chapter 7-gaya antar molekul
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Chapter 7
Covalent Bonds and Molecular
Structure
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The Covalent Bond
• Covalent bond – formed by the sharing of
electrons between two nonmetal atoms
– Forces involved in the bond
• lectrostatic attraction between proton and
electron
• lectrostatic repulsion between electron and
electron
– !hen will a bond form"
http://1covalent_bond.rm/http://1covalent_bond.rm/
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Strengths of Bonds
• Bond dissociation energy – the amount of
energy re#uired to brea$ a bond
– nergy increases as the length of the bond gets
shorter
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%nternuclear distance
&bond length'
Covalent
radius
%nternuclear distance
&bond length'
Covalent
radius
%nternuclear distance
&bond length'
Covalent
radius
%nternuclear distance
&bond length'
Covalent
radius
Bond length and covalent radius.
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Strengths of Bonds
– Single bonds ( double bonds ( triple bonds
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)roblem
• *rrange the following bonds in order of
increasing bond strength+
– *+ C,% - C,Br - C,Cl - C,F
– B+ C,F - C,Cl - C,Br - C,%
– C+ C,Br - C,% - C,Cl - C,F
– .+ C,% - C,Br - C,F- C,Cl
– + none of these orders is correct
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)roblem
• Select the strongest bond in the following
group+
– *+ C,S
– B+ C,/
– C+ C0C
– .+ C12
– + C,F
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lectron .ot Structures
• lectron dot symbols
– *id in understanding the formation of bonds between atomic nuclei
– lemental symbol represents the type of element and all core electrons3
the valence electrons are represented by dots around the symbol
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lectron .ot Structures
• * metal in an ionic loses its electrons to achievean octet or pseudo,octet &transition elements' inits outermost shell
• * nonmetal in an ionic compound gainselectrons to achieve an octet in its outermostshell
• )eriod 4 and 5 elements of a covalentcompound share enough electrons to achievean octet
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lectron,.ot Structures
• Ionic:
• Covalent:
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lectron .ot Structures
• 6sing electron dot symbols build
– 58 5/8 C98 /58 258 C28 C/5• :east electronegative atom is often central &e;cept
'
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2aming Binary Molecular
Compounds
• lectronegativity – indicates how well an
elements nuclei attract the electrons in a
covalent bond
http://electronegavity.avi/http://electronegavity.avi/
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The )eriodic Table and
lectronegativity
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lectron .ot Structures
• 6sing electron dot symbols build
– 58 5/8 C98 /58 258 C28 C/5• :east electronegative atom is often central &e;cept
'
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lectron .ot Structures
• Single bond: * covalent bond formed by
sharing one electron pair+
• Double bond: * covalent bond formed by
sharing two electron pairs+• Triple bond: * covalent bond formed by sharing
three electron pairs+
• Single bonds are longer &wea$er' than double bonds
• .ouble bonds are longer &wea$er' than triple bonds
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lectron,dot Structures
• Step 4< Count the total valence electrons+
• Step 5< %dentify the central atom
, /ften least electronegative
• Step =< )lace all other atoms around the central
atom
• Step 9< .raw a single bond between each
e;ternal atom and the central atom
subtracting 5 electrons for each bond
drawn from the total valence electrons+
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lectron,dot Structures
• Step >< .istribute remaining valence
electrons around the e;ternal
atoms giving the e;ternal atoms
an octet
• Step ?< %f valence electrons still remain8
place them on the central atom in pairs
• Step 7< @erify that each atom has an octetA ydrogen needs only 5 electrons
A Boron needs only ? electrons
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lectron,.ot Structures
• Step < %f the central atom does not have an
octet8 form a multiple bond by bringing a
pair of electrons in from the e;ternal
atom
• Step < Calculate formal charge and minimiDe
the formal charge if acceptable
A )eriod = elements and greater can have e;panded octets ifone is necessary to minimiDe formal charge
A Formal charge 0 E valence electrons for the atom – 4 for
every dot on the atom – 4 for every line around the atom
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)roblems
• BF=
• )F=
• C5?
• %=F
• 29F
• S/95,
• GCl/=
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lectron,dot Structures and
Hesonance
• ow is the double bond formed in /="
• The correct answer is that both are correct 8
but neither is correct by itself +
O O O
O O O
O O O
or
Or from thisoxygen?
Move lone pair from
this oxygen?
