octet Rule

The principle that atoms bond to have eight electrons in their outermost electron valence shell.

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Lone Pairs

Two valence electrons that are not shared with another atom and exist as a pair.

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Bonding Pairs

A pair of electrons shared between two atoms, which forms a covalent bond between them.

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Electronegativity

An atom's ability to attract shared electrons towards itself when forming a chemical bond.

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What Are Lewis Dot Structures?

• Lewis Dot Structures show bonding and lone electron pairs in molecules.

• They help predict a molecule's shape and how it will react.

• Each dot represents a valence electron from the atom's outermost shell.

• Atoms follow the octet rule, aiming for 8 valence electrons for stability.

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Solved Example 1
Using electronegativity values (H = 2.20, Br = 2.96), calculate the electronegativity difference for a hydrogen bromide (HBr) bond and determine its bond type.

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Step 1: Analyze and Sketch the Problem

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Solved Example 1
Using electronegativity values (H = 2.20, Br = 2.96), calculate the electronegativity difference for a hydrogen bromide (HBr) bond and determine its bond type.

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Step 2: Solve for the Unknown

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Solved Example 1
Using electronegativity values (H = 2.20, Br = 2.96), calculate the electronegativity difference for a hydrogen bromide (HBr) bond and determine its bond type.

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Step 3: Evaluate the Answer

• The calculated difference of 0.76 falls within the defined range for a polar covalent bond.

• This result is logical, as a bond between two different nonmetals typically involves unequal electron sharing, resulting in a polar covalent bond.

Multiple Choice

What is the primary purpose of drawing a Lewis Dot Structure?

• To determine the total number of protons in an atom.

• To predict the shape of a molecule and its bonding patterns.

• To calculate the molar mass of a compound.

• To show the exact location of electrons in their orbitals.

Understanding Bonds and Electron Pairs

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Single Bond

• A single bond shares one pair of electrons between two atoms.

• A total of two electrons are shared in this type of bond.

• It is represented by a single dash (—) between the atoms.

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Double Bond

• A double bond shares two pairs of electrons between two atoms.

• A total of four electrons are shared in this type of bond.

• It is represented by two dashes (=) between the two atoms.

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Triple Bond

• A triple bond shares three pairs of electrons between two atoms.

• A total of six electrons are shared in this type of bond.

• It is represented by three dashes (≡) between the two atoms.

Multiple Choice

How many total electrons are shared in a double bond, and how is it represented in a Lewis structure?

• 4 electrons, represented by two lines.

• 4 electrons, represented by four dots.

• 6 electrons, represented by three lines.

• 2 electrons, represented by one line.

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Drawing Lewis Dot Structures

Ionic Bonds

• First, draw each atom with its valence electrons to show the initial state before bonding.

• Next, show the transfer of electrons from the metal atom to the non-metal atom.

• Finally, draw the resulting ions in brackets with their respective charges shown outside.

Covalent Bonds

• Find the total number of valence electrons and identify the central atom.

• Connect atoms with single bonds and complete the octets on the outer atoms.

• If needed, form double or triple bonds to satisfy octets for all atoms.

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Solved Example 3
Create the Lewis dot structure for a carbon dioxide molecule (CO₂), following the established rules for drawing covalent molecules.

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Step 1: Analyze and Sketch the Problem

• Goal: Draw the Lewis structure for CO₂.

• Knowns: The molecule has 1 Carbon atom (4 valence electrons) and 2 Oxygen atoms (6 valence electrons each).

• Unknown: The final arrangement of atoms, bonds, and lone pair electrons.

• Formula: Follow the 5-step procedure for drawing covalent Lewis structures.

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Solved Example 3
Create the Lewis dot structure for a carbon dioxide molecule (CO₂), following the established rules for drawing covalent molecules.

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Step 2: Solve for the Unknown

• Find total valence electrons: (1 C × 4e^-) + (2 O × 6e^-) = 4 + 12 = 16 total valence electrons.

• Identify central atom: Carbon is less electronegative, so it is the central atom.

• Connect atoms with single bonds: O-C-O. This uses 4 electrons.

• Complete outer octets: Add 6 electrons (3 lone pairs) to each Oxygen. This uses all 16 electrons.

• Check octets and adjust: Each Oxygen has 8 electrons, but Carbon only has 4. To give Carbon an octet, move one lone pair from each Oxygen to form two double bonds.

