4.3 Covalent Structures
IB Chemistry SL
Mrs. Page
Essential Idea
Lewis (electron dot) structures show the electron
domains in the valence shell and are used to predict
molecular shape.
Nature of Science
Scientists use models as representatives of the real
world – the development of the model of molecular
shapes (VSEPR) to explain observable properties.
Understandings
1. Lewis (electron dot) structures show all the valence electrons in a
covalently bonded species
2. The “octet rule” refers to the tendency of atoms to gain a valence
shell with a total of eight electrons.
3. Some atoms, like Be and B, might form stable compounds with
incomplete octets of electrons.
4. Resonance structures occur when there is more than one
possible position for a double bond in a molecule.
5. Shapes of species are determined by the repulsion of electron
pairs according to the valence shell electron pair repulsion
(VSEPR) theory.
6. Carbon and silicon form covalent network (giant covalent)
structures.
Application & Skills
1. Deduction of Lewis (electron dot) structures of molecules and ions
showing all valence electrons for up to four electron pairs on each
atom.
2. The use of VSEPR theory to predict the electron domain
geometry and the molecular geometry for species with two, three,
and four electron domains.
3. Prediction of bond angles from molecular geometry and presence
of non-bonding pairs of electrons
4. Prediction of molecular polarity from bond polarity and molecular
geometry
5. Deduction of resonance structures, examples include but are not
limited to C6H6CO32- and O3
6. Explanation of the properties of covalent networks (giant covalent)
compounds in terms of their structures.
U1 &2: LEWIS STRUCTURES
Multiple ways to show the same molecule
XX
H N H
H
H N H
H
H N H
H N H
H
H
U1& 2: Lewis Structures & Ions
 We can use Lewis structures of ionic
compounds
 Must use brackets around cation and
anion (electrostatic force bonds ions)
NH NO
4
3
http://chemwiki.ucdavis.edu/
NH4Cl
U1 & 2: Rules for Lewis Structures
 Add up the total number of valence electrons in the molecule.
 Draw the skeletal structure.
 Use a line between each element to symbolize an electron
pair.
 Distribute the remaining electrons around the elements in
pairs to form octets. (Hydrogen can only ever have 2
electrons.)
 If you do not have enough to form octets, make double or
triple bonds.
 Ions must have square brackets around them with the charge
notated in the top right hand corner.
 To be a correct Lewis structure, ALL electrons must be shown.
U1 &2: Lewis Structures
 Tell us about covalent bonds
 Bonding pairs vs. Lone pairs
 Single, double or triple bonds
 They do not tell us about the
actual shape of the molecules
U5: VSEPR
•
•
•
•
Valence Shell Electron Pair Repulsion theory.
States that pairs of valence electrons repel each
other and are therefore arranged as far apart
from each other as possible.
So far we have dealt with structural formulas
which only show the types of atoms, bonds and
lone pairs of electrons. They do not show the
shape of the molecule.
