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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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11-!

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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11-#

"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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11-

,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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11-

,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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11-!5

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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11-!1

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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11-!!

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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11-!"

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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11-!#

Types of Covalent onds

 ( sigma 

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11-!

,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.

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11-!

,igure 11!1< continued

There is relati)ely e)en distri$ution o electron density o)er all σ $onds.

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11-!/

,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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11-"5

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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11-"!

,igure 11!1- 8estricted rotation around a &ond!

cis31=)3+ichloroethylene

trans31=)3+ichloroethylene

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11-""

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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11-"#

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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11-"

,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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11-"

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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11-"/

,igure 11!10 M% diagram for ')!

H! $ond order B E

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11-"2

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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11-"3

,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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11-#1

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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11-#!

,igure 11!12 onding in s3&loc homonuclear diatomic molecules!

Li)

Li) &ond order 1

e)

e) &ond order <

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11-#"

,igure 11!)< Shapes and energies of and M%s from

com&inations of ) p atomic or&itals!

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11-##

,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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11-#

,igure 11!)* The paramagnetic properties of %)!

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11-#/

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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11-#2

?!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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11-5

,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!