apchapt8.ppt

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Chapter 8

Bonding


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What is a Bond? A force that holds atoms together.

Why?

We will look at it in terms of energy. Bond energy the energy required to

break a bond.

Why are compounds formed? Because it gives the system the lowest

energy.


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onic Bonding An atom with a low ioni!ation energy

reacts with an atom with high electron

affinity.

"he electron moves.

#pposite charges hold the atoms

together.


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Coulomb$s %aw&' (.)* + *,-* / 0 nm12*2(34r

2 is the charge.

r is the distance between the centers. f charges are opposite5 & is negative

e+othermic

6ame charge5 positive &5 requires

energy to bring them together.


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What about covalent

compounds? "he electrons in each atom are attracted

to the nucleus of the other.

"he electrons repel each other5 "he nuclei repel each other.

"he reach a distance with the lowest

possible energy.

"he distance between is the bond length.


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0

E n e r g y

Internuclear Distance


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0

E n e r g y



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0

E n e r g y



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0

E n e r g y



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0

E n e r g y


Bond Length


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0

E n e r g y


Bond Energy


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Covalent Bonding&lectrons are shared by atoms.

"hese are two e+tremes.

n between are polar covalent bonds. "he electrons are not shared evenly.

#ne end is slightly positive5 the other

negative.

ndicated using small delta δ.


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H - Fδ7 δ-


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H - F

δ7 δ-

H - F

δ 7

δ - H

- F δ 7 δ -

H -

F

δ 7

δ -

H

- F δ 7 δ -

H - F δ 7 δ -

H

- F

δ 7

δ -

H -

F

δ 7

δ -


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H - F

δ7 δ-

H - F

δ 7

δ - H

- F δ 7 δ -

H -

F

δ 7

δ -

H

- F δ 7 δ -

H - F δ 7 δ -

H

- F

δ 7

δ -

H -

F

δ 7

δ -

7-


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H - F

δ7 δ-

H - F

δ7 δ-

H - Fδ7 δ- H - F

δ7 δ-

H - Fδ7 δ-

H - Fδ7 δ-

H - Fδ7 δ- H - F

δ7 δ-

- 7


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&lectronegativity "he ability of an electron to attract

shared electrons to itself.

auling method maginary molecule 9:

&+pected 9-: energy ' 9-9 energy

7 :-: energy ( ∆ ' 19-:3 actual - 19-:3e+pected


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&lectronegativity ∆ is known for almost every element

;ives us relative electronegativities ofall elements.

"ends to increase left to right.

decreases as you go down a group.

ifference in electronegativity between

atoms tells us how polar.


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Electronegativity

difference

Bond

Type

Zero

Intermediate

Large

ovalent

!olar

ovalent

Ionic

ov

alenth

aracter

de

cr eas

es

Ionic

h

ara

c

terinc

re

ases


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>ipole oments A molecule with a center of negative

charge and a center of positive charge

is dipolar 1two poles35

or has a dipole moment.

Center of charge doesn=t have to be

on an atom.Will line up in the presence of an

electric field.


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9ow t is drawn

H - Fδ7 δ-


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Which olecules 9ave "hem? Any two atom molecule with a polar

bond.

With three or more atoms there aretwo considerations.

"here must be a polar bond.

;eometry can=t cancel it out.


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;eometry and polarity "hree shapes will cancel them out.

%inear


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;eometry and polarity "hree shapes will cancel them out.

lanar triangles

"#0$


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;eometry and polarity#thers don=t cancel

Bent


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;eometry and polarity#thers don=t cancel

"rigonal yramidal


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ons Atoms tend to react to form noble gas

configuration.

etals lose electrons to form cations


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onic CompoundsWe mean the solid crystal.

ons align themselves to ma+imi!eattractions between opposite charges5

and to minimi!e repulsion between likeions.

Can stabili!e ions that would be unstableas a gas.

@eact to achieve noble gas configuration


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6i!e of ions on si!e increases down a group.

Cations are smaller than the atomsthey came from.

Anions are larger.

across a row they get smaller5 andthen suddenly larger.

irst half are cations.

6econd half are anions.


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eriodic "rends Across the period nuclear charge

increases so they get smaller.

&nergy level changes between anionsand cations.

%i7*

Be7(

B7)

C7


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6i!e of soelectronic ions so - same

so electronic ions have the same of

electrons Al7) g7(


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6i!e of soelectronic ionsositive ions have more protons so

they are smaller.

Al

7)

g7(


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orming onic Compounds Lattice energy - the energy associated

%ith ma&ing a solid ionic compound

from its gaseous ions'

71g3 7 :-1g3 → :1s3 This is the energy that (pays) for

ma&ing ionic compounds'

Energy is a state function so %e can getfrom reactants to products in a round

a*out %ay'


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Calculating %attice &nergy %attice &nergy ' k12*2( 4 r3

k is a constant that depends on the

structure of the crystal.2=s are charges.

r is internuclear distance.

%attice energy is greater with morehighly charged ions.


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Calculating %attice &nergy "his bigger lattice energy HpaysI for

the e+tra ioni!ation energy.

Also HpaysI for unfavorable electronaffinity.