VSEPR is a model that allows us to look at
shapes of molecules
U5: VSEPR – Basic Shape
•
To determine the shape you must look at the
electron domains (areas where there are pairs
of electrons)
U5: VSEPR – Basic Shape
•
•
•
Linear: central atom with two electron domains
(no lone pairs)
Bond angle 180°
Ex: BeCl2, CO2
https://www.boundless.com/
U5: VSEPR – Basic Shape
•
•
•
Trigonal Planar: central atom with three electron
domains (no lone pairs)
Bond angle 120°
Ex: BeF3, NO3-
https://www.boundless.com/
U5: VSEPR – Basic Shape
•
•
•
Tetrahedral: central atom has four electron
domains (no lone pairs)
Bond Angle 109.5°
Ex: CH4, [NH4]+
Solid Wedge = bond in front
Dashed Wedge = bond toward back
https://www.boundless.com/
Effect of Lone Pairs
✘ Molecular Geometry gives the shape of the
molecule
✘Electron Domain Geometry is based on the number
of electron domains
✘Electron domains can be bonded pairs or lone pairs
✘Lone pair electrons occupy more space than bonding
pairs and therefore alter the bond angles from
molecular geometry
✘The more lone pairs, the greater the repulsion and
therefore the greater the impact on the bond angle
U5: VSEPR – V-Shaped (Bent)
•
•
•
•
e- Geometry: Trigonal Planar
Molecular Geometry: (V-shaped or bent) :
central atom has three electron domains (1 lone
pair)
Bond angle is <120°
Ex: SO2, SF2, [NO2]-
U5: VSEPR – Trigonal Pyramidal
•
•
•
•
e- Geometry: Tetrahedral
Molecular Geometry: Trigonal Pyramidal
central atom has four electron domains (1 lone
pair)
Bond angle is <109.5°
Ex: NF3, NH3, [H3O]+, [SO3]2-
U5: VSEPR – Tetrahedral Bent
•
•
•
•
e- Geometry: Tetrahedral
Molecular Geometry: Bent (V-Shaped) central
atom has four electron domains (2 lone pairs)
Bond angle is <109.5°
Ex: H2O, [SO3]2-
Bonding
Groups on
Central Atom
Lone Pairs on
Central Atom
e- Domain
Geometry
Molecular
Geometry
Bond Angle
2
0
Linear
Linear
180
3
0
Trigonal Planar
Trigonal Planar
120
4
0
Tetrahedral
Tetrahedral
109.5
Bent (Vshaped)
<120
2
1
Trigonal Planar
3
1
Tetrahedral
Trigonal
Pyramidal
<109.5
2
2
Tetrahedral
Bent (Vshaped)
<109.5
Example
Other Effects on Bond Angles
•
Bond angle depends on the electron domains but
is not the exact same measure for all molecules
with the same domains
• Bond angle is effected by the type of atoms,
electronegativity differences, and multiple bonds as well
• You do not need to know EXACT bond angles however
should be able to predict which molecular and which
electron domain geometry
• NOTE: You should also be able to predict geometries of
oxoanions (polyatomic ions containing oxygen)
Using Lewis Structures; Predict the molecular
geometry, the electron domain geometry and bond
angles for the following
•
•
•
•
•
•
CCl4
NH4+
NF3
SF2
[NO2][SO3]2-
Check answers in
book, pp. 110-114
QUIZ (15 minutes)
Using Lewis Structures (2); Predict the molecular
geometry (1), the electron domain geometry (1)
and bond angles (1) for the following (15 marks
total)
• XeO3
• CH3+
• ClO4-
U3:Exceptions to Octet Rule
• Hydrogen will never have more than 2 electrons.
• Some elements such as Be and B may have an
incomplete octet when bonding
• Be has only 4 electrons in BeCl2 bond
• Boron only has 6 valence electrons in BF3
• Some elements like S and P can have expanded
octets which hold more than 8 electrons.
U3:Exceptions to Octet Rule
• Coordinate covalent bonds are formed when
both electrons originate from the same atom.
• An arrow is used to denote the direction in a
coordinate covalent bond showing the atom
from which both electrons originated.
Your Turn
Draw the Lewis structures showing the
coordinate (dative) bond
1. CO3 22. CO
3. NO3 -
U4 & A5: Resonance Structures
 Resonance is a concept used to describe the structures
when there are multiple ways to depict the same
molecule.
 If you can put a double bond in more than one position,
you will be expected to draw the resonance structures.
 The electrons are actually delocalized in the areas of
the double bonds and are spread out equally among all
bonding positions.
 Bond strength and length are in between that of single
and double bonds.
U4 & A5: Resonance Structures
 Resonance structures allow us to depict all the
possible positions of the double bonds.
 The true structure, however, is an intermediate
form known as a resonance hybrid.
