2b. shapes of molecules (1).pptx

8/18/2019 2b. Shapes of Molecules (1).pptx

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SHAPES OF

MOLECULESChemistry


2/78

Recall that:Covalent bonds are formed from theoverlap of two atomic orbitals

Each pair of covalent bond contains a pairof electrons

Two types of covalent bonds:1. Sigma (σ) bonds

. !i bonds (") bonds


3/78

Electron Pairs#n molec$les thereare types ofelectron pairs

1. %onding !airs

. &on'bonding orlone pairs

. The bonding pairmay berepresented by aline


4/78

What is the shape of the

molecule?e will $se the geometric arrangementof atoms around a central atom in acovalent compo$nd (or charged ion) to

determine the shape of the molec$le

pi ' bonds have a very limited impact on

shape

sigma'bonds have a m$ch more importanteect


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alence Shell Electron PairRepulsion !SEPR" #heor$

The VSEPR theory proposes that shape of themolecule is determined solely by the

repulsions between electron pairs present inthe valence shell of the central atom

• The n$mber of electron pairs aro$nd the centralatom can be determined by writing the *ewisstr$ct$re for the molec$le

• +SE!, theory is a model that helps $s to$nderstand the -' str$ct$re of molec$les


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#he SEPR Mo%el

• +SE!, theory can be $sed to predict the shapesand bond angles of molec$les and molec$lar ions

•

!airs of electrons that s$rro$nd the central atom ofa molec$le or ion are arranged as far apart aspossible to minimi/e electron'electron rep$lsion

•

0pplicable only to covalent compo$nds (#onicbonds are non'directional while covalent bondshave a preferred direction in space)


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SEPR #heor$ an% Repulsion

Electrostatic force of rep$lsion betweenelectron pairs

,ep$lsion occ$rs to attain most

comfortable arrangement Three types of rep$lsion to consider:

1. %onding pair ' %onding pair rep$lsion

. *one pair ' *one pair rep$lsion-. %onding pair *one pair rep$lsion


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Stren&th of Repulsion• epends on pro2imity of electron pairs to central atom

• %onding pairlocated between two n$clei

• *one pairs closer to central atom

they are delocali/ed

have no other n$cle$s to attract them

• *one pairs p$sh bond pairs closer0s bonds are p$shed closer together the bond angle decreases

LP'LP REPULS(O)S * LP'+P REPULS(O)S * +P'+P REPULS(O)S


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Molecular ,eometries

#-o Electron Pairs

#hree Electron Pairs

Four Electron Pairs


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Fi.e ElectronPairs

Si/ ElectronPairs


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Molecular ,eometries !Cont0%"

Se.en Electron Pairs


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Usin& SEPR theor$ to pre%ict the shapeof the molecule

1. ecide which is the central atom in a molec$le.#n cases of ambig$ity3 pic4 the leastelectronegative atom (aside from hydrogen) asthis atom will be better able to share its

electrons with the other atoms in the molec$le

. Co$nt $p the valence electrons on the centralatom.

-. Co$nt $p the electrons $sed by the o$ter atomsto ma4e bonds with the central atom


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Usin& SEPR theor$ to pre%ict the shapeof the molecule

5. The s$m of steps () 6 (-) divided by two givesthe +alence Shell Electron !air (+SE!) co$nt

The VSEP count gives the number of electron pairs around the central atom

7. 0ssign one electron pair for each bond (or twopairs for each do$ble bond). 0ny $nassignedelectron pairs are lone pairs and are placed onthe CENTRA atom.


