CHEMISTRY 103 – Help Sheet #15 Molecular Geometry-Part I

Do the topics appropriate for your course Prepared by Dr. Tony Jacob

https://clc.chem.wisc.edu (Resource page) Nuggets: VSEPR; Dipole Moment/Molecular Polarity VALENCE SHELL ELECTRON PAIR REPULSION MODEL (VSEPR): A model used to determine molecular shape of the molecule. VSEPR shape will be wrong if you have an incorrect Lewis structure. Method to determine VSEPR structure:

1. Draw correct Lewis Dot Structure.

2. Add the #lone pairs of e- around the central atom + #atoms bonded to the central atom. 3. Compare to VSEPR table (last page of this handout)

Electron Domain Geometry (EDG) refers to the geometry when all domains are considered; All domains = #lone pairs of e- + #atoms bonded to the central atom. (Electron domain geometry, EDG, can also be called electron region geometry, ERG, and electron pair geometry, EPG; they all mean the same thing!)

Molecular Geometry (MG) refers to the actual arrangement of atoms in space. In general, all lone pairs on atoms attached to the central atom are not included. (Sometimes MG diagrams include lone pairs of electrons on the central atom; sometimes these lone pairs are not included; check with your instructor.)

Repulsions: lone pair e-–lone pair e- > lone pair e-–bonding pair e- > bonding pair e-–bonding pair e- MOLECULAR POLARITY/DIPOLE MOMENT: The sum of all of the bond polarities.

If DEN bond vectors ( ) cancel Þ nonpolar molecule; dipole moment (measure of polarity) = 0 If DEN bond vectors ( ) don’t cancel Þ polar molecule; dipole moment (measure of polarity) ≠ 0 Polar molecules can absorb microwave radiation and heat up in a microwave oven. Non-polar molecules do not absorb microwave radiation and do not heat up in a microwave oven.

Example 1: a. Draw the Lewis dot structure for CO2. b. Draw and name the electron domain geometry (EDG). c. Draw and name the molecular geometry (MG). d. Using the electron domain geometry diagram, draw the polarity vectors for each bond. Is the molecule polar?

Answer 1: a. b. c. d. Example 2: a. Draw the Lewis dot structure for H2O. b. Draw and name the electron domain geometry (EDG). c. Draw and name the molecular geometry (MG). d. Using the electron domain geometry diagram, draw the polarity vectors for each bond. Is the molecule polar?

Answer 2: a. b. c. or d. Example 3: a. Draw the Lewis dot structure for IF2

-. b. Draw and name the electron domain geometry (EDG). c. Draw and name the molecular geometry (MG). d. Using the electron domain geometry diagram, draw the polarity vectors for each bond. Is the molecule polar?

Answer 3: a. b. c. or d.

O C O

O C OElectron Domain Geometry (EDG):

2 atoms + 0 lone pairs = linear

O C OMolecular Geometry (MG):

2 atoms + 0 lone pairs = linear

O C Ovectors point to the more EN atom; vectors cancel;

nonpolar molecule

OH HElectron Domain Geometry (EDG): 2 atoms + 2 lone pairs = tetrahedral

H

OH

H

OH

Molecular Geometry (MG): 2 atoms + 2 lone pairs = tetrahedral

H

OH

H

OH

net dipole

vectors point to the more EN atom; vectors don't cancel; polar molecule

IF F-

I

F

F

-

Electron Domain Geometry (EDG): 2 atoms + 3 lone pairs =

trigonal bipyramid

I

F

F

-

Molecular Geometry (MG): 2 atoms + 3 lone pairs = linear

I

F

F

- I

F

F

-

vectors point to the more EN atom; vectors cancel;

nonpolar molecule

Is the molecular Polar or Nonpolar? 1. If VSEPR = 2|1, 3|1, 2|2, 4|1, 3|2 and 5|1 Þ polar (always) (#atoms|#lone pairs e- on central atom)

2. If VSEPR = 2|0, 3|0, 4|0, 5|0 and 2|3, 6|0 and 4|2, AND all the atoms around the central atom are the same atom Þ nonpolar; if atoms around the central atom are different Þ polar

3. Molecules which are 5|0 (trigonal bipyramid), 6|0 (octahedral) and 4|2 (square planar) with different atoms around the central atom can be nonpolar or polar depending on the atom locations around the central atom. For example, PCl3F2 (5|0) can be nonpolar if both F atoms are in the axial positions but polar if one F is axial and one is equatorial (see below). Likewise, SF4Cl2 (6|0) can be nonpolar if the two Cl atoms are 180˚ apart (on the same axes) but polar if they are 90˚ apart (on different axes; see below). Finally, XeF2Cl2 (4|2) can be nonpolar if the two Cl atoms are 180˚ apart (on the same axes) but polar if they are 90˚ apart (on different axes; see below).

vs ; vs ; vs 4. Hydrocarbons (only hydrogen and carbon) – nonpolar 5. Binary atom molecules: X–X atoms: nonpolar; X–Y atoms: polar 6. Molecules with N or P with 3 groups attached or O or S with 2 groups attached are polar:

DRAWING STRUCTURES: EDG structures have all e- pairs drawn; MG structures often have all e- pairs removed. (Note: The angles shown are idealized values. For example, the T-shaped molecule (3|2-MG) does not reflect that the 180˚ angle for the axial atoms is reduced somewhat due to the lone pair electrons occupying a larger volume causing the two axial B atoms to have an angle less than 180˚ apart.)