 Double arrows are placed between all resonance
structures.
https://www.studyblue.com/notes/note/n/final-exam-prep/deck/2821905
A5:Resonance & Benzene
http://chemistry.tutorvista.com/organic-chemistry/benzene.html
A5:Resonance & Carbonate
http://chemwiki.ucdavis.edu/
A5:Resonance & Ozone
http://www.mikeblaber.org/oldwine/chm1045/notes/Bonding/Resonan/Bond07.htm
A5: Resonance Practice
Draw all resonance structures for the following polyatomic
molecules and ions. Be sure to draw polyatomic ions in
brackets and include the charge on each.
1.
2.
3.
4.
5.
6.
Formate ion, CHO2Cyclobutadiene, C4H4
Ozone, O3
Sulfur Dioxide, SO2
Carbonate ion, CO32Chlorate ion, ClO3-
A4: Molecular Polarity
• Bond polarity refers to a specific bond
within a compound
• Molecular polarity is the polarity of the
molecule as a whole
• We can have a non-polar molecule even if
the molecule contains polar bonds
A4: Molecular Polarity
Steps to deduce molecular polarity:
• Determine molecular geometry with
VSEPR
• Determine polarity of each bond using
electronegative values ∆p, show with
vectors and dipole moment - +
• Add vectors to determine if there is a net
dipole moment, . If so the molecule is
polar
A4: Molecular Polarity
Try it:
• SF2
• BF3
• CO2
• NH3
• H 2O
• CS2
Check solutions in
book pp. 115-116
U6 & A6: Covalent Networks
• Allotropes: different structures of the same element
• Carbon has 4 allotropes; graphite, diamond,
graphene, and C60 fullerene
• 3 of these allotropes, graphite, diamonds and
graphene are examples of covalent network solids
– also known as giant 3-D covalent structures
• C60 fullerene is a molecule
* Another example of a covalent network solid is quartz
(SiO2)
U6 & A6: Covalent Networks
Properties of Covalent Network Solids
• High melting points (>1000°C) due to MANY strong
covalent bonds
• Generally poor conductors (except graphite and
graphene)
• Usually insoluble in most substances
• Generally very hard (except graphite – sheets
slide)
U6 & A6: Covalent Networks
Graphite
• Each carbon is bonded to 3 other carbon atoms in a
trigonal planar geometry – these covalent bonds are
strong
• Carbon atoms form layers of hexagonal rings
• Layers connected by weak intermolecular forces called
London forces
• Electrons delocalized allowing conduction
• Often used as a lubricant because layers can slide past
each other due to weak London forces
• Pencil lead
U6 & A6: Covalent Networks
Diamonds
• Each carbon is bonded to 4 other carbon atoms in a
tetrahedral geometry
• Form a lattice structure
• One of hardest substance on earth due to bonding
and interlocking tetrahedrons
• Very high melting & boiling point (strong bonds)
• No delocalized electrons (no conduction)
• Insoluble
U6 & A6: Covalent Networks
Graphene
• Each carbon is bonded to 3 other carbon atoms
• Form a lattice structure that is densely packed
• Atoms arranged hexagonally and one atom thick
• One of thinnest and strongest materials known
• Excellent conductor
• Transparent
• Flexible
U6 & A6: Covalent Networks
C60 fullerene (buckyballs)
• 60 carbon atoms
• 20 hexagonal surfaces
• 12 pentagonal surfaces
• Each carbon bonded to 3 other carbons
• A molecule (not a covalent network solid)
• Black solids
• Insoluble in water but soluble in non-polar solvants
• Not conductive
U6 & A6: Covalent Networks
Silicon Dioxide (Quartz)
• Another example of a covalent network solid
• Each silicon bonded to 4 oxygens
• Each oxygen bonded to 2 silicon atoms
• High melting and boiling points (strong covalent
bonds)
• Not conductive
EXTRA PRACTICE
Formula
SO3
BeCl2
PO4 3SO2
N3 XeO3
NH2 NO2 ClO2 H3O+
NH3
Lewis
Structure
(Show
Resonance
Structures)
Complete the following:
Molecular
Geometry
e- Domain
Geometry
Bond Angle
Polar
Molecule?