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SEP Count

The predicted geometry of the molec$le is basedon the n$mber of +SE! co$nt i.e. the arrangementof E! aro$nd the central atom (both lone pair andbonded pair)

Eg. Calc$late the +SE! co$nt for the watermolec$le

1. Central 0tom is 82ygen. &$mber of valence electrons 9 e'

-. &$mber of electrons $sed byperipheral atoms to bond to centralatom 9 (1) 9 e'

+SE! co$nt 9 ( e') 6 ( e') ; 9 5 e' pairsThe ! central atom has " e# pairsaround it

$ bonding pairs and $ non#bonding


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Pre%ictin& the Shapes of

Molecules%eClhat is the +SE! co$nt< =ow many bonding pairsand lone pairs<Central 0tom is %erylli$m

&$mber of valence electrons 9 e'

&$mber of electrons $sed by peripheral atoms tobond to central atom 9 (1) 9 e'

+SE! co$nt 9 ( e') 6 ( e') ; 9 e' pairs

•The 'e central atom has $ e# pairs around it

•$ bonded pairs and N! non#bonded or lone pairs


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Molecules+eCl1hat is the shape of themolec$le< %ond angle<

%ond 0ngle 9 1>??

*inear Shape

#n generalmolec$les 3 0@3 with

$ bonding pairsand no lone pairs

are *inear.


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Linear Molecules !A21"

hen applying the +SE!, model3 we count a double ortriple bond as a single electron domaino$ble and triple bonds are linear3 and behave very m$chli4e a single bond when it comes to rep$lsions

What is the structure of C1H1?


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Molecules%A-hat is the +SE! co$nt< =ow many bonding pairsand lone pairs<Central 0tom is %oron

&$mber of valence electrons 9 - e'

&$mber of electrons $sed by peripheral atoms tobond to central atom 9 - (1) 9 - e'

+SE! co$nt 9 (- e') 6 (- e') ; 9 - e' pairs

•The ' central atom has ( e# pairs around it

•( bonding pairs and N! non#bonding or lone

pairs


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Molecules+F3hat is the shape of themolec$le< %ond angle<

%ond 0ngle 9 1??

Trigonal !lanar

#n generalmolec$les 3

0@-3 with (bonding pairs and no

lone pairs are Trigonal

Planar


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#ri&onal Planar !A23"

)* Show that %a& N!(# and %b& S!(

display a trigonal planar

arrangement

E2amples incl$de &8-'3 S8-3 C8-' andC=5


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MoleculesC=5hat is the +SE! co$nt< =ow many bonding pairsand lone pairs<Central 0tom is Carbon

&$mber of valence electrons 9 5 e'

&$mber of electrons $sed by peripheral atoms tobond to central atom 9 5 (1) 9 5 e'

+SE! co$nt 9 (5 e') 6 (5 e') ; 9 5 e' pairs

•The C central atom has " e# pairs around it

• " bonding pairs and N! non#bonding or lone

pairs


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MoleculesCH4hat is the shape of themolec$le< %ond angle<

%ond 0ngle 9 1?B.7?

Tetrahedral

#n general molec$les 30@53 with " bonding

pairs and no lone pairs are Tetrahedral


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#ri&onal Planar !A24"

)* Show that the following moleculeshave a tetrahedral structure+

a&N, "-

b&S! "$#

c& P! "(#

d&Ni%C!& "e&C, "


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Pre%ictin& the Shapes ofMolecules&=-hat is the +SE! co$nt< =ow many bonding pairsand lone pairs<Central 0tom is &itrogen


&$mber of electrons $sed by peripheral atoms tobond to central atom 9 - (1) 9 - e'

+SE! co$nt 9 (7 e') 6 (- e') ; 9 5 e' pairs

•The N central atom has " e# pairs around it

•( bonding pairs and . non#bonding or lone pair


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Pre%ictin& the Shapes ofMolecules)H3

hat is the shape of themolec$le<

%ond 0ngle 9 1??

Trigonal !yramidal

#n general molec$les 30@-E3 with ( bonding

pairs and . lone pairare Trigonal Pyramidal


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#ri&onal P$rami%al

)* Show that the following moleculeshave a trigonal pyramidal structure+

a&P,(

b&S!($#


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Pre%ictin& the Shapes ofMolecules=8hat is the +SE! co$nt< =ow many bonding pairsand lone pairs<Central 0tom is 82ygen



+SE! co$nt 9 ( e') 6 ( e') ; 9 5 e' pairs

•The ! central atom has " e# pairs around it

•$ bonding pairs and $ non#bonding or lone pairs


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Pre%ictin& the Shapes ofMoleculesH1O

hat is the shape of themolec$le< %ond angle<

%ond 0ngle 9 1?5.7?

