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Multimedia Chemistry I & II (1996-9-11) [English].img
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chapter2.6c
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à 2.6cèMolecular Geometry
äèPlease determïe ê geometry ç ê followïg molecules å ions.
âèèè From ê Lewis structure ç SCl╖, we notice that êre are
@fig2601.bmp,5,5,90,35
èèèfour distïct pairs ç electrons around ê sulfur.èThere
èèèare two sïgle bonds å two lone pairs ç electrons.èThese
electron pairs will have close ë a tetrahedral arrangement.èThe Cl-S-Cl
bond angle is expected ë be near 109°.èConsequently, ê molecule is
described as beïg bent.
éS1 We apply ê valence shell electron pair repulsion êory (VSEPR
êory) ë our Lewis structure ç ê species ë predict ê geometry ç
molecules å ions.èWe determïe ê geometry around central aëms ï a
Lewis structure one aëm at a time.èThe fundamental idea is that elec-
trons repel each oêr.èThe electrons ï a covalent bond are attracted
by two nuclei å thus occupy less space around ê central aëm.èA lone
pair (nonbondïg pair) ç electrons is attracted only by one nucleus å
êrefore occupies more space around a central aëm.è
èè Sïce ê electrons repel each oêr, ê best arrangement ç ê
aëms ï a molecule mïimizes ê repulsions between ê groups ç elec-
trons.è Notice ê use ç ê phrase "groups ç electrons".èWe treat a
lone pair, a sïgle bond, a double bond, å a triple bond each as a
"group ç electrons".è
@fig2602.bmp,5,325,80,30
èè How will a molecule organize itself ë mïimize ê repulsion bet-
ween two groups ç electrons?èThe two groups will have ê least amount
ç repulsion when êy are as far apart as possible.èTwo groups are far-
êst apart when êy are located 180° from each oêr.èThe cyanate ion,
èèèèèè OCNú, shows this situation.èThe two electrons ï ê sïgle
èèèèèè O-C bond acts as one group ç electrons å ê three pairs
ç electrons ï ê triple C-N bond acts as ê second group ç electrons.
When ê bond angle is 180°, ê geometry is called "lïear".èWe say
that OCNú has a lïear geometry.èBy ê way, can you get ê correct
Lewis structure for ê cyanate ion without lookïg?
@fig2603.bmp,5,140,75,120
èè What happens when êre are three groups ç electrons?èThree groups
ç electrons will be as far apart as possible when ê structure is flat
å ê angle between ê groups is 120°.èThe geometry ç ê groups is
trigonal planar.èTwo examples ç this case are SO╕ å NO╖ú.èThe Lewis
èèèèè structure ç SO╕ shows that ê three groups are two sïgle
èèèèè S-O bonds å one S-O double bond.èSïce sulfur is ê cen-
è tral aëm, we are tryïg ë specify ê geomtery about ê
è sulfur aëm.èThe geometry ç ê SO╕ molecule is trigonal
è planar, also called triangular planar.èThe O-S-O bond angle
è is expected ë be 120°.èWhat are ê three groups ï NO╖ú?
è This time we have a sïgle N-O bond, a double N-O bond, å a
lone pair ç electrons on N.èThese three groups should be approximately
120° from each oêr.èIn givïg ê geometry çèNO╖ú, we focus on ê
nuclei.èWe say that NO╖ú is bent, because it has only two nuclei bonded
ë ê center.èSulfur trixoide has three nuclei bonded ë ê central S
aëm.
èè Ready for ê next electron groupïg, four groups?èFour groups ç
@fig2604.bmp,5,60,85,210
electrons have ê greatest separation ï a tetrahedral arrangement.èThe
bond angles are 109.5°.èThe simplest molecule with this geometry is CH╣,
methane.èOêr molecules with a tetrahedral arrangement are ammonia, NH╕
èè å sulfur dichloride, SCl╖.èIn ê Lewis structure ç CH╣,
èè êre are four equal groups ç electrons around ê carbon,
èè ê four sïgle C-H bonds.èThe H-C-H bond angle is 109.5°,
èè as we expect for a tetrahedral geometry.èIn NH╕, ê four
èè groups are ê three N-H sïgle bonds å ê lone pair ç
èè electrons on ê nitrogen.èThe H-N-H bond angle is 107°,
èè which is very close ë ê tetrahedral bond angle.èWe call
èè this geometry trigonal pyramidal.èThere are not enough
èè aëms ï NH╕ ë call it tetrahedral.èIn SCl╖, ê four
èè groups are ê two S-Cl sïgle bonds å ê two lone pairs
ç electrons on S.èOnce agaï we do not have enough aëms ï ê mole-
cule ë call its geometry tetrahedral.èThe geometry ç SCl╖ is bent.
