The Shapes of Molecules · SAMPLE PROBLEM 1 Writing Lewis Structures for Molecules with One Central Atom SOLUTION: PROBLEM: Write a Lewis structure for CCl 2 F 2, one of the compounds

10-1

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

The Shapes of Molecules

10-2


The steps to follow in converting a molecular formula into LEDS.

1st: Place the atoms relative to each other. [Atom withthe lowest electronegativity(EN)]

2nd: Determine the total number of valence electronsavailable. (Recall that the number of valence e-equals the A-group number)

3rd: Draw a single bond from each surrounding atomto the central atom and subtract two valenceelectrons for each bond.

4th: Distribute the remaining electrons in pairs so thateach atom ends up with 8 electrons (or 2 for H).First place the lone pairs to the surrounding (moreelectronegative) atoms to give each an octet.

10-3


The steps to follow in converting a molecular formula into LEDS.

5th : CASES INVOLVING MULTIPLE

BONDS

If after step 4, a central atom still does NOT have an octet, make MULTIPLE bond by changing a lone pair from one of the surrounding atoms into a bonding pair in the central atom.

10-4


Figure 10.1

The steps in converting a molecular formula into a Lewis structure.

Molecular

formula

Atom

placement

Sum of

valence e-

Remaining

valence e-

Lewis

structure

Place atom

with lowest

EN in center

Add A-group

numbers

Draw single bonds.

Subtract 2e- for each bond.

Give each

atom 8e-

(2e- for H)

Step 1

Step 2

Step 3

Step 4

10-5


Molecular

formula

Atom

placement

Sum of

valence e-

Remaining

valence e-

Lewis

structure

NF3

NFF

F

N 5e-

F 7e-

X 3 = 21e-Total 26e-

:

: :

:

: :

:: ..

N 5e- X 1 = 5e-

F 7e-

NF

F

F

:

10-6


SAMPLE PROBLEM 1 Writing Lewis Structures for Molecules with

One Central Atom

SOLUTION:

PROBLEM: Write a Lewis structure for CCl2F2, one of the compounds

responsible for the depletion of stratospheric ozone.

PLAN: Follow the steps outlined in Figure 10.1 .

Step 1: Carbon has the lowest EN and is the central atom.

The other atoms are placed around it.C

Steps 2-4:

C has 4 valence e-, Cl and F each have 7. The

sum is 4 + 4(7) = 32 valence e-.

Cl

Cl F

F

C

Cl

Cl F

F

Make bonds and fill in remaining valence

electrons placing 8e- around each atom.

:

::

::

:

:

::

: ::

10-9


There are some compounds that undergo theprocess of chemical bonding that form more or lessthan 8 electrons are are considered EXCEPTIONSTO THE OCTET RULE.

1) Electron Deficient molecules

- gaseous compounds containing Be or B as the central atom.

Ex. BF3; BeCl2

2) Odd-Electron molecules

-most have central atoms from an odd-numbered group.

Ex. N (Group 5A -15); Cl (Group 7A -17)

NO2 ( free radical contain a lone electron.)

10-10


There are some compounds that undergo theprocess of chemical bonding that form more or lessthan 8 electrons are are considered EXCEPTIONSTO THE OCTET RULE.

3) Expanded Valence Shells

- molecules having MORE than 8 valence electrons

around the central atom.

- occur around a central NON METAL atom from

period 3 or higher, those in which d orbitals are available.

Ex. SF6

10-12


• VSEPR focuses not only on

electron pairs it also focus on

electron groups as a whole.

• An electron group is an

electron pair, a lone pair, a

single unpaired electron, a

double bond or a triple bond

on the central atom.

The valence-shell electron-pair repulsion (VSEPR)

theory states that electron pairs repel each other

whether they are in bond pairs or in lone pairs. Electron

pairs will spread themselves as far from each other as

possible to minimize repulsion.

A model for predicting the geometry of molecules

10-13


VSEPR - Valence Shell Electron Pair Repulsion Theory

has a its general formula.

A Xm En

A - central atom

X -surrounding atom

or

Bonding pairsE -nonbonding

valence electron-

group (lone pairs)

integers

Understanding the molecular structure of a compound can help determine

the polarity, reactivity, phase of matter, color, magnetism, as well as the

biological activity.

10-14


The actual shape of a molecule can be determined by

the location of the nuclei and the distribution of

electrons.

CATEGORIES:

1) Electron-group geometry

is determined by the

NUMBER of

ELECTRON GROUPS

2) Molecular Geometry

(specific geometry)

depends on the NUMBER

OF LONE PAIRS.

