4.3 Covalent Structures

4.3 CovalentStructures

Pg 104

Understandings• Lewis (electron dot) structures show all the valence electrons in

covalently bonded species

• The “octet rule” refers to the tendency of atoms to gain a valenceshell with a total of eight electrons.

• Some atoms, like Be and B, might form stable compounds withincomplete octets of electrons

• Resonance structures occur when there is more than one possibleposition for a double bond in a molecule

• Shapes of species are determined by the repulsion of electronpairs according to the valence shell electron pair repulsion(VSEPR) theory

• Carbon and silicon form covalent network (giant covalent)structures

• Lewis (electron dot) structures show all the valence electrons incovalently bonded species

• The “octet rule” refers to the tendency of atoms to gain a valenceshell with a total of eight electrons.

• Some atoms, like Be and B, might form stable compounds withincomplete octets of electrons

• Resonance structures occur when there is more than one possibleposition for a double bond in a molecule

• Shapes of species are determined by the repulsion of electronpairs according to the valence shell electron pair repulsion(VSEPR) theory

• Carbon and silicon form covalent network (giant covalent)structures

Applications and skills• Deduction of Lewis (electron dot) structure of molecules and ions to

show all valence electrons for up to four electron pairs on each atom.

• The use of VSEPR theory to predict the electron domain geometry andthe molecular geometry for species with two, three and four electrondomains.

• Prediction of bond angles from molecular geometry and presence ofnon-bonding pairs of electrons

• Prediction of molecular polarity from bond polarity and moleculargeometry.

• Deduction of resonance structures, examples include but are nonlimited to C6H6, CO3

2- and O3.

• Explanation of the properties of covalent network (giant covalent)compounds in terms of their structures.

• Deduction of Lewis (electron dot) structure of molecules and ions toshow all valence electrons for up to four electron pairs on each atom.

• The use of VSEPR theory to predict the electron domain geometry andthe molecular geometry for species with two, three and four electrondomains.

• Prediction of bond angles from molecular geometry and presence ofnon-bonding pairs of electrons

• Prediction of molecular polarity from bond polarity and moleculargeometry.

• Deduction of resonance structures, examples include but are nonlimited to C6H6, CO3

2- and O3.

• Explanation of the properties of covalent network (giant covalent)compounds in terms of their structures.

Drawing Lewis structures1. Calculate total number of valence electrons

• Multiply group number by number of atoms informula

• Total these

2. Draw symbols of elements showing how the atoms arelinked

3. Draw one bond between each atom• pair of crosses, dots or a single line

4. Add more electrons to complete octet• Except hydrogen and helium

5. Form double or triple bonds if there are not enoughelectrons to form octets

6. Double check the total number of electrons

1. Calculate total number of valence electrons• Multiply group number by number of atoms in

formula• Total these

2. Draw symbols of elements showing how the atoms arelinked

3. Draw one bond between each atom• pair of crosses, dots or a single line

4. Add more electrons to complete octet• Except hydrogen and helium

5. Form double or triple bonds if there are not enoughelectrons to form octets

6. Double check the total number of electrons

Practice

• CF4

• H2O

• CO2

• C2H4

• CF4

• H2O

• CO2

• C2H4

Dative bonds(Coordinate covalent)• Dative bonds (coordinate bonds) – both electrons

in a bond come from one atom• Sometimes shown with an arrow

Examples:

H3O+ NH4+ CO

• Dative bonds (coordinate bonds) – both electronsin a bond come from one atom

• Sometimes shown with an arrow

Examples:

H3O+ NH4+ CO

The octet rule is not alwaysfollowed• Incomplete octet – fewer than 8 valence electrons

• BeCl2 and BCl3 are examples• These have very small central atoms

• Incomplete octet – fewer than 8 valence electrons• BeCl2 and BCl3 are examples

• These have very small central atoms

• Expanded octet – more than 8 valence electrons• Example PCl5 or SF4• Central atoms are large from period 3 and beyond• CNOF (can not over fill)

• Discussed more in HL

• Expanded octet – more than 8 valence electrons• Example PCl5 or SF4• Central atoms are large from period 3 and beyond• CNOF (can not over fill)

• Discussed more in HL

VSEPR TheoryVSEPR Theory

VSEPR

• The shape of a molecule is determined by repulsionbetween electron pairs

• VSEPR – valance shell electron pair repulsion

• Electron pairs found in the outer energy level orvalence shell of atoms repel each other and thusposition themselves as far apart as possible

