Lewis Structures of Molecules: Lesson 4.2

Lesson 4.
Lewis Structures of
Molecules
Objective
At the end draw the Lewis dot
of the 1 structures of atoms and
lesson, you molecules.
should be
able to:
Learn about It!
Lewis Structures
• Gilbert Newton Lewis was an American physical chemist
who proposed the idea of covalent bonding and the
importance of the electron pairs in bonding.
• He devised the use of Lewis electron-dot symbols, simply known as
Lewis symbols.
• An element’s Lewis symbol is basically the element’s chemical symbol
surrounded by dots, which represent the element’s valence electrons.
• These dots are located on the top, right, bottom, and left of the symbol
with each side having up to two electrons in place.
Learn about It!
Lewis Structures
The first four valence electrons must occupy all sides off the chemical symbol
before pairing up.
Learn about It!
Electronegativity and Lewis Structures

• Electronegativity is the ability of an atom to attract
electrons towards its own density cloud.
• Nonmetals have high electronegativity values which make
them very resistive in totally transferring their electrons.
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Electronegativity and Lewis Structures
Pauling’s Electronegativity values of elements.

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Octet Rule and Covalent Bonds

• The octet rule states that an element must gain, lose or
share electrons to attain eight electrons in its valence shell
during chemical bonding.
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• A covalent compound is formed when two atoms share
electrons to achieve stability. The bond holding the two
atoms is called a covalent bond.
• Nonmetals participate in the formation of covalent bonds
since they have high ionization energies and negative
electron affinities. This ensures that they do not give out
electrons to another nonmetal, instead share what they
have.
Learn about It!

• For example, chlorine has a valence electron configuration
of 3s2 3p5 and needs an additional electron to become
stable. When chlorine forms a bond with another chlorine
atom, both atoms donate one electron to form the covalent
bond for both atoms to achieve octet.
• The bond in the chlorine molecule is composed of a shared
pair of electrons. This electron pair is formed when two
electrons from two separate orbitals of two atoms pair up,
occupying a space between the two atoms.
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Each chlorine atom shares one electron to form the chlorine molecule.
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• The pair of electrons that forms the bond (bonding
electrons) is called a bonding pair while the pair of
electrons that do not take part in bonding (nonbonding
electrons) is called lone pair.
• Note that lines could be drawn between atoms that are
bonded instead of using dots.
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Lewis structure of a chlorine molecule.

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Formation of Multiple Bonds

• Sometimes, atoms connected with single bonds do not
meet the eight-rule requirement.
• One way to reinforce octet rule in these atoms is to form
multiple bonds such as double bonds and triple bonds.
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Formation of Multiple Bonds
Lewis structure of CO2.

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Exceptions to the Octet Rule

1. Atoms possessing less than eight electrons such as
hydrogen, boron and beryllium follows reduced electron
requirements.
2. Atoms possessing odd number of electrons do not meet
the octet requirements.
3. Atoms possessing more than eight electrons exhibit
hypervalency where the elements go even higher than ten.
Try it!
Consider the molecule carbon tetrachloride,

CCl4.
Try it! Solution

CCl4.
Step 1: Determine the number of valence electrons in each
constituent element of the compound.
Carbon has 4 valence electrons. Chlorine has 7 valence

electrons.
Try it! Solution

CCl4.
Step 2: Determine the total number of valence electrons
present in the covalent compound.
The total number of valence electrons in CCl4 is 32.

Try it! Solution

CCl4.
Step 3: Draw the skeleton structure of the molecule.
Skeleton structure of CCl4

Try it! Solution

CCl4.
Step 4: Distribute the valence electrons to form the Lewis
structure.
Lewis structure of CCl4

Try it! Solution

CCl4.
Step 5:If the valence electrons seem to be deficient to satisfy
the octet rule for each element, multiple bonds can be drawn
to satisfy the rule.
In this compound, all atoms follow octet rule.

Key Points
An element’s Lewis symbol is basically the element’s

1 chemical symbol surrounded by dots, which represent the
element’s valence electrons.
When nonmetal atoms combine to form molecules, they

2 follow the octet rule and share electrons to have eight
electrons in the valence shell.
Key Points
Some compounds show exceptions to the octet rule.

These include:
● electron-deficient atoms which can only achieve duet
3 and sextet;
● electron-rich atoms which can exhibit hypervalence;
and
● odd-electron atoms which forms radicals.
Sometimes, octet rule can be reinforced by forming

4 multiple bonds.
Check Your Understanding
Draw the Lewis symbols of the following

substances.
1. Mg
2. Sb
3. PH3
4. SCl6
Challenge Yourself
Why is the least electronegative atom usually the

central atom?
Bibliography
Brown T.L. et al. 2012. Chemistry: The Central Science. Pearson Prentice Hall.
Bettelheim F.A. et al. 2015. Introduction to General, Organic and Biochemistry. Boston: Cengage
Learning.
Ebbing, Darrell and Gammon, Steven. 2016. General Chemistry. Boston: Cengage Learning.
Reger D.L. et al. 2009. Chemistry: Principles and Practice. Boston: Cengage Learning.
Spencer J.N. et al. 2010. Chemistry: Structure and Dynamics. New Jersey: John Wiley & Sons.

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Lesson 4.

Electronegativity and Lewis Structures

Electronegativity and Lewis Structures

Pauling’s Electronegativity values of elements.

Octet Rule and Covalent Bonds

Octet Rule and Covalent Bonds

Octet Rule and Covalent Bonds

Octet Rule and Covalent Bonds

Octet Rule and Covalent Bonds

Octet Rule and Covalent Bonds

Lewis structure of a chlorine molecule.

Formation of Multiple Bonds

Formation of Multiple Bonds

Lewis structure of CO2.

Exceptions to the Octet Rule

Consider the molecule carbon tetrachloride,

Consider the molecule carbon tetrachloride,

Carbon has 4 valence electrons. Chlorine has 7 valence

Consider the molecule carbon tetrachloride,

The total number of valence electrons in CCl4 is 32.

Consider the molecule carbon tetrachloride,

Skeleton structure of CCl4

Consider the molecule carbon tetrachloride,

Lewis structure of CCl4

Consider the molecule carbon tetrachloride,

In this compound, all atoms follow octet rule.

An element’s Lewis symbol is basically the element’s

When nonmetal atoms combine to form molecules, they

Some compounds show exceptions to the octet rule.

Sometimes, octet rule can be reinforced by forming

Draw the Lewis symbols of the following

Why is the least electronegative atom usually the

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