formal charge l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Formal Charge PowerPoint Presentation
Download Presentation
Formal Charge

Loading in 2 Seconds...

play fullscreen
1 / 33

Formal Charge - PowerPoint PPT Presentation


  • 1854 Views
  • Uploaded on

Formal Charge. Edward A. Mottel Department of Chemistry Rose-Hulman Institute of Technology. H. •. •. H. O. H. C. •. •. H. Formal Charge. Assume bonding electrons belong 1/2 to each bonding atom. The formal charge is the net charge on each atom. e – shared. 2. H. •. •. H. O.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Formal Charge' - gerda


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
formal charge

Formal Charge

Edward A. Mottel

Department of Chemistry

Rose-Hulman Institute of Technology

formal charge2

H

H

O

H

C

H

Formal Charge

Assume bonding electrons

belong 1/2 to each bonding atom.

The formal charge is the net charge on each atom.

formal charge3

e–shared

2

H

H

O

H

C

H

Formal Charge

unshared

electrons

formal

charge

-

= e–donated -

formal charge

of oxygen

6 valence

electrons

4/2 shared

electrons

4 unshared

electrons

=

-

-

formal charges of atoms methanol

4 - 8/2 - 0 = 0

H

H

C

H

O

1 - 2/2 - 0 = 0

H

6 - 4/2 - 4 = 0

C: 4 - 8/2 - 0 = 0

H: 1 - 2/2 - 0 = 0

O: 6 - 4/2 - 4 = 0

Formal Charges of AtomsMethanol

Formal charge =

valence electrons - electrons shared/2 - electrons not shared

Atoms with low formal charges generally indicate the

correct bonding pattern of chemically stable molecules.

incorrect diagrams for methanol

H

H

H

C

H

H

O

H

C

O

C: 4 - 6/2 - 2 = -1

C: 4 - 4/2 - 4 = -2

H

H

H: 1 - 2/2 - 0 = 0

H: 1 - 2/2 - 0 = 0

O: 6 - 6/2 - 2 = +1

O: 6 - 8/2 - 0 = +2

Incorrect Diagrams for Methanol

What are the formal charges for each atom

in the following structures?

Validity Check

The sum of the formal charges always equals

the charge on the molecule or ion.

Validity Check

The sum of the formal charges always equals

the charge on the molecule or ion.

incorrect diagrams for methanol6

H

H

H

H

H

C

C

H

H

H

O

H

C

O

O

C: 4 - 6/2 - 2 = -1

C: 4 - 4/2 - 4 = -2

H

H

H

H: 1 - 2/2 - 0 = 0

H: 1 - 2/2 - 0 = 0

O: 6 - 6/2 - 2 = +1

O: 6 - 8/2 - 0 = +2

Incorrect Diagrams for Methanol

If either of these structures was stable,

which one would more likely be stable?

cyanide ion

C

C

N

N

C: 4 - 6/2 - 2 = -1

total charge = -1

Cyanide Ion

Lewis Dot diagram

Skeleton diagram

C 4 valence e–

N 5 valence e–

charge 1 e–

total 10 valence e–

Convert to a

Lewis Dot diagram

N: 5 - 6/2 - 2 = 0

5 e– pairs

What are the formal charges for each atom in this ion?

ammonium ion

+

H

N 5 valence e–

+

H

H 4 valence e–

H

N

H

H

N

H

charge -1 e–

H

H: 1 - 2/2 - 0 = 0

H

total 8 valence e–

N: 5 - 8/2 - 0 = +1

total charge = +1

Ammonium Ion

Lewis Dot diagram

Skeleton diagram

Nitrogen usually

forms 3 bonds

4 e– pairs

Which atoms have a non-zero formal charge?

In an ion, some atom must have a non-zero formal charge.

Why?

formal charges
Formal Charges
  • What are the formal charges on the atoms in
    • hydroxide ion
    • hydronium ion (oxonium ion)
chlorate ion

O

O

O

Cl

Chlorate Ion

Skeleton diagram

usually the unique atom is in the center

two possibilities

Cl-O-O-O –

chain structures

are uncommon

correct

structure

chlorate ion11

O

O

O

O

O

O

Cl

Cl

Cl: 7 - 6/2 - 2 = +2

O: 6 - 2/2 - 6 = -1

O: 6 - 2/2 - 6 = -1

O: 6 - 2/2 - 6 = -1

total charge = -1

Chlorate Ion

Lewis Dot diagram

Skeleton diagram

Cl 7 valence e–

O 18 valence e–

charge 1 e–

total 26 valence e–

Only 6 electrons

shown

(3 pairs)

13 e– pairs

Complete the Lewis Dot diagram

Why is chlorate ion a good oxidizing agent?

perchlorate ion

Cl 7 valence e–

O

O

O 24 valence e–

O

O

Cl

charge 1 e–

Cl: 7 - 8/2 - 0 = +3

O

O

total 32 valence e–

O: 6 - 2/2 - 6 = -1

O: 6 - 2/2 - 6 = -1

O: 6 - 2/2 - 6 = -1

O: 6 - 2/2 - 6 = -1

total charge = -1

Perchlorate Ion

Lewis Dot diagram

Skeleton diagram

O

O

Cl

Only 8 electrons

shown

(4 pairs)

16 e– pairs

Why are

organic perchlorate salts

potentially explosive?

pair activity formal charges
Pair ActivityFormal Charges
  • Develop a procedure to determine the formal charges on a molecule or ion.
    • Initially, one person should be responsible for writing all the information.
    • The other person should be responsible for saying what should be written.
pair activity formal charges14
Pair ActivityFormal Charges
  • On a card or a piece of paper
    • write each sequential step necessary to determine the formal charges.
    • number each step.
    • amend and update the list.
  • Determine the formal charges on
    • acetic acid, CH3COOH
    • acetate ion, CH3COO–
acetic acid and acetate ion

