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Lecture PowerPoint
ChemistryThe Molecular Nature ofMatter and Change
Sixth Edition
Martin S. Silberberg
Copyright © The McGraw-Hill Companies, Inc. Permission required or reproduction or display.
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Chapter 11
Theories of Covalent onding
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11-"
Theories of Covalent onding
11!1 "alence ond #"$ Theory and %r&ital 'y&ridi(ation
11!) Modes of %r&ital %verlap and the Types of Covalent onds
11!* Molecular %r&ital #M%$ Theory and Electron +elocali(ation
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"alence ond #"$ Theory
The $asic principle o %& theory'
( co)alent $ond orms when the or$itals o two atoms
overlap and a pair o electrons occupy the o)erlap region.
The space ormed $y the o)erlapping or$itals canaccommodate a maximum of two electrons and these
electrons must ha)e opposite (paired) spins.
The greater the or$ital o)erlap, the stronger the $ond.
*+tent o or$ital o)erlap depends on or$ital shape and direction.
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,igure 11!1 %r&ital overlap and spin pairing in ')!
( co)alent $ond results rom the o)erlap o or$itals rom two atoms.
The shared space is occupied $y two electrons, which ha)e opposite spins.
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,igure 11!) %r&ital orientation and maximum overlap!
Hydrogen luoride, H. luorine, !.
The greater the e+tent o or$ital o)erlap, the stronger the $ond.
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" Theory and %r&ital 'y&ridi(ation
The or$itals that orm when $onding occurs are different
rom the atomic or$itals in the isolated atoms.
I no change occurred, we could not account or the molecular shapes
that are o$ser)ed.
(tomic or$itals 0mi+ or hybridize when $onding occurs to
orm hybrid orbitals.
The spatial orientation o these hy$rid or$itals correspond with
o$ser)ed molecular shapes.
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,eatures of 'y&rid %r&itals
The number o hy$rid or$itals ormed equals the num$er
o atomic or$itals mi+ed.
The type o hy$rid or$itals ormed varies with the types oatomic or$itals mi+ed.
The shape and orientation o a hy$rid or$ital maximizes o)erlap with the other atom in the $ond.
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,igure 11!* ,ormation and orientation of sp hy&rid or&itals
and the &onding in eCl)!
or$ital $o+ diagrams
atomic
or$itals
hy$rid
or$itals
4ne !s and one ! p atomic or$ital mi+ to orm two sp hy$rid or$itals.
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,igure 11!* continued
$o+ diagram with or$ital contours
4)erlap o &e and Cl or$itals to orm &eCl!
.
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,igure 11!- The sp2 hy&rid or&itals in ,*!
Mi+ing one s and two p or$itals gi)es three sp!
hy$rid or$itals.The third ! p or$ital remains unhy$ridi6ed.
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,igure 11!- continued
The three sp! or$itals point to the corners o an equilateral triangle,
their a+es 1!57 apart.
*ach hal-illed sp! or$ital o)erlaps with the hal-illed ! p or$ital o a
atom.
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,igure 11!. The sp3 hy&rid or&itals in C'-!
The our sp" or$itals adopt a
tetrahedral shape.
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,igure 11!/ The sp3 hy&rid or&itals in N'*!
The 8 lone pair occupies an sp"
hy$rid or$ital, gi)ing a trigonal
pyramidal shape.
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,igure 11!/ continued The sp3 hy&rid or&itals in ')%!
The 4 lone pairs occupy sp"
hy$ridor$itals, gi)ing a $ent shape.
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,igure 11!0 The sp3d hy&rid or&itals in PCl.!
The ormation o more than our$onding or$itals requires d or$ital
in)ol)ement in hy$ridi6ation.
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,igure 11! The sp3d 2 hy&rid or&itals in S,/!
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Ta&le 11!1 Composition and %rientation of 'y&rid %r&itals!
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,igure 11!2 ,rom molecular formula to hy&rid or&itals!