%ent (or +'shaped)

#n general

molec$les 3 0@E3

with $ bonding pairs and $ lone

pairs are 'ent or V#shaped


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+ent or 'shape% molecules

)* Show that the following moleculeshave a bent structure+

a&,$Sb&S!$

c& ,$!


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Pre%ictin& the Shapes ofMolecules!Cl7hat is the +SE! co$nt< =ow many bonding pairsand lone pairs<Central 0tom is !hosphor$s


&$mber of electrons $sed by peripheral atoms tobond to central atom 9 7 (1) 9 7 e'

+SE! co$nt 9 (7 e') 6 (7 e') ; 9 7 e' pairs

•The P central atom has / e# pairs around it

•/ bonding pairs and N! non#bonding or lone

pairs


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Pre%ictin& the Shapes ofMoleculesPCl5hat is the shape of themolec$le< %ond angle<

%ond 0ngle 9 B?? and 1??

Trigonal %ipyramidal

#n general

molec$les 3 0@73 with

/ bonding pairsand 0 lone pairs

are Trigonal'ipyramidal


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Pre%ictin& the Shapes ofMoleculesSAhat is the +SE! co$nt< =ow many bonding pairsand lone pairs<Central 0tom is S$lph$r



+SE! co$nt 9 ( e') 6 ( e') ; 9 e' pairs

•The S central atom has 1 e# pairs around it

•1 bonding pairs and N! non#bonding or lone

pairs


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Pre%ictin& the Shapes ofMoleculesSF6hat is the shape of themolec$le< %ond angle<

The A'S'A %ond 0ngles areall B??

8ctahedral

#n general molec$les 3 0@3

with 1 bonding pairs and 0lone pairs are !ctahedral


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SHAPES A)7 +O)7A),LES OF S(MPLEOR,A)(C COMPOU)7S


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Or&anic

Compoun%s!rganic compounds belong to a large classof chemical compounds whose moleculescontain carbon %e2cept C!$ and carbonates&

The simple pict$re of overlap of half'Dlledatomic orbitals cannot be $sed to e2plain thegeometry of all molec$les especially organicmolec$les

e $se a concept 4nown as hybridi3ation toe2plain bonding in carbon compo$nds


36/78

Methane !CH4"Carbon needs to have fo$r singlebonds

&eeds fo$r single electrons

!romotion of an electron from the s

orbital to the p orbital

i2ing the s and p orbitals(hybridi/ation)


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H$8ri%isation• s electronsspontaneo$sly promotedto empty p orbital

• FsG orbital and the T=,EE FpG orbitals mi2to form A8H, hybridorbitals (sp-) with four

single electrons

• number of orbitals is conserved

during hybridisation


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E/cite% Car8on Atom

Ao$r identical sp- orbitals formed 0ll the same shape3 si/e and energy


39/78

sp3 H$8ri% Or8italsEach hybrid orbital contains 7I scharacter and 7 I p character.

The A8H, sp- hybrid orbitals eachcontains one electron

=ybrid orbitals arrange themselves in athree dimensional space to get as farapart as possible (to minimi/e rep$lsion)

0rrangement gives rise to a tetrahedralstr$ct$re bond angle of 1?B.7


40/78

Sp3 H$8ri%i9ation !Methane"

Overlapping of sp3 orbitals on carbon

with s orbital of hydrogen gives rise to a

sigma bond


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Sp3 H$8ri%i9ation !Ethane"

sp3 or8itals ofcar8on o.erlap-ith s or8italof h$%ro&en to&i.e a si&ma

8on%

sp3 or8itals on8oth car8onso.erlap to &i.e

a si&ma 8on%


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Sp2 Hybridization


43/78

Sp1 H$8ri%i9ation


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Sp1 h$8ri%i9ationAo$nd in compo$nds s$ch as al4enes (eg ethene)

Each carbon needs to form three bonds (twohydrogens and one carbon)

Electron promotion still occ$rs in carbon followed bymi2ing of the s and p orbitals

=owever3 only two of the p orbitals are mi2ed withthe s orbital to form three sp hybrid orbitals