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Bonding


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+ I

o n i c

h a r a c t e r

Electronegativity difference

#,+

,0+

,+


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9ow do we deal with it?f bonds can=t be ionic5 what are ionic

compounds?

And what about polyatomic ions? An ionic compound will be defined as

any substance that conducts electricity

when melted. Also use the generic term salt.


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"he Covalent Bond

"he forces that cause a group of atoms tobehave as a unit.

Why?

>ue to the tendency of atoms to achieve the

lowest energy state.

t takes *DF( k/ to dissociate a mole of C9

into its ions

6ince each hydrogen is hooked to the carbon5we get the average energy ' *) k/4mol


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C9)Cl has ) C-95 and * C - Cl

the C-Cl bond is )) k/4mol"he bond is a human invention.

t is a method of e+plaining the energy

change associated with forming

molecules.

Bonds don=t e+ist in nature5 but areuseful.

We have a model of a bond.


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What is a odel?&+plains how nature operates.

>erived from observations.

t simplifies the and categori!es theinformation.

A model must be sensible5 but it has

limitations.


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roperties of a odel A human inventions5 not a blown up picture

of nature.odels can be wrong5 because they are

based on speculations and

oversimplification.Become more complicated with age.

Kou must understand the assumptions in

the model5 and look for weaknesses.

We learn more when the model is wrong

than when it is right.


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Covalent Bond &nergies

We made some simplifications indescribing the bond energy of C9

&ach C-9 bond has a different energy.

C9 →

C9) 7 9 ∆

9 ' )F k/4molC9) → C9( 7 9 ∆9 ' F) k/4mol

C9( → C9 7 9 ∆9 ' (F k/4mol

C9→ C 7 9 ∆9 ' )) k/4mol&ach bond is sensitive to its

environment.


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Averages

9ave made a table of the averages ofdifferent types of bonds pg. )DF

single bond one pair of electrons is

shared.double bond two pair of electrons are

shared.

triple bond three pair of electrons areshared.

ore bonds5 shorter bond length.


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Lsing Bond &nergies

We can find∆

9 for a reaction.t takes energy to break bonds5 and end

up with atoms 173.

We get energy when we use atoms toform bonds 1-3.

f we add up the energy it took to breakthe bonds5 and subtract the energy we getfrom forming the bonds we get the ∆9.

&nergy and &nthalpy are state functions.


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ind the energy for this

# H# . HH/

#1H/ 2#

# H# . H 1

3 H#2

-H 4"/ &56mol

. 3"4&56mol1-H /7" &56mol

2-H 43 &56mol

2.2 47, &56mol 1 87" &56mol

- /4 &56mol


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%ocali!ed &lectron odel

6imple model5 easily applied. A molecule is composed of atoms that

are bound together by sharing pairs ofelectrons using the atomic orbitals of the

bound atoms. "hree arts

*3 Malence electrons using %ewis structures

(3 rediction of geometry using M6&@

)3 >escription of the types of orbitals1Chapt 3


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@ules6um the valence electrons.Lse a pair to form a bond between

each pair of atoms.

Arrange the rest to fulfill the octet rule

1e+cept for 9 and the duet3.

9(#

A line can be used instead of a pair.


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A useful equations1happy-have3 4 ( ' bonds

#Cl) is central atom

6#-( 6 is central atom6#)-( 6 is central atom

#-(

6 is central atom6Cl( 6 is central atom


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&+ceptions to the octetB9

)

Be and B often do not achieve octet

9ave less than and octet5 for electron

deficient molecules.6D

"hird row and larger elements can

e+ceed the octetLse )d orbitals?

)-


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&+ceptions to the octetWhen we must e+ceed the octet5 e+tra

electrons go on central atom.

Cl):e#)

Cl-

BeCl(


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@esonance6ometimes there is more than one valid

structure for an molecule or ion.


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ormal Chargeor molecules and polyatomic ions

that e+ceed the octet there are several

different structures.

Lse charges on atoms to help decidewhich.

"rying to use the o+idation numbers to

put charges on atoms in moleculesdoesn=t work.


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ormal Charge"he difference between the number of

valence electrons on the free atom andthat assigned in the molecule.

We count half the electrons in each

bond as HbelongingI to the atom.6#-(

olecules try to achieve as low a formal

charge as possible.


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M6&@%ewis structures tell us how the atoms

are connected to each other.

"hey don=t tell us anything about

shape.

"he shape of a molecule can greatly

affect its properties.Malence 6hell &lectron air @epulsion

"heory allows us to predict geometry


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M6&@

olecules take a shape that putselectron pairs as far away from eachother as possible.

9ave to draw the %ewis structure todetermine electron pairs.

bonding

nonbonding lone pair %one pair take more space.

ultiple bonds count as one pair.


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M6&@&lectron

pairs

Bond

Angles

Lnderlying

6hape

( *8,O %inear

) *(,O "rigonal lanar *,.FO "etrahedral

F ,O P*(,O "rigonalBipyramidal

D ,O #ctagonal


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Actual shape

&lectro

nairs

Bondin

gairs


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Actual 6hape

&lectro

nairs

Bondin

gairs


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9ow well does it work?>oes an outstanding Qob for such a

simple model.

redictions are almost always

accurate.

%ike all simple models5 it has

e+ceptions.

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