• The shape of a molecule is determined by repulsionbetween electron pairs

• VSEPR – valance shell electron pair repulsion

• Electron pairs found in the outer energy level orvalence shell of atoms repel each other and thusposition themselves as far apart as possible

How to predict the shape of amolecule1. Bonding and non-bonding pairs of electrons repel

2. Double and triple bonded electrons behave as asingle unit called a electron domain

3. Total number of electron domains around thecentral atom determine the geometricarrangement of electrons

1. Bonding and non-bonding pairs of electrons repel

2. Double and triple bonded electrons behave as asingle unit called a electron domain

3. Total number of electron domains around thecentral atom determine the geometricarrangement of electrons

4. The shape of the molecule is determined by thebond angles between atoms

5. The order of repulsion is as follows(the stronger the repulsion thefurther apart the pair of electrons will be)

• Lone pair - lone pair strongest repulsion

• Lone pair – bonding pair• Bonding pair – bonding pair weakest repulsion

4. The shape of the molecule is determined by thebond angles between atoms

5. The order of repulsion is as follows(the stronger the repulsion thefurther apart the pair of electrons will be)

• Lone pair - lone pair strongest repulsion

• Lone pair – bonding pair• Bonding pair – bonding pair weakest repulsion

Two Domain Centers• Linear shape• Charge centers will be 180o apart

• Examples:

CO2: O=C=O or Cl – Be – Cl

• Linear shape• Charge centers will be 180o apart

• Examples:

CO2: O=C=O or Cl – Be – Cl

Three Electron DomainCenters

• Planar triangular shape• Charge centers will be 120o apart

• If one of the charge centers is a lone pair of electrons itwill be bent. This will slightly alter the bond angles.

• Planar triangular shape• Charge centers will be 120o apart

• If one of the charge centers is a lone pair of electrons itwill be bent. This will slightly alter the bond angles.

Four Electron Domain Centers• Tetrahedral shape• Charge centers will be 109.5o apart

• One lone pair trigonal pyramidal• Two lone pairs – Bent or v-shaped

• Tetrahedral shape• Charge centers will be 109.5o apart

• One lone pair trigonal pyramidal• Two lone pairs – Bent or v-shaped

Molecules with polarbonds are not always

polar

Molecules with polarbonds are not always

polar

• Polar bond – a bond that unequally shares electrons(differ in electronegativity by up to about 1.8)

• Polar molecule – a molecule that has a partiallypositive and a partially negative side

• Depends on:• The polar bonds it contains• The orientation of the polar bonds

• Polar bond – a bond that unequally shares electrons(differ in electronegativity by up to about 1.8)

• Polar molecule – a molecule that has a partiallypositive and a partially negative side

• Depends on:• The polar bonds it contains• The orientation of the polar bonds

• Molecules that contain polar bonds will be non-polarmolecule if the dipoles cancel out

• Draw a structural diagram for the followingmolecules, include partial charges:

CO2 BF3

CBr4 NH3

Note: these molecules are non-polar because theirdipoles cancel out!

• Molecules that contain polar bonds will be non-polarmolecule if the dipoles cancel out

• Draw a structural diagram for the followingmolecules, include partial charges:

CO2 BF3

CBr4 NH3

Note: these molecules are non-polar because theirdipoles cancel out!

• The molecule will be polar if:a) It contains polar bonds

b) The bonds are not symmetrical or have differentpolarities

c) The molecule has a slightly negative and a slightlypositive side

Draw a structural diagram for the following molecules,include partial charges:

CO2 BF3 CBr4 NH3

• The molecule will be polar if:a) It contains polar bonds

b) The bonds are not symmetrical or have differentpolarities

c) The molecule has a slightly negative and a slightlypositive side

Draw a structural diagram for the following molecules,include partial charges:

CO2 BF3 CBr4 NH3

Resonance StructuresResonance StructuresPg 115

• Resonance involves using two or more Lewisstructures to represent a particular molecule or ion.

• Can not be fully described with one Lewis structure

Some covalentsubstances formcrystalline solids

Some covalentsubstances formcrystalline solids

• Crystalline solid• Crystalline structure which contain covalent bonds• Repeating pattern• Sometimes called a giant molecular structure or

macromolecule

• Allotrope – different forms of an element in the samephysical state

• Different bonding patterns• Different physical properties

• Crystalline solid• Crystalline structure which contain covalent bonds• Repeating pattern• Sometimes called a giant molecular structure or

macromolecule

• Allotrope – different forms of an element in the samephysical state

• Different bonding patterns• Different physical properties

Allotropes of Carbon - Graphite• Each carbon atom bonds to 3 other

carbon atoms forming hexagons in parallel layers

• Bond angle 120c (360 ÷ 3)

• The fourth electron pair becomes delocalised over the wholeof the sheet of atoms

• Graphite can conduct electricity because of these mobile electrons

• Layers are held together by weak van der Waals’ forces

• Physical properties: dull, grey, solid, brittle, lower density thatdiamond due to spaces between layers.