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lectron,.ot Structures and
Hesonance
• !hen multiple structures can be drawn8 the actual
structure is an average of all possibilities+
• The average is called a resonance hybrid + *
straight double,headed arrow indicates resonance+
O O O O O O
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)roblem
• S=
• )/9=,
• C/=5,
• 2/5
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Molecular Shapes< The @S)H
Theory
• The appro;imate shape of molecules
is given by
Valence-Shell Electron-Pair Repulsio
n (VSEPR)
.
http://01vsepr.rm/http://01vsepr.rm/http://01vsepr.rm/http://01vsepr.rm/http://01vsepr.rm/http://01vsepr.rm/
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Molecular Shapes< The @S)H
TheoryMolecular
formula
:ewis
structure
lectron,group
arrangement
Bond
angles
Molecular
shape
&*Imn'
Count all e, groups around central
atom &*' – Single8 .ouble and Triple
bonds are all counted as 4 e, group
2ote lone pairs and
double bonds
Count bonding
and nonbonding e,
groups separately+
Step 1
Step 2
Step 3
Step 4
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)roblem
• .etermine the shape of the molecules for
which :ewis Structures have been
developed+
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@alence Bond Theory
• %f8 in order for a bond to form8 a pair of
electrons must be shared8 then how does
C form molecules with 9 bonds"
• @alence Bond Theory – hybrid orbitals
http://01hybrid_orbitals.rm/http://01hybrid_orbitals.rm/http://01hybrid_orbitals.rm/http://01hybrid_orbitals.rm/
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@alence Bond Theory
Basic )rinciple
* covalent bond forms when the orbtials of two atoms overlap
and are occupied by a pair of electrons that have the highest
probability of being located between the nuclei+
Themes
* set of overlapping orbitals has a ma;imum of two electrons
that must have opposite spins+
The greater the orbital overlap8 the stronger &more stable' the
bond+
The valence atomic orbitals in a molecule are different from
those in isolated atoms+
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@alence Bond Theory
The number of hybrid orbitals obtained e#uals the number of
atomic orbitals mi;ed+
The type of hybrid orbitals obtained varies with the types of
atomic orbitals mi;ed+
Gey )oints
sp sp5 sp= sp=d sp=d5
Types of ybrid /rbitals
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@alence Bond Theory
The conceptual steps fro olecular forula to the h!brid orbitals
used in bonding.
"olecular
forula
#e$is
structure
"olecular shape
and e% group
arrangeent
&!brid
orbitals
Step 1 Step 2 Step 3
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)roblems
• Carbon uses JJJJJJ hybrid orbitals in
ClC2+
– *+ sp
– B+ sp5
– C+ sp=
– .+ sp=d
– + sp=d 5
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The bonds in ethane.
both C are sp= hybridiDeds,sp= overlaps to σ bonds
sp=,sp= overlap to form a σ bond
relatively even
distribution of electron
density over all σ bonds
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The and bonds in eth!lene 'C2&4(
overlap in one position , σ
p overlap , π
electron density
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The and bonds in acet!lene 'C2&2(
overlap in one position , σ
p overlap ,π
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)olar Covalent Bonds<
lectronegativity
• lectronegativity – represents the ability of anatom to attract a shared pair of electrons
• igher the 2 – the more the electrons in abond will be pulled toward the atom – Most electronegative atom is F
• 2 K down a group• 2L across a period from left to right w few
e;ceptions
http://electronegavity.avi/http://electronegavity.avi/
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)olar Covalent Bonds<
lectronegativity
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)roblem
• !hich of the following elements is the
most electronegative"
– *+ S
– B+ Hu
– C+ Si
– .+ Te
– + Cs
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)roblem
• *rrange calcium8 rubidium8 sulfur8 andarsenic in order of decreasingelectronegativity+
– *+ S ( *s ( Hb ( Ca
– B+ S ( *s ( Ca ( Hb
– C+ *s ( S ( Hb ( Ca – .+ *s ( S ( Ca ( Hb
– + 2one of these orders is correct+
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)olar Covalent Bonds<
lectronegativity
• ) Ionic Character: *s a general rule for two
atoms in a bond8 we can calculate an
electronegativity difference &N2 '< N2 0
2&O' – 2&I' for I–O bond+
%f N2 - P+> the bond is covalent+
%f N2 P+> , - 5+P the bond is polar covalent+ %f N2 ( 5+P the bond is ionic+
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)roblem
• Select the most polar bond amongst the
following+
– *+ C,/
– B+ Si,F
– C+ Cl,F
– .+ C,F
– + C,%
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Molecular /rbital Theory
• The molecular orbital (MO) model provides a
better e;planation of chemical and physical
properties than the valence bond (VB) model +
– *tomic /rbital
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Molecular /rbital Theory
• *dditive cobination of orbitals &σ' is
lower in energy than two isolated 4s orbitals
and is called a bonding molecular orbital +
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Molecular /rbital Theory
• Subtractive cobination of orbitals &σ∗' is
higher in energy than two isolated 4s
orbitals and is called an antibonding
molecular orbital +
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Molecular /rbital Theory
• "olecular +rbital Diagra for &2:
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Molecular /rbital Theory
• "olecular +rbital Diagras for &2 , and&e2:
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Molecular /rbital Theory
• *dditive and subtractive combination of p
orbitals leads to the formation of both sigma
and pi orbitals+
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Molecular /rbital Theory
• Second%-o$ "+ nerg! #evel Diagras:
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Molecular /rbital Theory
• Bond +rder is the number of electron pairs
shared between atoms+
• Bond +rder is obtained by subtracting the
number of antibonding electrons from the
number of bonding electrons and dividing by 5+
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