Using Octet Rule Chapter 10: The Shapes of Moleculeszakarian/az_personal_web_cz/CHM1050... · Chapter 10: The Shapes of Molecules ¥ Lewis Structures for Molecules and Ions ... form

Chapter 10: The Shapes of

Molecules

• Lewis Structures for

Molecules and Ions

• Shapes of Molecules: a Certain Theory

• Polarity of Molecules

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)

Using Octet Rule

Using Octet Rule

Step 1. Place the atoms relative to each other. NF3; SOCl2

N

F F

F

S

ClCl

O

for compounds such as ABn, place the atom with lower group

number in the center.

Using Octet Rule

Step 1. Place the atoms relative to each other. NF3; SOCl2

N

F F

F

S

ClCl

O

for compounds such as ABn, place the atom with lower group

number in the center.

lowest EN

Using Octet Rule

Step 2. Determine the total number of valence electrons

available.

N

F F

F

S

ClCl

O

(for ions, add one e- for each negative charge and subtract one

each positive charge).

Using Octet Rule


available.

N

F F

F

S

ClCl

O

(for ions, add one e- for each negative charge and subtract one

each positive charge).

3x7 + 5 = 26 2x7 + 2x6 = 26

Using Octet Rule

Step 3. Draw a single bond from each surrounding atom to

the central atom.

3x7 + 5 = 26 2x7 + 2x6 = 26

N

F

F F

SClCl

O

Using Octet Rule


the central atom.

Step 3a. Subtract 2e- for each single bond from the total number

of valence electrons.

3x7 + 5 = 26 - 6 = 20 2x7 + 2x6 = 26- 6 = 20

N

F

F F

SClCl

O

Using Octet Rule

Step 4. Distribute the remaining electrons in pairs so that

each atom obtains eight electrons (or two for H).

Start with more electronegative atoms.

3x7 + 5 = 26 - 6 = 20 2x7 + 2x6 = 26- 6 = 20

N

F

F F

SClCl

O

Using Octet Rule



Start with more electronegative atoms.

3x7 + 5 = 26 - 6 = 20- 18 = 2

2x7 + 2x6 = 26- 6 = 20- 18 = 2

N

F

F F

SClCl

O

Using Octet Rule



If any electrons remain, place them around the central atom

3x7 + 5 = 26 - 6 = 20- 18 = 2

2x7 + 2x6 = 26- 6 = 20- 18 = 2

N

F

F F

SClCl

O

Using Octet Rule



If any electrons remain, place them around the central atom

3x7 + 5 = 26 - 6 = 20- 18 = 2

2x7 + 2x6 = 26- 6 = 20- 18 = 2

N

F

F F

SClCl

O

Using Octet Rule

Step 1. Place the atoms relative to each other.


available.


the central atom



Works for C, N, O as the central atom, single bonds only!

Using Octet Rule

BONUS!

Step 5. If, after step 4. The central atom still does not have

an octet, make a multiple bond by changing a lone pair

from one of the surrounding atoms into a bonding pair to

the central atom.

Let’s analyze CO2

Using Octet Rule

BONUS!

Step 5. If, after step 4. The central atom still does not have

an octet, make a multiple bond by changing a lone pair

from one of the surrounding atoms into a bonding pair to

the central atom.

Let’s analyze CO2

HCN, CO, N2

Resonance

S

O

Cl Cl

S

O

Cl Cl

thionyl chloride

O

O

O O

O

O

ozone

O C O

O C O

Resonance

O

O

O O

O

O

S

O

Cl Cl

S

O

Cl Cl

O

O

O

thionyl chloride

ozone

S

O

Cl Cl

Resonance

A

B

C C

B

A C

B

A

general situation:

Resonance

A

B

C C

B

A C

B

A

A

B

A A

B

A A

B

A

general situation:

when symmetrical:

bond order is 1.5

Resonance

BOND ORDER = number of electron pairs

number of atom-to-atom linkages

C

O

O

O

2-

N

O

O

O

-

P

O

O

O

3-

O

carbonate nitrate phosphate

Formal Charge

the charge an atom would have if the electrons

were shared equally

FC of atom = no. of v. e– – no. of unshared v. e– – 1/2 of shared v. e–

Formal Charge


were shared equally


S

O

Cl Cl

S

O

Cl Cl

thionyl chloride

Formal Charge


were shared equally


S

O

Cl Cl

S

O

Cl Cl

thionyl chloride

0 0

0

0

Formal Charge


were shared equally


S

O

Cl Cl

S

O

Cl Cl

thionyl chloride

0 0

0

0

0 0

+1

-1

Formal Charge


were shared equally


S

O

Cl Cl

S

O

Cl Cl

thionyl chloride

0 0

0

0

0 0

+1

-1

the structure with the lowest FC has a higher contribution

Formal Charge


were shared equally

Most important resonance structures will have

• smaller FC

• no like charges on adjacent atoms

• more negative FC on a more electronegative atom

Formal Charge


were shared equally

Most important resonance structures will have

• smaller FC

• no like charges on adjacent atoms

• more negative FC on a more electronegative atom

EXAMPLES: H2CN2 (diazomethane)