H

H

H

O

O

O

H

H

C

H

C

H

C

C

C

C

O

H

H

H

O

O

C: 4 - 8/2 - 0 = 0

C: 4 - 8/2 - 0 = 0

H: 1 - 2/2 - 0 = 0

H: 1 - 2/2 - 0 = 0

O: 6 - 4/2 - 4 = 0

O: 6 - 2/2 - 6 = -1

O: 6 - 4/2 - 4 = 0

O: 6 - 4/2 - 4 = 0

total charge = 0

total charge = -1

Acetic Acid and Acetate Ion
formal charges16
Formal Charges
  • What are the formal charges on the atoms in
    • ozone
    • sulfite ion
    • sulfur trioxide
carbonate ion

C 4 valence e–

2-

2-

O

O

O 18 valence e–

O

C

O

charge 2 e–

O

C

O

total 24 valence e–

Carbonate Ion

Lewis Dot diagram

“Connection” diagram

Which oxygen atom

has the double bond?

Convert to a

Lewis Dot diagram

12 e– pairs

resonance

Resonance

Edward A. Mottel

Department of Chemistry

Rose-Hulman Institute of Technology

resonance19

O

O

O

C

O

C

O

C

O

O

O

O

Resonance

For some skeleton and dot structures

there is not an unique, correct answer.

Carbonate ion has three resonant configurations.

Individually,

none of these structures

represents the actual structure.

But the average of these

structures is a good

representation of the ion.

resonance20

O

O

O

C

O

C

O

C

O

O

O

O

Resonance

What is the carbon-oxygen bond order?

(1+1+2) / 3 = 1.33

resonance21

O

O

O

C

O

C

O

C

O

O

O

O

Resonance

Where is the negative charge located

in each contributing structure?

On the oxygen atoms with a single bond.

resonance22

O

O

O

C

O

C

O

C

O

O

O

O

Resonance

What is the average formal charge

on each atom in the carbonate ion?

carbon: 0; oxygen: -2/3

thiocyanate ion

N

N

S

C

S

C

Thiocyanate Ion

Skeleton diagrams

S 6 valence e–

C 4 valence e–

N 5 valence e–

charge 1 e–

Only 8 electrons

shown

(4 pairs)

total 16 valence e–

thiocyanate ion24

-

-

-

N

N

N

S

C

S

C

S

0

0

-1

+1

0

-2

-1

0

0

A “double headed” arrow is the symbol for resonance.

Thiocyanate Ion

Possible Lewis Dot structures

-

C

What carbon-sulfur and carbon-nitrogen bond orders

are predicted for this ion?

Formal charges favor which of these structures?

C-S: 1.5; C-N: 2.5

(nitrogen is slightly more electronegative than sulfur: 3.1, 2.4)

resonance structures
Resonance Structures
  • Factors to consider when picking the best resonance structures:
  • Zero or low formal charges are the best.
  • Formal charges are consistent with the electronegativity of the elements involved.
  • Alternating positive and negative formal charges are better than highly polarized patterns.
sulfate ion

2-

2-

O

O

O

O

O

O

S

S

O

O

S: 6 - 8/2 - 0 = +2

O: 6 - 2/2 - 6 = -1

O: 6 - 2/2 - 6 = -1

O: 6 - 2/2 - 6 = -1

O: 6 - 2/2 - 6 = -1

total charge = -2

Sulfate Ion

Skeleton diagram

Lewis Dot diagram

S 6 valence e–

O 24 valence e–

charge 2 e–

total 32 valence e–

16 e– pairs

Is sulfate ion a

good oxidizing agent?

Is it explosive?

rationalizing the inertness of the sulfate ion
Rationalizing the Inertness of theSulfate Ion
  • The octet rule (8 electrons around an atom) is based on use of one s-orbital and three p-orbitals.
  • If d-orbitals are included, the octet rule is broken.
    • Expanding the valence results in illegal Lewis Dot structures, but may explain the inert chemistry of the sulfate ion.
formal charges of atoms sulfate ion

2-

O

O

O

S

S: 6 - 12/2 - 0 = 0

O

O: 6 - 2/2 - 6 = -1

O: 6 - 4/2 - 4 = 0

O: 6 - 2/2 - 6 = -1

O: 6 - 4/2 - 4 = 0

total charge = -2

Formal Charges of AtomsSulfate Ion

Where are the two

negative charges located?

How many different resonance

configurations can be drawn?

resonance forms of sulfate ion

2-

2-

2-

O

O

O

O

O

S

O

O

S

O

O

S

O

O

O

2-

2-

2-

O

O

O

O

O

O

O

S

O

S

O

S

O

O

O

Resonance Forms of Sulfate Ion

What is the sulfur and oxygen bond order?

Why is sulfate ion relatively inert?

What is the average formal charge on each atom?

benzene c 6 h 6

H

H

H

H

H

H

C

C

C

C

C

C

C

C

C

C

H

H

C

C

H

H

C 24 valence e–

H

H

H 6 valence e–

It is assumed that there are enough hydrogen

atoms at each corner to fulfill carbon’s valence.

Benzene, C6H6

Shorthand notation

Even shorter notation

Cyclic structure

total 30 valence e–

In this structure, what does the circle represent?

What is the carbon-carbon bond order?

cyclopentadiene

H

H

H

What is the shorthand notation for the

cyclopentadienide ion?

Cyclopentadiene

Cyclopentadiene reacts with hydroxide ion

to make the cyclopentadienide ion.

+ OH–

+ H2O

One of several

resonance structures

What is the carbon-carbon bond order in C5H5–?

What is the formula of cyclopentadiene?