Molecular
,ormula
igure 15.15
Step )
Molecular shape
and e3 group
arrangement
Lewis
structure
igure 15.1
Step 1
'y&rid or&itals
Step *
Ta$le 11.1
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Sample Pro&lem 11!1 Postulating 'y&rid %r&itals in a Molecule
S%L4T5%N6
PL7N6 9e use the molecular ormula to draw the :ewis structure and
determine the electron-group arrangement around eachcentral atom. 9e then postulate the type o hy$rid or$itals
required and write a partial or$ital diagram.
P8%LEM6 ;se partial or$ital diagrams to descri$e how mi+ing o
the atomic or$itals o the central atom
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Sample Pro&lem 11!1
9:
!s
! p
↑ ↑
isolated C atom
↑ ↑ ↑ ↑
sp3
hy$ridi6ed C atom
C has our hal-illed sp" or$itals.
9:
!sisolated 4 atom
↑: ↑: ↑ ↑
sp3
hy$ridi6ed 4 atom
! p
↑: ↑ 9
The 4 atom has two hal-illed sp"
or$itals and two illed with lone pairs.
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Sample Pro&lem 11!1
#a$ ># The electron-group arrangement is
trigonal $ipyramidal, so the central> atom is sp3d hy$ridi6ed.
9:
"s
isolated > atom hy$ridi6ed > atom
" p
↑: ↑ 9
"d
↑ ↑ ↑ ↑
sp3d
9
"d
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Limitations of the 'y&ridi(ation Model
Hy$ridi6ation is not always consistent with o$ser)ed
molecular shapes.
This is particularly true or the $onding o larger atoms.
The $ond angle in H!> is closer to the angle$etween unhy$ridi6ed p or$itals.
d -4r$itals do not hy$ridi6e eecti)ely with s and p or$itals,
which are much lower in energy and more sta$le.
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Types of Covalent onds
( sigma
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,igure 11!1< The &onds in ethane #C)'/$!
&oth C are sp*
hy&ridi(ed
&ond formed &y s3sp3
overlap
End3to3end sp*3sp* overlap to
form a &ond
( ? $ond is cylindrically symmetrical, with its
highest electron density along the $ond a+is.
Copyright A The McGraw-Hill Companies, Inc. Permission required or reproduction or display.
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,igure 11!1< continued
There is relati)ely e)en distri$ution o electron density o)er all σ $onds.
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,igure 11!11 The and &onds in ethylene #C)'-$!
7 &ond has two regions ofelectron density!
unhy&ridi(ed ) p or&itals
Copyright A The McGraw-Hill Companies, Inc. Permission required or reproduction or display.
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11-!2
Each C is sp hy&ridi(ed and has
two unhy&ridi(ed p or&itals!
,igure 11!1) The and &onds in acetylene #C)')$! Copyright A The McGraw-Hill Companies, Inc. Permission required or reproduction or display.
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Sample Pro&lem 11!) +escri&ing the Types of onds in
Molecules
S%L4T5%N6
P8%LEM6 escri$e the types o $onds and or$itals in acetone,
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Sample Pro&lem 11!)
The sp" hy$ridi6ed C atoms orm ? $onds using sp" hy$rid or$itals.
The sp! hy$ridi6ed C and 4 atoms orm ? $onds using sp! hy$rid
or$itals, and the π $ond o the CB4 dou$le $ond is ormed using p
or$itals.
&ond #shown with molecule
rotated 2$!
; &onds
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,igure 11!1- 8estricted rotation around a &ond!
cis31=)3+ichloroethylene
trans31=)3+ichloroethylene
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Molecular %r&ital #M%$ Theory
The com$ination o or$itals to orm $onds is )iewed as thecom$ination o wave functions.
Atomic wa)e unctions
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7mplitudes of wave
functions added
,igure 11!1.
7n analogy &etween light waves and atomic wave functions!
7mplitudes of
wave functions
su&tracted
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,igure 11!1/ Contours and energies of ') &onding and
anti&onding M%s!
