IB Chemistry on Resonance, Delocalization and Formal Charges

http://lawrencekok.blogspot.com

Prepared by Lawrence Kok

Tutorial on Resonance, Delocalization andFormal Charges.

FORMAL CHARGE (FC)

Tool/Model for comparing which Lewis structures is more acceptable

Lewis structure SO2 Which is acceptabl

e?

Lewis structure SO3

✓ ✓

Formal charge concept

FORMAL CHARGE (FC)



e?

Lewis structure SO3

Formal Charge•Treats covalent bond with equal electron distribution no EN diff bet atom•Electronegative atom has negative while least electronegative atom has positive formal charge.

Formula formal charge

V - valence electrons of atom L – Lone pair electronB - electrons shared in covalent bonds in the molecule

✓ ✓

L +


FORMAL CHARGE (FC)



e?

Lewis structure SO3

Formal Charge•Treats covalent bond with equal electron distribution no EN diff bet atom•Electronegative atom has negative while least electronegative atom has positive formal charge.


Click here video formal chargesClick here video formal charges

V - valence electrons of atom L – Lone pair electronB - electrons shared in covalent bonds in the molecule

✓ ✓

All resonance structure contribute to electronic structure. Real structure is combination of them.Lowest formal charge (stable), contribute more than less stable structure.Sum of formal charges must be zero for neutral or equal to charge on ion.

L +


FORMAL CHARGE (FC)

Formal Charge•Tool/Model for comparing which Lewis structures is more acceptable•Treats covalent bond with equal electron distribution no electronegativity differences bet atom•Electronegative atom has negative while least electronegative atom has positive formal charge


V - valence electrons of atom L – lone pair electronB – bonding electron molecule

✓Formal charge carbon dioxide

formal charge for O

L +

formal charge for Oformal charge for C

L + L + L +

FORMAL CHARGE (FC)




✓Formal charge carbon dioxide

formal charge for O

V- Valence electron O = 6L- Lone pair electron O = 4B- Bonding electron O = 4

L +

FC = 6 – (4+2) = 0

formal charge for O


FC = 6 – (4+2) = 0

formal charge for C

V- Valence electron C = 4L - Lone pair electron C = 0B - Bonding electron C = 8

FC = 4 – (0+4) = 0

✓ Lowest formal charge is preferred

L + L + L +

Formal charge carbon dioxide

Formal charge for O


FC = 6 – (4+2) = 0

Formal charge for O


FC = 6 – (4+2) = 0

Formal charge for C


FC = 4 – (0+4) = 0

Different Lewis structures for CO2

Which is acceptable?

L +L + L +

Formal charge carbon dioxide

Formal charge for O


FC = 6 – (4+2) = 0

Formal charge for O


FC = 6 – (4+2) = 0

Formal charge for C


FC = 4 – (0+4) = 0

✓Lowest formal charge is preferredDifferent Lewis structures for CO2


0 0 0 0-1 +1 -1 +2 -1 0 -2+2

Lowest formal charge is preferred

✓

Click here video CO2

Lowest formal charge – more stable - contribute more to diff resonance structures.Sum of formal charges must be zero for neutral or equal to charge on ion.

L +L + L +

Formal charge dinitrogen oxide

Formal charge for N Formal charge for N Formal charge for O

V- Valence electron N = 5L- Lone pair electron N = 2B- Bonding electron N = 6



FC = 5 – (2+3) = 0 FC = 5 – (0+4) = +1 FC = 6 – (6+1) = -1

+10 -1

Different Lewis structures for N2O


L + L + L +

Formal charge dinitrogen oxide

Formal charge for N Formal charge for N Formal charge for O




FC = 5 – (2+3) = 0 FC = 5 – (0+4) = +1 FC = 6 – (6+1) = -1

+10 -1

Different Lewis structures for N2O

-2 +1 +1+1 -10 0 +1 -1 -1 +2 -1 -2 +2 0


✓

Click here to view video


Lowest formal charge is preferred ✓


L + L + L +

Delocalization of electrons

Resonance • Describing delocalization of electrons within a molecule/polyatomic ion where bonding cannot be express by ONE single Lewis structure•Delocalization of π bond – π electrons spread over more than 2 nuclei•π electrons are shared•π electrons spread – more stable