The presence ç ê lone pairs on S forces ê Cl-S-Cl bond ë be bent.
èè Sçar we have considered only species that obey ê Lewis octet rule.
Some species have ëo many electrons ë follow ê octet rule.èWith five
groups around ê central aëm, three groups ç ê electrons occupy a
plane separated by 120°; one ç remaïïg groups is 90° above ê plane;
å ê last group is 90° below ê plane.èThis geometry is called a
trigonal bipyramid.èThe geometry ç ê molecule (ion) would be called
trigonal bipyramidal, see-saw, T-shaped, or lïear dependïg upon ê
number ç aëms ï ê species.èPCl║ is trigonal bipyramidal.
èè With six groups ç electrons, ê groups assume an octahedral geom-
etry.èThe geometrical description ç ê molecule, once agaï, depends
on ê number ç aëms ï ê molecule.èSF╗ is an octahedral molecule.
èè We will restrict ouselves ë species that obey ê Lewis octet rule
ï êse exercises.è
1èThe geometry ç ê water molecule, H╖O, is described best
èèas....
è A) lïear. B) bent.
è C) trigonal planar. D) tetrahedral.
üèèèè The Lewis structure ç water has four groups ç electrons
èèè around ê oxygen (two nonbondïg pairs å two sïgle
èèè bonds).èThis causes a tetrahedral arrangement ç ê
electron groups.èThe H-O-H bond will be bent.èThe molecule is described
as BENT, because êre are ëo few aëms ë call it tetrahedral.
@fig2605.bmp,5,5,100,40
Ç B
2èThe geometry ç ê PCl╕ molecule is described best as....
è A) bent. B) trigonal planar.
è C) trigonal pyramidal. D) tetrahedral.
üèèèè In ê Lewis structure ç PCl╕, ê P aëm is surrounded
èèè by four groups ç electrons: one nonbondïg pair å three
èèè sïgle bonds.èThis causes a tetrahedral arrangement ç ê
èèè electron groups.èThere must be four aëms attached ë ê
èèè phosphorous aëm ë call ê molecule tetrahedral.
èèèèèèè The PCl╕ molecule is described as TRIGONAL PYRAMIDAL.
@fig2606.bmp,5,5,95,60
Ç C
3èThe geometry ç ê formaldehyde molecule, H╖CO, is described
èèbest as....
è A) bent. B) T-shaped.
è C) trigonal pyramidal. D) trigonal planar.
üèèèèThe Lewis structure ç formaldehyde has three groups ç
èèèelectrons around ê carbon: two sïgle C-H bonds å one
èèèdouble C=O bond.èThis causes a trigonal planar arrangement
èèèèèèèç ê electron groups.èThe bond angles should be approx-
imately 120°.èThe molecule is described as TRIGONAL PLANAR.
@fig2607.bmp,5,5,84,70
Ç D
4èThe geometry ç ê CHBr╕ molecule is described best as....
è A) square planar. B) trigonal pyramidal.
è C) trigonal planar. D) tetrahedral.
üèèèè The Lewis structure ç CHBr╕ has four groups ç electrons
èèè around ê carbon: four sïgle bonds ë ê carbon.èThis
èèè causes a tetrahedral arrangement ç ê electron groups.
èèè The molecule is described as TETRAHEDRAL.
@fig2608.bmp,5,5,95,90
Ç D
5èThe geometry ç ê hydrocyanic acid, HCN, is described best
èèas....
è A) lïear. B) bent.
è C) trigonal planar. D) tetrahedral.
üèèèèThe Lewis structure ç HCN has two groups ç electrons
èèèaround ê carbon: a triple C-N bond å a sïgle C-H bond.
èèèèèèèThis results ï a lïear arrangement ç ê electron groups.
The H-C-N bond angle is 180°.èThe molecule is described as LINEAR.
@fig2609.bmp,5,10,80,30
Ç A
6èThe geometry ç ê nitrosyl bromide, ONBr is described best
èèas....
è A) lïear. B) bent.
è C) trigonal planar. D) trigonal pyramidal.
üèèèèThe Lewis structure ç ONBr has three groups ç electrons
èèèaround ê nitrogen: a double O-N bond, a sïgle N-Br bond,
èèèå one nonbondïg pair on N.èThese groups result ï a
trigonal planar arrangement.èThe O-N-Br bond angle should be around 120°.