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Solved Example 3
Create the Lewis dot structure for a carbon dioxide molecule (CO₂), following the established rules for drawing covalent molecules.

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Step 3: Evaluate the Answer

• The electron count is verified: 8 bonding electrons + 8 lone pair electrons = 16 electrons. This matches the total valence electrons.

• All atoms satisfy the octet rule. Each oxygen has 4 bonding and 4 lone pair electrons (total 8), and the carbon atom has 8 bonding electrons.

Multiple Choice

What is the key difference in the process of drawing Lewis structures for ionic versus covalent bonds?

• Covalent structures are only for metals, and ionic structures are for non-metals.

• Ionic structures show electron transfer, while covalent structures show electron sharing.

• Ionic structures always involve double bonds, while covalent structures do not.

• Covalent structures use brackets and charges, while ionic structures do not.

Exceptions to the Octet Rule

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Fewer Than 8 Electrons

• Some elements are stable with fewer than eight valence electrons.

• Hydrogen is stable with two, Beryllium with four valence electrons.

• Boron is an exception, being stable with six valence electrons.

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Odd Number of Electrons

• A small group of molecules has an odd number of electrons.

• This makes a full octet on each atom totally impossible.

• Examples include NO and NO₂, which have unpaired electrons.

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Expanded Octets

• Central atoms with atomic number 14 or greater can expand.

• They can accommodate more than eight electrons in their valence shell.

• Phosphorus can have 10 electrons and Sulfur can have 12.

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Solved Example 4
Draw the Lewis Dot Structure for Phosphorus Pentachloride (PCl₅), which is an exception to the octet rule, showing the arrangement of all valence electrons.

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Step 1: Analyze and Sketch the Problem

• Goal: Draw the Lewis structure for PCl₅.

• Knowns: Phosphorus (P) has 5 valence electrons. Chlorine (Cl) has 7 valence electrons. There are five Cl atoms.

• Unknown: The final arrangement of electrons in the PCl₅ molecule.

• Formula: Total Valence Electrons = (Valence e^- of P) + 5 × (Valence e^- of Cl).

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Solved Example 4
Draw the Lewis Dot Structure for Phosphorus Pentachloride (PCl₅), which is an exception to the octet rule, showing the arrangement of all valence electrons.

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Step 2: Solve for the Unknown

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Solved Example 4
Draw the Lewis Dot Structure for Phosphorus Pentachloride (PCl₅), which is an exception to the octet rule, showing the arrangement of all valence electrons.

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Step 3: Evaluate the Answer

Multiple Choice

Which of the following elements is known to be stable with an "under-filled" octet of only 6 valence electrons when it is the central atom?

• Sulfur (S)

• Boron (B)

• Oxygen (O)

• Carbon (C)

Electronegativity and Bond Polarity

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Nonpolar Covalent Bond

• ​Electrons are shared equally between two atoms in this type of bond.

• ​​The electronegativity difference between atoms is very small, from 0 to 0.4.

• ​Atoms involved have a similar ability to attract the bonding electrons.

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Polar Covalent Bond

• ​Electrons are shared unequally, favoring one atom over the other one.

• ​​The electronegativity difference is intermediate, ranging from 0.41 to 1.69.

• ​Electrons spend more time near the more electronegative atom in the bond.

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Ionic Bond

• ​Electrons are completely transferred from one atom to the other one.

• ​​This occurs with a large electronegativity difference, typically greater than 1.7.

• ​The transfer of electrons results in the formation of positive and negative ions.

Multiple Choice

If the electronegativity difference between two bonded atoms is 1.9, what type of bond is formed?

• Polar Covalent Bond

• Nonpolar Covalent Bond

• A double bond

• Ionic Bond

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Molecular Geometry and VSEPR Model

No Lone Pairs

With Lone Pairs

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Solved Example 6
What are the molecular geometry and predicted bond angle of a water molecule (H₂O)? Use the VSEPR model and the molecule's Lewis structure to explain your reasoning.

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Step 1: Analyze and Sketch the Problem

• Goal: Determine the molecular geometry and bond angle of a water molecule (H₂O).

• Knowns: The molecule consists of two Hydrogen atoms (1 valence electron each) and one Oxygen atom (6 valence electrons).