The $n$sed p orbital remains p$re ($nhybridi/ed)


45/78

sp1 H$8ri%i9ation !C1H4"

Two sp orbitals will overlap with hydrogen 1sorbitalJ the third sporbital will overlap with asimilar sp orbital on the other carbon atom

Each carbon has a p$re3 $n$sed p orbitalcontaining an electron

The $n$sed p orbitals are perpendic$lar to theplane of the sp orbitalsJ proKects above and below

the plane

close pro2imity of p orbitals ca$ses sidewaysoverlap forming a pi bondJ !i bonds are wea4erthan sigma bonds


46/78

Sp2 Hybridization (Ethene)


47/78

Ethene


48/78

+en9ene

Si2 carbon atoms in a ring

Shows resonance hybrid

=e2agonal in shape ' at each ape2 there is acarbon bonded to a hydrogen

Each carbon is bonded to three other atomsJ ahydrogen and two other carbon atoms

Each carbon $ses sp hybrid orbitals

Each carbon contains a p$re p orbital perpendic$larto the plane of the ring


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+en9ene

Each $nhybridi/ed p orbital overlaps with twoother p orbitals3 one on each of the twoneighbo$ring carbon atoms

0 large circ$lar pi'type bond is formed aboveand below the plane

Electrons are delocali/ed in the ben/ene ring


50/78

+en9ene

8verlapping of p orbitals


51/78

+en9ene


52/78

+en9ene

Canonical forms


53/78

+en9ene

=ybridi/ed str$ct$re


54/78

LE!"#E

Shapes of $olec%les

&' hemistry ower oints

!opic

Bonding

www.pedagogics.ca


55/78

*reat than+s to

,O-.!H.- HO!O- / 0-O0H.#1 "'L&SH&-*

www.knockhardy.org.uk/sci.htm

uch taken from

AN INTRODUCTION TO

BONDING

and

SHAPES OF

OECUES

.LE-E SHELL ELE!#O- .


56/78

.LE-E SHELL ELE!#O- .

#E"LS&O- (SE#) !HEO#

4!HE SH.E .1O!E1 ' . S&$LE $OLE"LE O# &O- &S

!H.! 5H&H 0EES #E"LS&E 6O#ES !O . $&-&$"$7

Molecules containco.alent 8on%s Asco.alent 8on%s consistof a pair of electrons;each 8on% -ill repelother 8on%s

.l'onds are f%rther

apart so rep%lsive

forces are less

'onds are closer

together so rep%lsive

forces are greater

.l

.ll bonds are

e8%ally spaced

o%t as far apartas possible

-ote 9 yo% m%st

thin+ of spacing

in 31 not :%st 21

+on%s -ill thereforepush each other as farapart as possi8le to

re%uce the repulsi.eforces

+ecause the repulsionsare e


57/78

.11&-* .-O!HE# .!O$ ; .-&$.!&O-

.11&-* .-O!HE# .!O$ ; .-&$.!&O-


58/78

.11&-* .-O!HE# .!O$ ; .-&$.!&O-


59/78

2 regions of electron density

2 'onding airs; 'E#LL&"$ HLO#&1E


60/78

2 'onding airs; 'E#LL&"$ HLO#&1E

l l'e?'O-1 .S 2

LO-E .S >

'O-1 .-*LE@@@

*eometry @@@

?

L&-E.#

l'e 'e ll

'erylli%m ; has two electrons to pair %p

hlorine ; needs < electron for AoctetB

!wo covalent bonds are formed

'erylli%m still has an incomplete shell

$OLE"LES 5&!H 1O"'LE 'O-1S


61/78

!reat as a single region of electron density

O OO

arbon ; needs fo%r electrons to complete its shell

OCygen ; needs two electron to complete its shell

!he atoms share two electrons

each to form two do%ble bonds

1O"'LE 'O-1 .S 2

LO-E .S >

'O-1 .-*LE@@@

*eometry @@@

?

L&-E.#

O O

?