2 2|0-EDG 2|0-MG

3

3|0-EDG 3|0-MG 2|1-EDG 2|1-MG

4

4|0-EDG 4|0-MG 3|1-EDG 3|1-MG 2|2-EDG 2|2-MG

5

5|0-EDG 5|0-MG 4|1-EDG 4|1-MG 3|2-EDG 3|2-MG 2|3-EDG 2|3-MG

6

- or -

6|0-EDG 6|0-MG 5|1-EDG 5|1-MG 4|2-EDG 4|2-MG

PF

Cl

ClF

F

Nonpolar

PCl

F

ClF

F

Polar

S

F

F

Cl Cl

F

F

Nonpolar

S

Cl

F

F Cl

F

F

Polar

XeCl Cl

F

F

Nonpolar

XeCl F

F

Cl

Polar

N P O S

B A B B A B

A

B

B BA

B

B BA

B B

AB B

B

AB

BB

B

AB

BB

AB

BB

AB

BB

B

AB

B

AB

AB

B

BB

BAB

B

BB

BA

B

BB

BA

B

BB

BAB

B

B

AB

B

B

A

B

B

A

B

B

A

B

B

B B

B

B

A

B

B

B B

B

B

A

B

B B

B

B

A

B

B B

B

BAB B

B

B

AB B

B

B

AB B

B

B

1. Predict the electron domain geometry (EDG) and the molecular geometry (MG) of the following molecules. a. IF4

+ b. SF5+ c. SeH2 d. NF3 e. NH2

- f. BrF3 g. BrF5 h. SCl4 i. PF6- j. IF4

- 2. What is the electron domain geometry (EDG) around sulfur in the molecule SF4? a. tetrahedral b. octahedral c. square planar d. trigonal planar e. trigonal bipyramid 3. Which molecule(s) below would be nonpolar? There may be more than one molecule. a. H2S b. PF3 c. TeF6 4. How many of the following molecules will have one lone pair of electrons on the central atom? NF3 CF4 TeF4 N3

- a. 0 molecules b. 1 molecule c. 2 molecules d. 3 molecules e. 4 molecules 5. How many of the following molecules will have an electron domain geometry (EDG) of tetrahedral? XeF4 CH4 SeF2 SF3

+ a. 0 molecules b. 1 molecule c. 2 molecules d. 3 molecules e. 4 molecules 6. For each molecule a) draw the Lewis dot structure. b) in 3D, draw the electron domain geometry (EDG); name the geometry. c) in 3D, draw the molecular geometry (MG); name the geometry. d) redraw the EDG in 3D and draw in vectors representing the bond dipoles. e) after drawing in the vectors in part “d”, draw the net vector representing the net dipole if the molecule is polar; if the molecule is nonpolar write “no net dipole”. i) BrF3 ii) BCl2F iii) GeCl4-2 iv) NH3 v) CF4 vi) COF2

ANSWERS 1. a. electron domain geometry: trigonal bipyramid; molecular geometry: seesaw

{draw Lewis dot structure: ; 4 atoms + 1 lone pair = 5; EDG = trigonal bipyramid; MG = seesaw} b. electron domain geometry: trigonal bipyramid; molecular geometry: trigonal bipyramidal

{draw Lewis dot structure: ; 5 atoms + 0 lone pairs = 5; EDG = trigonal bipyramid; MG = trigonal bipyramid} c. electron domain geometry: tetrahedral; molecular geometry: bent

{draw Lewis dot structure: ; 3 atoms + 1 lone pair = 4; EDG = tetrahedral; MG = bent} d. electron domain geometry: tetrahedral; molecular geometry: trigonal pyramidal

{draw Lewis dot structure: ; 3 atoms + 1 lone pair = 4; EDG = tetrahedral; MG = trigonal pyramid} e. electron domain geometry: tetrahedral; molecular geometry: bent

{draw Lewis dot structure: ; 2 atoms + 2 lone pairs = 4; EDG = tetrahedral; MG = bent} f. electron domain geometry: trigonal bipyramid; molecular geometry: T-shaped

{draw Lewis dot structure: ; 3 atoms + 2 lone pairs = 5; EDG = trigonal bipyramid; MG = T-shaped} g. electron domain geometry: octahedral; molecular geometry: square pyramid

{ ; 5 atoms + 1 lone pair = 6; EDG = octahedral; MG = square pyramid} h. electron domain geometry: trigonal bipyramid; molecular geometry: seesaw

{draw Lewis dot structure: ; 4 atoms + 1 lone pair = 5; EDG = trigonal bipyramid; MG = seesaw} i. electron domain geometry: octahedral; molecular geometry: octahedral

{draw Lewis dot structure: ; 6 atoms + 0 lone pairs = 6; EDG = octahedral; MG = octahedral} j. electron domain geometry: octahedral; molecular geometry: square planar