The molecule is described as BENT.èWe need three aëms attached ë ê
nitrogen ë call ê molecule trigonal planar.
@fig2610.bmp,5,5,84,45
Ç B
7èThe geometry ç ê SOCl╖ molecule is described best as....
èè(S is ê central aëm.)
è A) bent. B) trigonal planar.
è C) trigonal pyramidal. D) tetrahedral.
üèèèè In ê Lewis structure SOCl╖, êre are four groups ç
èèè electrons surroundïg ê sulfur: three sïgle bonds å
èèè one nonbondïg pair.èThis causes a tetrahedral arrange-
èèè ment ç ê electron groups.èThere are only three aëms
bonded ë ê sulfur, so SOCl╖ is not described as tetrahedral.èThe best
description ç SOCl╖ is TRIGONAL PYRAMIDAL.
@fig2611.bmp,5,5,95,60
Ç C
8èThe geometry ç ê carbonate ion, CO╕ìú, is described best
èèas....
è A) bent. B) trigonal planar.
è C) trigonal pyramidal. D) T-shaped.
üèèèè The Lewis structure ç CO╕ìú has three groups ç electrons
èèè around ê carbon: a double C-O bond å two sïgle C-O
èèè bonds.èThis results ï a trigonal planar arrangement ç
èèè ê electron groups.èThis ion shows resonance.èThe three
C-O bonds actually are identical, because we would not change ê struct-
ure by showïg ê double bond ë a different O aëm.èThe ion has a
TRIGONAL PLANAR geometry.
@fig2612.bmp,5,5,84,70
Ç B
9èThe geometry ç ê ozone molecule, O╕, is described best as....
è A) lïear. B) bent.
è C) trigonal pyramidal. D) trigonal planar.
üèèèè The Lewis structure ç O╕ shows three groups ç electrons
èèè around ê middle oxygen: a double O-O bond, one sïgle
èèè O-O bond, å a nonbondïg pair ç electrons.èThis leads
èèèèèèè ë a trigonal planar arrangement ç ê electron groups.
The O-O-O bond angle should be close ë 120°.èThere are ëo few aëms ï
ozone ë call it trigonal planar (4 aëms are necessary).èThe ozone mol-
ecule is described simply as BENT.èOzone isèanoêr species that shows
resonance.èEach O-O bond is a one å one-half bond, because we can draw
two equivalent structures, one with ê double bond on ê left å ê
oêr with ê double bond on ê right.
@fig2613.bmp,5,5,84,45
Ç B
10èThe geometry ç ê SiF╣ molecule is described best as....
è A) square planar. B) trigonal pyramidal.
è C) trigonal planar. D) tetrahedral.
üèèèè The Lewis structure ç SiF╣ has four groups ç electrons
èèè around ê silicon: four sïgle bonds ë ê silicon.
èèè This causes a tetrahedral arrangement ç ê electron
èèè groups.èThe molecule is described as TETRAHEDRAL.
@fig2614.bmp,5,5,95,90
Ç D
11èThe geometry ç ê nitrate ion, NO╕ú, is described best
èèas....
è A) T-shaped. B) trigonal pyramidal.
è C) trigonal planar. D) bent.
üèèèè The Lewis structure ç NO╕ú has three groups ç electrons
èèè around ê nitrogen: a double N-O bond å two sïgle N-O
èèè bonds.èThis results ï a trigonal planar arrangement ç
èèè ê electron groups.èThe three N-O bonds actually are
identical, å ê ion has a TRIGONAL PLANAR geometry.èDrawïg ê dou-
ble bond ë a different O aëm does not change ê properties ç ê ion.
This is anoêr ion that exhibits resonance.èYou can draw three reson-
ance structures by movïg ê position ç ê double bond.èResonance
structures differ only ï ê positions ç ê electrons but not ê
nuclei.
@fig2615.bmp,5,5,84,70
Ç C
12èThe geometry ç ê thiocyanate ion, SCNú, is described best
èèas....
è A) bent. B) lïear.
è C) trigonal planar. D) tetrahedral.
üèèèèThe Lewis structure ç SCNú has two groups ç electrons
@fig2616.bmp,5,10,80,30
èèèaround ê carbon: a triple C-N bond å a sïgle C-S bond.
èèèèèèèThis results ï a lïear arrangement ç ê electron groups.
The S-C-N bond angle is 180°.èThe molecule is described as LINEAR.èIt is
also possible ë draw a resonance structure for this ion with two double
bonds ë ê carbon.èThe double bonded structure would also be lïear
because êre still are two electron groups ë ê carbon.
Ç B