Number of electron

groups

Name of electron

group geometry

2 linear

3 trigonal-planar

4 tetrahedral

5trigonal-

bipyramidal

6 octahedral

ELECTRON GROUP GEOMETRY

10-15


Number

of

Electron

Groups

Electron-

Group

Geometry

# of

Lone

Pairs

VSEPR

Notation

(Type of

Shape)

Molecular

Geometry

Ideal

Bond

Angles

Examples

2 linear 0 AX2 180° BeH2

3Trigonal

planar

0 AX3 120° CO32-

1 AX2E 120° O3

10-16


Number

of

Electron

Groups

Electron-

Group

Geometry

# of

Lone

Pairs

VSEPR

Notation

(Type of

Shape)

Molecular

Geometry

Ideal

Bond

Angles

Examples

4

Tetrahe

-dral

0 AX4 109.5° S042-

1 AX3E 109.5° H3O+

2 AX2E2 109.5° H2O

10-17

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.No. of

Electro

n

Groups

Electron-

Group

Geometry

# of

Lone

Pairs

VSEPR

Notation

(Type of

Shape)

Molecular

Geometry

Ideal

Bond

Angles

Examples

5Trigonal-

Bipyramidal

0

AX5 90°,

120° PF5

1 AX4E

90°,

120° TeCl4

2 AX3E2 90°

3 AX2E3180°

I3-

10-18

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.No. of

Electro

n

Groups

Electron-

Group

Geometry

# of

Lone

Pairs

VSEPR

Notation

(Type of

Shape)

Molecular

Geometry

Ideal

Bond

Angles

Examples

6 Octahedral

0 AX6 90° PF6-

1AX5E

90° SbCl52-

2 AX4E290° ICl4

-

10-19


Figure 10.2

Electron-group repulsions and the five basic molecular shapes.

linear trigonal planar tetrahedral

trigonal bipyramidal octahedral

10-20


Figure 10.3 The single molecular shape of the linear electron-group

arrangement.

Examples:

CS2, HCN, BeF2

10-21


Figure 10.4 The two molecular shapes of the trigonal planar electron-

group arrangement.

Class

Shape

Examples:

SO3, BF3, NO3-, CO3

2-

Examples:

SO2, O3, PbCl2, SnBr2

10-22


Factors Affecting Actual Bond Angles

Bond angles are consistent with theoretical angles when the atoms

attached to the central atom are the same and when all electrons are

bonding electrons of the same order.

C O

H

Hideal

1200

1200

larger EN

greater

electron

density

C O

H

H

1220

1160

real

Lone pairs repel bonding pairs

more strongly than bonding pairs

repel each other.

Sn

Cl Cl

950

Effect of Double Bonds

Effect of Nonbonding(Lone) Pairs

10-23


Figure 10.5 The three molecular shapes of the tetrahedral electron-

group arrangement.

Examples:

CH4, SiCl4,

SO42-, ClO4

-

NH3

PF3

ClO3

H3O+

H2O

OF2

SCl2

10-24


Figure 10.6 Lewis structures and molecular shapes.

10-25


Figure 10.7 The four molecular shapes of the trigonal bipyramidal

electron-group arrangement.

SF4

XeO2F2

IF4+

IO2F2-

ClF3

BrF3

XeF2

I3-

IF2-

PF5

AsF5

SOF4

10-26


Figure 10.8 The three molecular shapes of the octahedral electron-

group arrangement.

SF6

IOF5

BrF5

TeF5-

XeOF4

XeF4

ICl4-

10-27


Figure 10.9 A summary of common molecular shapes with two to six

electron groups.

10-28


Figure 10.10 The steps in determining a molecular shape.

Molecular

formula

Lewis

structure

Electron-group

arrangement

Bond

angles

Molecular

shape

(AXmEn)

Count all e- groups around central

atom (A)

Note lone pairs and double

bonds

Count bonding and

nonbonding e-

groups separately.

Step 1

Step 2

Step 3

Step 4

See Figure

10.1

10-29


SAMPLE PROBLEM 10.6 Predicting Molecular Shapes with Two, Three,

or Four Electron Groups

PROBLEM: Draw the molecular shape and predict the bond angles (relative

to the ideal bond angles) of (a) PF3 and (b) COCl2.

SOLUTION: (a) For PF3 - there are 26 valence electrons, 1 nonbonding pair

PF F

F

The shape is based upon the tetrahedral arrangement.

The F-P-F bond angles should be <109.50 due

to the repulsion of the nonbonding electron

pair.

The final shape is trigonal pyramidal.

PF F

F

<109.50

The type of shape is

AX3E

10-30


SAMPLE PROBLEM 10.6 Predicting Molecular Shapes with Two, Three,

or Four Electron Groups

continued

(b) For COCl2, C has the lowest EN and will be the center atom.

There are 24 valence e-, 3 atoms attached to the center atom.

CCl O

Cl

C does not have an octet; a pair of nonbonding

electrons will move in from the O to make a

double bond.

The shape for an atom with three atom

attachments and no nonbonding pairs on the

central atom is trigonal planar.CCl

O

Cl The Cl-C-Cl bond angle will

be less than 1200 due to

the electron density of the

C=O.

C

Cl

O

Cl

124.50

1110

Type AX3

10-31


SAMPLE PROBLEM 10.7 Predicting Molecular Shapes with Five or Six

Electron Groups

PROBLEM: Determine the molecular shape and predict the bond angles

(relative to the ideal bond angles) of (a) SbF5 and (b) BrF5.

SOLUTION: (a) SbF5 - 40 valence e-; all electrons around central

atom will be in bonding pairs; shape is AX5 - trigonal

bipyramidal.

F

SbF

F F

FF Sb

F

F

F

F

(b) BrF5 - 42 valence e-; 5 bonding pairs and 1 nonbonding pair on central

atom. Shape is AX5E, square pyramidal.

BrF

F F

F

F

Documents

The Shapes of Molecules · SAMPLE PROBLEM 1 Writing Lewis Structures for Molecules with One Central Atom SOLUTION: PROBLEM: Write a Lewis structure for CCl 2 F 2, one of the compounds