• Example: used in pencils

• Each carbon atom bonds to 3 othercarbon atoms forming hexagons in parallel layers

• Bond angle 120c (360 ÷ 3)

• The fourth electron pair becomes delocalised over the wholeof the sheet of atoms

• Graphite can conduct electricity because of these mobile electrons

• Layers are held together by weak van der Waals’ forces

• Physical properties: dull, grey, solid, brittle, lower density thatdiamond due to spaces between layers.

• Example: used in pencils

Allotropes of Carbon -Graphene• First two dimensional crystal ever discovered• Different from graphite because it consists of a

single planer sheet of carbon atoms arrangedhexagonally

• Only one atom in thickness• Very conductive• Graphene rolled up forms carbon nanotubes

• First two dimensional crystal ever discovered• Different from graphite because it consists of a

single planer sheet of carbon atoms arrangedhexagonally

• Only one atom in thickness• Very conductive• Graphene rolled up forms carbon nanotubes

Allotropes of Carbon - Diamond

• Each carbon atom is covalently bondedto 4 other diamonds

• Tetrahedral arrangement in arepeating pattern

• Physical properties: lustrous crystal, does not conductelectricity (no mobile electrons), extremely hard

• Examples: used in jewellery or in tools for cuttingglass

• Each carbon atom is covalently bondedto 4 other diamonds

• Tetrahedral arrangement in arepeating pattern

• Physical properties: lustrous crystal, does not conductelectricity (no mobile electrons), extremely hard

• Examples: used in jewellery or in tools for cuttingglass

Allotropes of Carbon – Fullerene C60aka buckeyball (buckminsterfullerene)

• Consists of 60 carbon atoms

• Each carbon is bonded to three others

• 12 pentagons and 20 hexagons

• Forms a spherical shape

• Easily accepts electrons to form negative ions

• Semi conductor (Some electron mobility)

• Consists of 60 carbon atoms

• Each carbon is bonded to three others

• 12 pentagons and 20 hexagons

• Forms a spherical shape

• Easily accepts electrons to form negative ions

• Semi conductor (Some electron mobility)

Diamond Graphite Fullerene C60

Bonding Covalentbonding

Covalent bonding andweak van der Waals’

Covalent bonding andweak van der Waals’

Delocalizedelectrons

No delocalizedelectrons

Delocalized electrons Delocalized electrons

Structure Giantstructure / 3D

Layered structure /2D / planar

Consists of molecules

Spheres made of atomsarranged in hexagons andarranged in hexagons andpentagons

Bond angles 1090 120o Between 109-120o

Number ofatoms eachcarbon is bondedto

4 3 3

Bond types(more to comelater)

sp3 sp2 sp2 and sp3

• http://www.youtube.com/watch?v=fuinLNKkknI

Silicon

• Forms a giant lattice (similar to diamond)• Group 4 : 4 valence electrons• Tetrahedrally bonded structure

Silicon dioxide (silica or quartz)• Forms giant covalent structure

• Know the ratio of Si:O but not the specific number

• Tetrahedrally bonded structure around carbons

• Each silicon atom is covalently bonded to four oxygenatoms

• Each oxygen atom is covalently bonded to two siliconatoms

• Forms giant covalent structure• Know the ratio of Si:O but not the specific number

• Tetrahedrally bonded structure around carbons

• Each silicon atom is covalently bonded to four oxygenatoms

• Each oxygen atom is covalently bonded to two siliconatoms

Structure and Bonding of Carbon AllotropesDiamond Graphite Fullerene C60

Bonding Covalentbonding

Covalent bonding andweak van der Waals’

Covalent bonding andweak van der Waals’

Delocalizedelectrons

No delocalizedelectrons

Delocalized electrons Delocalized electrons

Structure Giantstructure / 3D

Layered structure /2D / planar

Consists of molecules

Spheres made of atomsarranged in hexagons andpentagons

Consists of molecules

Spheres made of atomsarranged in hexagons andpentagons

Bond angles 1090 120o Between 109-120o

Number ofatoms eachcarbon is bondedtoo

4 3 3

Bond types(more to comelater)

sp3 sp2 sp2 and sp3