Download lecture# 34

CHEM 1711 & 1811 Lecture # 34
MOLECULAR GEOMETRY
•
3-D shape of molecules affects physical and chemical properties
•
arrangement of valence e- around a central atom determines shape
•
geometric isomers have different properties
•
visual models (ball/stick, foam, computer, etc.) are very helpful
VALENCE-SHELL ELECTRON-PAIR RESULSION (VSEPR) MODEL
1) draw the Lewis formula to determine how atoms are connected
2) focus on a central atom (has two or more atoms connected to it)
3) count # of regions of e- density around this central atom
a region
(domain)
=
=
a lone pair
a bond (single, double or triple)
resonance is not an issue
4) e- repulsion maximizes angles between regions
-
consider cases for 2, 3, 4, 5, or 6 regions
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learn names for these arrangements
5) lone-pair regions need more space than bonding regions
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keep lone-pair regions apart when possible
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put lone-pair regions on equator (5-region arrangement)
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location of bond regions (atoms) determines shape
6) repeat for other central atoms to find geometry of larger molecules
1
CHEM 1711 & 1811 Lecture # 34
CASE I:
2
only one bond (diatomics)
arrangement
shape
not important
LINEAR
examples
CASE II:
don't need VSEPR
HCl
CO
two regions
arrangement
shape
LINEAR
LINEAR
bond angle
BeCl2 (16 e-)
examples
180°
CO2
HCCH
{ SHOW LINEAR models }
CASE III A:
three regions – three bonds
arrangement
shape
TRIGONAL PLANAR
TRIGONAL PLANAR
examples
BF3
(24 e-)
bond angle
120°
CO32-
C2H4
{ SHOW TRIGONAL PLANAR models }
CASE III B:
three regions – two bonds
arrangement
shape
examples
{ SHOW BENT models }
TRIGONAL PLANAR
BENT
bond angle ~ 115°
NO2- (18 e-)
CHEM 1711 & 1811 Lecture # 34
CASE IV A:
3
four regions – four bonds
arrangement
shape
TETRAHEDRAL
TETRAHEDRAL
examples
CH4
bond angle
CF2Cl2
SO42-
109.5°
C2H6
{ SHOW TETRAHEDRAL models DRAW wedge/ dash structures }
CASE IV B:
four regions – three bonds
arrangement
shape
TETRAHEDRAL
TRIGONAL PYRAMIDAL
examples
bond angle ~ 107°
NH3
{ SHOW TRIGONAL PYRAMIDAL models }
CASE IV C:
four regions – two bonds
arrangement
shape
examples
CASE V A:
TETRAHEDRAL
BENT
bond angle ~ 105°
H2O
{ SHOW BENT models }
five regions – five bonds
arrangement
TRIGONAL BIPYRAMIDAL
shape TRIGONAL BIPYRAMIDAL
examples
bond angles 120°
and
90°
PF5 (40 e-)
{ SHOW TRIGONAL BIPYRAMIDAL models
DRAW shape}
CHEM 1711 & 1811 Lecture # 34
CASE V B:
4
five regions – four bonds
arrangement
shape
TRIGONAL BIPYRAMIDAL
SEE-SAW
bond angles ~ 115°
and
~ 88°
SF4 (34 e-)
examples
{ SHOW SEE-SAW models }
CASE V C:
five regions – three bonds
arrangement
shape
TRIGONAL BIPYRAMIDAL
TEE
bond angles ~ 85°
BrF3 (28 e-)
examples
{ SHOW TEE models }
CASE V D:
five regions – two bonds
arrangement
shape
TRIGONAL BIPYRAMIDAL
LINEAR
bond angle 180°
I3- (22 e-)
examples
{ SHOW LINEAR models }
CASE VI A:
six regions – six bonds
arrangement
OCTAHEDRAL
shape
OCTAHEDRAL
examples
SF6
bond angles 90°
{ SHOW OCTAHEDRAL models }
CHEM 1711 & 1811 Lecture # 34
CASE VI B:
5
six regions – five bonds
arrangement
OCTAHEDRAL
shape
SQUARE PYRAMIDAL
examples
IF5 (42 e-)
bond angles 90°
{ SHOW SQUARE PYRAMIDAL models }
CASE VI C:
six regions – four bonds
arrangement
OCTAHEDRAL
shape
SQUARE PLANAR
examples
XeF4 (36 e-)
{ SHOW SQUARE PLANAR models }
bond angles 90°