Advanced Molecular Structures: Compounds With an Incomplete
Octet, Expanded Octet, Multiple Bonds, Polyatomic Ions, or
Resonance Structures
Drawing Basic Structural Formulas
1. Find the total number of valence electrons in the
polyatomic ion or molecule.
2. Write the symbol of the central atom (the LEAST
electronegative element – except Hydrogen)
3. Connect other atoms to the central atom with
SINGLE bonds (dash).
4. Count each single bond as a PAIR (2) of electrons.
5. Add electrons to the outer atoms to give each one 8
(except Hydrogen, which needs only 2).
6. Add electrons to the central atom to give it 8.
7. Check:
Does each atom have a full outer shell?
Have you used up all the valence electrons? Have
you used too many electrons?
Example: PCl3
Guidelines for Molecular Structures
1. ___________ atoms form only ______ bond.
2. ___________ atoms always form only _____ bond, and other halogens atoms usually form ONE
bond.
Exceptions occur when a halogen is the __________ atom that is bonded to smaller halogens or
to _____________.
1. Oxygen atoms normally form _______ bonds.
2. Nitrogen atoms normally form ______ bonds.
3. Carbon atoms normally form _______ bonds.
Example: Draw the molecular structure for nitrogen trichloride.
The Octet Rule in a Lewis Structure
__________________ atoms must not violate the ___________ rule (except hydrogen).
Certain central atoms MAY violate the octet rule:
________, __________, and _________ will have an INCOMPLETE OCTET because they are so small.
Elements belonging to the ________ period or higher may have an EXPANDED OCTET because they
have access to empty ________________ .
Atoms belonging to the ___________ or ___________ period will NOT have expanded octets because they
do not have d-orbitals
Example: Draw the molecular structure for boron trichloride
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Example: Draw the molecular structure for sulfur hexafluoride.
Multiple Bonds
More than _______ pair of electrons may be shared between two atoms.
Multiple bonds are ____________ than single bonds.
Multiple bonds are ____________ than single bonds.
Multiple bonds store more _____________ than single bonds.
Example: Sketch the molecular structure of carbon dioxide.
Polyatomic Ions
Many polyatomic ions contain _____________ atoms bonded covalently to each other in addition to
gaining (or losing) _____________ to make a ______________ particle.
The charge on a cation equals the number of electrons ___________________ from the original neutral
species.
The amount of negative charge on an anion represents the number of electrons ___________________.
Drawing Structure of Polyatomic Ions
Follow the same steps as before, but ____________ or ____________ electrons according to the charge of
the ion.
_______________ the structure as before.
Enclose the Lewis Structure in ____________ ___________ and write the charge for the ion
Example: Sketch the molecular structure for the ammonium ion.
Resonance Structures
Sometimes there is __________ _______ one correct Lewis structure that can be drawn.
Since they are _______________ structures but equally correct, they are considered to be
_________________, coexisting and merging together.
Electrons are said to be __________________ because they are not associated with any one pair of
bonded atoms, but rather “spread out” equally between the entire structure
Example: Sketch the molecular structure for the carbonate ion.
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Practice Problems
1. Hydrogen sulfide, H2S, is a poisonous, foul-smelling, and flammable gas. Why is a molecular
structure “ S – H – H “ incorrect for this molecule?
2. For each element listed below, decide whether it is likely to obey the octet rule, have an
incomplete octet, or could potentially have an expanded octet.
a. H
g. F
b. Be
h. Al
c. B
i. Si
d. C
j. P
e. N
k. S
f. O
l. Cl
3. The molecule of tetrafluoroethane is a building block of the synthetic material known as
Teflon ® . Tetrafluoroethane has the formula C2F4. Circle the Lewis structure for the MOST
LIKELY molecular structure.
4. Draw the Lewis Structure for a molecule of carbon tetrachloride, CCl4. Describe the number
of BONDING PAIRS of electrons and the number of NONBONDING PAIRS of electrons.