The $onding M4 is lower in energy and the anti$onding M4 is higher in
energy than the (4s that com$ined to orm them.
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Molecular %r&ital +iagrams
(n !" diagram, Dust li@e an atomic or$ital diagram,shows the relati)e energy and num$er o electrons in
each M4.
The M4 diagram also shows the (4s rom which eachM4 is ormed.
&ond order is calculated as ollows'
EF
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,igure 11!10 M% diagram for ')!
H! $ond order B E
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Electrons in Molecular %r&itals
K M4s are illed in order o increasing energy.
K (n M4 can hold a ma+imum o ! e- with opposite spins.
K 4r$itals o equal energy are hal-illed, with spins
parallel, $eore pairing spins.
*lectrons are placed in M4s Dust as they are in (4s.
( molecular electron coniguration shows the type oM4 and the num$er o e- each contains. or H! the
coniguration is
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,igure 11!1 M% diagram for 'e)? and 'e)!
He!L $ond order B E He! $ond order B 5
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11-#5
Sample Pro&lem 11!* Predicting Sta&ility of Species 4sing M%
+iagrams
P8%LEM6 ;se M4 diagrams to ind $ond orders and predict
whether H!L and H!J e+ist. I either e+ists, write its
electron coniguration.
PL7N6 >ince the 1s (4s orm the M4s, the M4 diagrams are similar
to the one or H!. 9e ind the num$er o electrons in each
species and distri$ute them one at a time to the M4s ollowing
the rules or or$ital illing. 9e calculate the $ond order and
predict sta$ility.
S%L4T5%N6
H!L
has one electron to place in its M4s while H!-
has three electronsto place.
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Sample Pro&lem 11!*
@1s
,or ')3= the &ond order is
A#) B 1$ AD
so we predict that ')3 exists!
The configuration is #;1s$)#; $1
,or ')?= the &ond order is
A#1 B
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,igure 11!12 onding in s3&loc homonuclear diatomic molecules!
Li)
Li) &ond order 1
e)
e) &ond order <
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,igure 11!)< Shapes and energies of and M%s from
com&inations of ) p atomic or&itals!
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,igure 11!)1 8elative M% energy levels for Period ) homonuclear
diatomic molecules!
M4 energy le)els
or 4!, !, and 8e!
without !s-! p
mi+ing
M4 energy le)els
or &!, C!, and 8!
with !s-! p
mi+ing
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,igure 11!)* The paramagnetic properties of %)!
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Sample Pro&lem 11!- 4sing M% Theory to Explain ond Properties
P8%LEM6 *+plain the ollowing data with diagrams showing the
occupancy o M4s'
PL7N6 The data show that remo)ing an electron rom each parent
molecule has opposite eects' 8!L has a wea@er longer $ond
than 8!, $ut 4!L has a stronger, shorter $ond than 4!. 9e
determine the )alence electrons in each species, draw the
sequence o M4 energy le)els
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?!s
? !s
π! p
π ! p
?! p
? ! p
↑:
8!
↑:
↑: ↑:
↑:
8!L
↑:
↑:
↑: ↑:
↑
?!s
? !s
π! p
π ! p
?! p
? ! p
4!
↑:
↑:
↑:
↑: ↑:
99
4!L
↑:↑:
↑:
↑:
↑:
9
Sample Pro&lem 11!- S%L4T5%N6
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11-#3
Sample Pro&lem 11!-
Calculating &ond orders6,or N) A# B )$ * ,or N)
? A#0 B )$ )!.
N)? has a longer= weaer &ond than N) &ecause to form N)
?= a
bonding electron is removed and the &ond order decreases!
,or %) A# B -$ ) ,or %)? A# B *$ )!.
%)? has a shorter= stronger &ond than %) &ecause to form %)
?= an
antibonding electron is removed and the &ond order increases!
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,igure 11!)-7 The M% diagram for ',!
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,igure 11!). The lowest energy 3&onding M%s in &en(ene
and o(one!
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