1o%ble bonds behave eCactly as single

bonds for rep%lsion p%rposes so the

shape will be the same as a molec%le with

two single bonds and no lone pairs@

!he shape of a compo%nd with a do%ble bond is calc%lated in the same way@ .

do%ble bond repels other bonds as if it was single e@g@ carbon dioCide


62/78

3 regions of electron density

3 'onding airs ; .L"$&-&"$ HLO#&1E


63/78

.l

3 'onding airs .L"$&-&"$ HLO#&1E

l

l

.l

?l

l

.l

l

l

l

'O-1 .S 3

LO-E .S >

'O-1 .-*LE@@@

*eometry @@@

?

!#&*O-.L L.-.#

.l%mini%m ; has three electrons to pair %p

hlorine ; needs < electron to complete AoctetB

!hree covalent bonds are formedD al%mini%m

still has an incomplete o%ter shell@


64/78

regions of electron density

'onding airs; $E!H.-E


65/78

'onding airs $E!H.-E

'O-1 .S

LO-E .S >

'O-1 .-*LE@@@

*eometry @@@

@F?

!E!#.HE1#.L

H H

H

H

H

@F?

H H

H

H

arbon ; has fo%r electrons to pair %p

Hydrogen ; < electron to complete shell

6o%r covalent bonds are formed

and H now have complete shells

regions of electron density ; .$$O-&.


66/78

g y

.-*LE@@@ G?

SH.E@@@ #.$&1.L

H- -H H

H

'O-1 .S 3

LO-E .S <

!O!.L .S

G?H

H

-

H

• !he shape is based on a tetrahedron b%t not all the rep%lsions are the same

• L;' #E"LS&O-S ';' #E"LS&O-S

• !he -;H bonds are p%shed closer together

• Lone pairs are not incl%ded in the shapeI

regions of electron density ; 5.!E#


67/78

.-*LE@@@ @F?

SH.E@@@ 'E-!

H

O

H

H OH

H

'O-1 .S 2

LO-E .S 2

!O!.L .S

O

H

O

H

@F?H

O

H

• !he shape is based on a tetrahedron b%t not all the rep%lsions are the same

• L;L #E"LS&O-S L;' #E"LS&O-S ';' #E"LS&O-S• !he O;H bonds are p%shed even closer together

• Lone pairs are not incl%ded in the shape

g y

HO5 !O 1E!E#$&-ESH.ES O6 &O-S


68/78

-

'O-1 .S 3 #.$&1.L

LO-E .S < H;-;H G?

'O-1 .S !E!#.HE1#.L

LO-E .S > H;-;H @F?

-

H

H

H

-JH

H

H

H -J

'O-1 .S 2 'E-!

LO-E .S 2 H;-;H @F?

-

H

H -

-HJ

-H2

;

-H3

F 'onding airs (HL only) ; HOSHO#"S() 6L"O#&1E


69/78

6

6

6

6

6

6

'O-1 .S F

LO-E .S >

'O-1 .-*LE@@@

*eometry @@@

? / >?

!#&*O-.L '&#.$&1.L

?

6

6

6

6

6

>?

hosphor%s ; has five electrons to pair %p

6l%orine ; needs one electron to complete AoctetB

6ive covalent bonds are formedD phosphor%s can

ma+e %se of d orbitals to eCpand its AoctetB

K 'onding airs (HL only) ; S"LH"#(&) 6L"O#&1E


70/78

g ( y) ( )

6S

'O-1 .S K

LO-E .S >

'O-1 .-*LE@@@

*eometry @@@

>?

O!.HE1#.L

S

6

6

6

6

6

6

S%lph%r ; has siC electrons to pair %p

6l%orine ; needs one electron to complete AoctetB

SiC covalent bonds are formedD s%lph%r can ma+e

%se of d orbitals to eCpand its AoctetB

6

6 6

6

S

6

6

>?

HL only ; E-O- !E!#.6L"O#&1E


71/78

y

6e e

6

6

6

6

6

6 6

6e

6

6 6

6

e

.-*LE@@@ >?