5. Draw the Lewis Structure for a molecule of ammonia, NH3. Describe the number of
BONDING PAIRS of electrons and the number of NONBONDING PAIRS of electrons.
6. Draw the Lewis Structure for a molecule of boron trifluoride. Consider the central atom in
your structure. Does it have a complete octet?
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7. Formaldehyde is used as a disinfectant, an embalming agent, and as a component in many
organic synthesis reactions. Draw the Lewis Structure for a molecule of formaldehyde,
CH2O. **HINT: Carbon is the central atom
8. Carbon monoxide is an invisible, odorless, and toxic gas that renders hemoglobin unable to
transport oxygen to body tissues. Draw the Lewis Structure for a molecule of CO.
9. Draw the Lewis Structures for each of the polyatomic ions below:
a. OH-1
b. SO4-2
**HINT: will have resonance structures
c. CN-1
10. Draw Lewis Structures for each of the following compounds in which the central atom has
an expanded octet.
a. PCl5
b. ICl4-1
c. SeBr4
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11. Draw Lewis Structures for each of the following compounds that have resonance
structures.
a. Ozone, O3
b. NO3-1
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Molecular Geometry: Going Beyond Tetrahedral
VSEPR Theory: V____________________ S_____________ E_______________ R__________________________ Theory
• Groups, or _______________, of valence electrons associated with a central atom will orient
themselves so as to _______________ the repulsion between them.
• Domains of electrons include _________ pairs, _____________ pairs, or _____________ pairs in a
double or triple bond.
• Lone pairs take up _____________ space than bonding pairs.
Balloons also naturally occupy spaces that reduce crowding.
Geometry Matters!
The arrangement of ________________ in the space around the central atom have a powerful influence
on the chemical and physical _______________ of the compound.
– Melting point
– pH
– Boiling point
– Conductivity, etc
– Solubility in water
Two Electron Domains
• Two groups of bonding electrons connect to the central atom
• Shape is ____________
• Bond angle is _______o
Examples: Please sketch each example below
BeCl2
CO2
HCN
Three Electron Domains
• Three regions, or domains, are occupied by
electrons.
• Shape depends on how many electron groups are
______________________ PAIRS and how many are
______________________ PAIRS.
Examples: Please sketch each example below
BF3
SO2
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Four Electron Domains : ____________________________ and Its Derivatives
• Four electron domains occupy the four corners of a regular tetrahedron.
• Shape depends on how many domains have BONDING PAIRS or LONE PAIRS.
Examples:
Methane
Ammonia
Water
Hydrogen Chloride
Five Electron Domains
• ______________________________ octets are only possible if the central atom belongs to period ____
(or greater) where there are empty ________________________________.
• Five electron domains are present
– three in a trigonal plane, called the ___________________ groups
– two in the __________________ groups above and below the trigonal plane.
Examples: Please sketch each example below
PCl5
SF4
ClF3
XeF2
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Six Electron Domains
• Six electron domains around the central atom point towards the corners of an octahedron.
Examples: Please sketch each example below
SF6
BrF5
ICl41-
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Practice Problems
1. What is the fundamental principal associated with VSEPR Theory?
2. Although methane, ammonia, and water each have four electron domains associated with
the central atom, the bond angles between atoms in each molecule are 109.5o, 107o, and
104.5o respectively. Explain why they are not all equal.
3. Sketch the molecular structure and determine the shape of the molecules below:
a. Tellurium tetrachloride, TeCl4
b. Carbon tetrachloride, CCl4
c. Phosphorus trifluoride, PF3
d. Sulfur dichloride, SCl2
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4. Complete the following table for each of the chemical species:
Lewis Structure
Name
Molecular Shape
5. Complete the following table for each of the chemical species:
Lewis Structure
Molecular Shape
Are There Polar
BONDS? (Yes/No)
Is the MOLECULE
Polar overall?
(Yes/No)
dichloromethane
phosgene
iodine pentafluoride
sulfur hexafluoride
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