N3– (azide anion)

NCO– (cyanate anion) in the textbook

SO32– (sulfite anion)

Exceptions to the Octet Rule

we’ve seen them before…

• Electron-Deficient Molecules

• Odd-Electron Molecules

• Expanded Valence Shell



Typically, groups 2 and 3

AlCl3, BF3, BeCl2

Al

Cl

Cl Cl



Typically, groups 2 and 3

AlCl3, BF3, BeCl2

Al

Cl

Cl Cl

B Al

Cl

Cl Cl

B


• Odd-Electron Molecules, aka free radicals

Typically, group 5 elements (NITROGEN)

NO

Example: NO (nitrogen monoxide)


• Odd-Electron Molecules, aka free radicals

Example: NO (nitrogen monoxide)

NO N ONO

N O

Typically, group 5 elements (NITROGEN)


• Expanded Valence Shell

ONLY period 3 or higher

Examples: SF6, PCl5

S

F F

F F

F

F

PCl Cl

Cl Cl

Cl

Using Bond Energies

To calculate heats of reactions:

!Horxn = !Ho

reactant bonds broken + !Hoproduct bonds formed

O OH H H H

HH

OH H

O!Horxn

enthalpy

H

Molecular Shape

groups or unshared electron pairs around a central atom

are located as far away as possible from each others

A is the central atom:

A X A

A

AXnEm

X is a group; E is an unshared pair of electrons

(E)

Valence-Shell Electron-Pair Repulsion Theory (VSEPR)

Molecular Shape

Valence-Shell Electron-Pair Repulsion Theory (VSEPR)

A X A

A

AXnEm

X is a group; E is an unshared pair of electrons

(E)

non-bonding pair > double bond > single bond

repulsion effect decreases

Molecular Shape

Molecular Shape

Molecular Polarity

molecular polarity is a vector sum of bond polarity

dipole momentum (µ) = charge " distance

1 D (debye) = 3.34 " 10-30 C•m

!+ !"

H Cl

Molecular Polarity

molecular polarity is a vector sum of bond polarity

dipole momentum (µ) = charge " distance

O C O

O

S

O

non-polar

H

O

H

Molecular Shape

Practice Problems

problem 10.4

What is required for an atom to expand its valence shell?

Which of the following atoms can expand its valence shell:

F, S, H, Cl, Al, Se, Ca

Molecular Shape

Practice Problems

problem 10.18 +

Draw a Lewis structure and calculate the formal charge of

each atom in

IF5

AlH4–

ClO–

CH3–

Molecular Shape

Practice Problems

problem 10.14 +

Draw a Lewis structure and the most important resonance

form for each ion, showing formal charges and oxidation

numbers of the atoms:

AsO4–

ClO2–

BrO3–

Molecular Shape

Practice Problems

follow-up problem 10.8

Draw molecular shapes and predict relative bond angles

of

(a) ICl2–

(b) ClF3

(c) SOF4

(d) IF4–

(e) ClO3–

Molecular Shape: Polyatomic

Practice Problemsproblem 10.57 and 10.58

State ideal values for each of the bond angles in each

molecule, and note where you expect deviations:

H C

H

N O H C O C

H

H

H

H

H

H

O B

O

O

H

HH

O N O

O

H CH C

H

H H

O H C

O

O H

Molecular Shape: Polyatomic


Which molecule in each pair has the greater dipole moment:

(a) ClO2 or SO2

(b) HBr or HCl

(c) BeCl2 or SCl2

(d) AsF3 or AsF5

Molecular Shape


Which molecule in each pair has the greater dipole moment:

(a) ClO2 or SO2

(b) HBr or HCl

(c) BeCl2 or SCl2

(d) AsF3 or AsF5

Molecular Shape

Practice Problemsproblem 10.82

(a) CO

(b) CO32–

(c) H2CO

(d) CH4O

(e) HCO3– (H is attached to O)

Like several other bonds, carbon-oxygen bonds have lengths

and strengths that depend on the bond order. Draw Lewis

structures for the following species, and arrange them in

order of increasing carbon-oxygen bond length.

Molecular Shape


Both aluminum and iodine form chlorides with bridging Cl

atoms. Lewis structures not showing lone pairs are:

Al

Cl

Al

Cl

Cl

Cl

Cl

Cl

I

Cl

I

Cl

Cl

Cl

Cl

Cl

(a) What is the formal charge on each atom?

(b) Which of these molecules has a planar shape?

Molecular Shape


Some scientists speculate that many organic molecules

required for life on Earth arrived on meteorites. The

Murchison meteorite that landed in Australia in 1969

contained 92 different amino acids, including 21 found in

Earth organisms. A skeleton of one of these extraterrestrial

amino acids is

Draw a Lewis structure, and identify atoms with a nonzero

formal charge.

H3NHC C

CH2 O

O

CH3

Using Octet Rule Chapter 10: The Shapes of Moleculeszakarian/az_personal_web_cz/CHM1050... · Chapter 10: The Shapes of Molecules ¥ Lewis Structures for Molecules and Ions ... form

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