• Unknowns: The final 3D shape (molecular geometry) and the H-O-H bond angle.

• Process: We will count the total valence electrons, draw the Lewis structure, and then apply VSEPR theory.

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Solved Example 6
What are the molecular geometry and predicted bond angle of a water molecule (H₂O)? Use the VSEPR model and the molecule's Lewis structure to explain your reasoning.

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Step 2: Solve for the Unknown

• First, count the total valence electrons: (2 H atoms × 1 e^-) + (1 O atom × 6 e^-) = 8 valence electrons.

• Draw the Lewis Structure with Oxygen as the central atom. Connect the two Hydrogen atoms with single bonds, using 4 electrons.

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

• The central Oxygen atom has 2 bonding pairs and 2 lone pairs. VSEPR theory predicts that this arrangement results in a Bent molecular geometry.

• The strong repulsion from the two lone pairs compresses the bond angle to approximately 104.5°.

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Solved Example 6
What are the molecular geometry and predicted bond angle of a water molecule (H₂O)? Use the VSEPR model and the molecule's Lewis structure to explain your reasoning.

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Step 3: Evaluate the Answer

• The final Lewis structure shows that each Hydrogen atom shares 2 electrons and the Oxygen atom has a full octet (4 shared + 4 lone pair = 8), which is a stable configuration.

• According to the VSEPR model rules provided, a central atom with 2 bonding pairs and 2 lone pairs corresponds to a bent shape with a bond angle of 104.5°. The calculated answer is consistent with these rules.

Multiple Choice

According to VSEPR theory, why does a water molecule (H₂O) have a bent shape instead of a linear one?

• The molecule is ionic.

• It forms a double bond.

• The two lone pairs on the central oxygen atom repel the bonding pairs.

• Hydrogen atoms are naturally bent.

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Common Misconceptions

Misconception	Correction
All atoms must strictly follow the octet rule.	Some elements, like Boron and Sulfur, are exceptions to the octet rule.
A molecule with polar bonds is always a polar molecule.	A symmetrical molecule like CCl4 can be nonpolar even with polar bonds.
Lone pairs and bonding pairs repel each other with equal force.	Lone pairs exert a stronger repulsive force, which compresses bond angles.

Multiple Choice

What is the very first step you should take when drawing the Lewis structure for a covalent molecule like PCl₃?

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

• Check if any atoms are exceptions to the octet rule.

• Place lone pairs around the outer atoms to satisfy the octet rule.

• Determine the total number of valence electrons for all atoms.

Multiple Choice

How does the representation of electrons in the Lewis structure of an ionic compound like NaCl differ from that of a covalent compound like H₂O?

• NaCl shows a transfer of an electron and resulting ions, while H₂O shows shared electrons.

• NaCl has a central atom, while H₂O does not.

• NaCl uses lines to show bonds, while H₂O uses only dots.

• NaCl shows a triple bond, while H₂O shows single bonds.

Multiple Choice

A central atom in a molecule has three bonding pairs and one lone pair of electrons. Predict the molecule's geometry based on VSEPR theory.

• Trigonal Planar, because there are three atoms bonded to the center.

• Bent, because the lone pair cancels out one of the bonds.

• Tetrahedral, because there are four total electron pairs.

• Trigonal Pyramidal, because the lone pair repels the bonding pairs.

Multiple Choice

A molecule of CO₂ contains two polar C=O bonds, yet the molecule itself is nonpolar. What does this observation imply about the structure of CO₂?

• The molecule must have a linear and symmetrical geometry.

• The molecule must be bent, like a water molecule.

• Carbon is an exception and cannot form polar bonds.

• The electronegativity difference between Carbon and Oxygen is zero.

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Summary

• Lewis structures show valence electrons, bonding pairs, and lone pairs.

• Most atoms follow the octet rule for stability, but some exceptions exist.

• Electronegativity difference determines if a bond is ionic or covalent.

• The VSEPR model predicts a molecule's 3D geometry by minimizing electron repulsion.

• Molecular polarity depends on bond polarities and the molecule's geometric shape.

• Ionic compounds transfer electrons, while covalent molecules share them.

Poll

On a scale of 1-4, how confident are you about drawing Lewis Dot Structures and predicting molecular geometry?

• 1

• 2

• 3

• 4