{draw Lewis dot structure: ; 4 atoms + 2 lone pairs = 6; EDG = octahedral; MG = square planar}

I FF

F

F+

SF

F

FF

F

+

SeH H

N

F

F F

NH H-

Br FF

F

BrF

F

FF

F

S

Cl

Cl

Cl Cl

P

F

FF

F

F

F

-

I FF

F

F-

2. e {draw Lewis dot structure: ; 4 atoms + 1 lone pair = 5: EDG = trigonal bipyramid} 3. TeF6 {Draw Lewis dot structures: : 2 atoms + 2 lone pairs = 4: MG = bent – always polar;

: 3 atoms + 1 lone pair = 4: MG = trigonal pyramid – always polar;

: 6 atoms + 0 lone pairs = 6: MG = octahedral; when all 6 atoms are the same – always nonpolar} 4. c {NF3 and TeF4 have lone pairs on the central atom;

Draw Lewis dot structures: ; ; ; }

5. d {CH4; SeF2; SF3+; Draw Lewis dot structures: : 4 atoms + 2 lone pairs = 6 = octahedral;

: 4 atoms + 0 lone pairs = 4 = tetrahedral; : 2 atoms + 2 lone pairs = 4 = tetrahedral;

: 3 atoms + 1 lone pairs = 4 = tetrahedral}

SF

F

FF

SH H

P

F

F F

Te

Cl

ClCl

Cl

Cl

Cl

N

F

F F

C FF

F

F

Te FF

F

F

N N N-

Xe FF

F

F

C HH

H

H

SeF F

S FF

F+

6. i) a. b. c. or d. e. polar

ii) a. b. c. d. e. polar

iii) a. b. c. or

d. e. polar

iv) a. b. c. or d. e. polar

v) a. b. c. d. e. nonpolar molecule

vi) a. b. c. d. e. polar

BrF F

LDSF

BrF

F

FEDG = trigonal bipyramidal

BrF

F

FMG = T-shaped

BrF

F

F

BrF

F

Fnet dipole

BCl Cl

LDSF

BF Cl

Cl

EDG = trigonal planar

BF Cl

Cl

MG = trigonal planar

BF Cl

Cl

net dipole

Ge

Cl

Cl

Cl Cl

-2

LDS

Ge

Cl

ClCl

Cl

-2

EDG = trigonal bipyramidal

Ge

Cl

ClCl

Cl

-2

MG = seesaw

Ge

Cl

ClCl

Cl

-2

Ge

Cl

ClCl

Cl

-2

net dipole

NH H

LDSH

NH

HH

EDG = tetrahedral

NH

HH

MG = trigonal pyramidal

NH

HH

net dipoleN

HH

H

C

F

F

F F

LDS

F

CF

FF

EDG = tetrahedral

F

CF

FF

MG = tetrahedral

F

CF

FF

no net dipole

CF F

LDSO

CF F

O

EDG = trigonal planar

CF F

O

MG = trigonal planar

CF F

O

net dipole

VSEPR(besttomemorize)Total = A + B

#atoms bonded to central atom

A

# lone pairs on central atom

B

Electron domain

geometry

Picture Molecular geometry

Picture Angles Ex Polar HO

2 2 0 Linear

Linear 180o BeCl2 NP* sp

3

3

0

Trigonal1

planar

Trigonal planar

120o

BF3

NP* sp2

2

1

Trigonal planar

Bent

~115o or

<120o

AlF2-

P sp2

4

4

0

Tetrahedral

Tetrahedral

109.5o

CH4

NP* sp3

3

1

Tetrahedral

Trigonal

pyramidal2

~107o or

<109.5o

NH3

P sp3

2

2

Tetrahedral

Bent

~105o or

<109.5o

H2O

P sp3

5

5

0

Trigonal

bipyramidal3

Trigonal

bipyramidal

90o, 120o

PCl5

NP**

4

1

Trigonal

bipyramidal

Seesaw

90o,

~115o

SF4

P

3

2

Trigonal

bipyramidal

T-shape

90o

ClF3

P

2

3

Trigonal

bipyramidal

Linear

180o

I3

-

NP*

6

6

0

Octahedral

Octahedral

90o

SF6

NP**

5

1

Octahedral

Square

pyramidal

90o

IF5

P

4

2

Octahedral

Square planar

90o

XeF4

NP**

P = polar molecule; NP = nonpolar molecule 1 = trigonal is also called triangular; 2 = pyramidal is also called pyramid; 3 = bipyramidal is also called bipyramid * = The nonpolar (NP) molecules assume the atoms around the central atom are identical; if not, the molecule will be polar ** = In the square planar or octahedral nonpolar molecules, the atoms around the central atom can be different atoms as long as the

atoms that are 180˚ from each other are the same. In the case of a trigonal bipyramid molecule the atoms that are 120˚ apart or the atoms that are 180˚ apart need to be the same for the molecule to be nonpolar (e.g., PCl2F3 can be polar or nonpolar depending on the positions of the Cl and F atoms).

Images: Public Domain from Wikipedia.org (Public Domain, https://commons.wikimedia.org/w/index.php?curid=1454616)