SH.E @@@ SM".#E L.-.#

• .s the total n%mber of electron pairs is KN the shape is '.SE1 on an octahedron

• !here are two possible spatial arrangements for the lone pairs

• !he preferred shape has the two lone pairs opposite each other

'O-1 .S

LO-E .S 2

!O!.L .S K

S"$$.#

6"-1.$E-!.L SH.ES l i


72/78

6"-1.$E-!.L SH.ES 9 no lone pairsMolecules; or ions; possessin& O)L=+O)7(), PA(RS of electrons >t into

a set of stan%ar% shapes All the8on% pair'8on% pair repulsions aree / lF

K O!.HE1#.L > S6K

'O-1 ELE!#O- 'O-1.S *EO$E!# .-*LE(S) E.$LE

. covalent bond will repel

another covalent bond

Effect of Lone airs on $olec%lar Shape


73/78

Effect of Lone airs on $olec%lar Shape

&f a molec%leN or ionN has lone pairs on the central atomN the shapes are slightly

distorted away from the reg%lar shapes@ !his is beca%se of the eCtra rep%lsion

ca%sed by the lone pairs@

'O-1 . ; 'O-1 . P LO-E . ; 'O-1 . P LO-E . ; LO-E .

OO O

.s a res%lt of the eCtra rep%lsionN bond angles tend to

be slightly less as the bonds are s8%eezed together@

S"$$.# ; .L"L.!&-* !HE SH.E O6 &O-S


74/78

!he shape of an ion or molec%le is determined by@@@

• calc%lating the n%mber of electrons in the o%ter shell of the central species Q

• pairing %p electronsN ma+ing s%re the o%ter shell maCim%m is not eCceeded• calc%lating the n%mber of bond pairs and lone pairs (regions of e; density)

• %sing ELE!#O- . #E"LS&O- !HEO# to calc%late shape and bond

angle(s)

-ote for ions

* the n%mber of electrons depends on the charge on the ion

* if the ion is positive yo% remove as many electrons as there are positive

charges

* if the ion is negative yo% add as many electrons as there are negative charges

e..g. for 6K

; add one electron to the o%ter shell of

for l

J remove one electron from the o%ter shell of

O!HE# E.$LES !O !#


75/78

'r6F

6

6 6

6

'r

6

'r63

6

6

'r

6

SO

2;

O S

O;

O;

O'O-1 .S

LO-E .S

SH.E

.-*LE

'O-1 .SLO-E .S

SH.E

.-*LE

'O-1 .S

LO-E .S

SH.E

.-*LE

O!HE# E.$LES !O !#


76/78

'r6F 'O-1 .S F

LO-E .S <

SM".#E #.$&1

.-*LES >? P>?

6

6 6

6

'r

6

66 6

6'r

6

'r63 'O-1 .S 3

LO-E .S 2

B!B SH.E1

.-*LE P>?

6

6

'r

6

6

6

'r

6

SO

2;

O

S

O;

O;

O

O S

O;

O;

O'O-1 .S

LO-E .S >

!E!#.HE1#.L

.-*LE @F?

!ES! M"ES!&O-S


77/78

'63

Sil

lK

;

H2S

SilK2;

l

J

6or each of the following ionsRmolec%lesN state the n%mber of bond pairs

state the n%mber of lone pairs

state the bond angle(s)

stateN or drawN the shape

!ES! M"ES!&O-S


78/78

3 bp > lp trigonal planar boron pairs %p all 3 electrons in

its o%ter shell

bp > lp @F tetrahedral silicon pairs %p all electrons in

its o%ter shell

bp > lp @F tetrahedral as ion is JN remove an electron

in the o%ter shell then pair %p

K bp > lp > octahedral as the ion is ; N add one electron to

the F in the o%ter shell then pair %p

K bp > lp > octahedral as the ion is 2;N add two electronsto the o%ter shell then pair %p

2 bp 2 lp 2 bent planar s%lph%r pairs %p 2 of its K

electrons in its o%ter shell ;

2 lone pairs are left

'63

Sil

lK

;

H2S

SilK2;

l

J

6or each of the following ionsRmolec%lesN state the n%mber of bond pairs

state the n%mber of lone pairs

state the bond angle(s)

stateN or drawN the shape

2b. shapes of molecules (1).pptx

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