Resonance structures carbonate ion

23CO

resonance structure 1 resonance structure 2 resonance structure 3

Carbonate Ion

C



Resonance structures carbonate ion

23CO


Resonance hybrid

• All bonds CO32- are identical in length and strength

• Hybrid of 3 resonance structures• Negative charge equally distributed over all oxygen• No O-O (single) or O=O (double) bonds found• Only O ----- O bond• Intermediate in character bet single and double bond

• Bond Order = 311

Carbonate Ion

• charge 2- delocalized into 2/3-• lower charge – more stable

Click here on video carbonate

C

FORMAL CHARGE (FC)




✓Formal charge carbonate ion

formal charge for O

L +


L + L + L +


FORMAL CHARGE (FC)




✓Formal charge carbonate ion

formal charge for O


L +

FC = 6 – (6 +1) = -1

formal charge for O


FC = 6 – (4+2) = 0

formal charge for C


FC = 4 – (0+4) = 0

-1

-1

0

0✓

L + L + L +


Resonance structures nitrate ion

3NO


Nitrate Ion



Resonance structures nitrate ion

3NO


resonance hybrid

• All bonds NO3- are identical in length and strength

• Hybrid of 3 resonance structures• Negative charge equally distributed over all oxygen• No N-O (single) or N=O (double) bonds found• Only N ----- O bond• Intermediate in character bet single and double bond

• Bond Order = 3

11

Nitrate Ion

charge of -1 delocalized into 1/3-lower charge – more stable



1/3

1/31/3

FORMAL CHARGE (FC)




✓Formal charge nitrate ion

formal charge for O

L +

formal charge for Oformal charge for N

L + L + L +


FORMAL CHARGE (FC)




✓Formal charge nitrate ion

formal charge for O


L +

FC = 6 – (6 +1) = -1

formal charge for O


FC = 6 – (4+2) = 0

formal charge for N

V- Valence electron N = 5L - Lone pair electron N = 0B - Bonding electron N = 8

FC = 5 – (0+4) = +1

-1-1+1

0

✓

L + L + L +


Resonance structures nitrite ion

2NO

resonance structure 1 resonance structure 2

Nitrite Ion



Resonance structures nitrite ion

2NO


resonance hybrid

• All bonds NO2- are identical in length and

strength• Hybrid of 2 resonance structures• Negative charge equally distributed over all

oxygen• NO N-O (single) or N=O (double) bonds found• Only N ----- O bond• Intermediate in character bet single and

double bond

• Bond Order =

211

Nitrite Ion

charge of -1 delocalized into 1/2-lower charge – more stable


Click here video nitrite

FORMAL CHARGE (FC)




✓Formal charge nitrite ion

formal charge for O

L +

formal charge for Oformal charge for N

L + L + L +


FORMAL CHARGE (FC)




✓Formal charge nitrite ion

formal charge for O


L +

FC = 6 – (4 +2) = 0

formal charge for O


FC = 6 – (6+1) = -1

formal charge for N

V- Valence electron N = 5L - Lone pair electron N = 2B - Bonding electron N = 6

FC = 5 – (2+3) = 0

-10 0✓

L + L + L +


Resonance structures sulfur dioxide

2SO


Sulfur Dioxide

S


resonance structure 3

How about structure 3?

resonance hybrid


Resonance structures sulfur dioxide

2SO


• All SO2 bonds are identical in length and strength

• Hybrid of 2 resonance structures• Negative charge equally distributed over all

oxygen• NO S-O (single) or S=O (double) bonds found• Only S ----- O bond• Intermediate in character bet single and

double bond

• Bond Order =

211

Sulfur Dioxide

Click here to view

S



How about structure 3?

resonance hybrid

FORMAL CHARGE (FC)




✓Formal charge sulfur dioxide

formal charge for O

L +

formal charge for Oformal charge for S

L + L + L +

FORMAL CHARGE (FC)




✓Formal charge sulfur dioxide

formal charge for O


L +

FC = 6 – (4 +2) = 0

formal charge for O


FC = 6 – (4+2) = 0

formal charge for S

V- Valence electron S = 6L - Lone pair electron S = 2B - Bonding electron S = 8

FC = 6 – (2+4) = 0

All resonance structure contribute to electronic structure. Real structure is combination of them.Lowest formal charge, contribute more than less stable structure.Sum of formal charges must be zero for neutral or equal to charge on ion.

✓00

0 ✓

L + L + L +

Formal charge Sulfur dioxide

Formal charge for O


FC = 6 – (4 +2) = 0

Formal charge for O


FC = 6 – (4+2) = 0

Formal charge for S


FC = 6 – (2+4) = 0

Different Lewis structures for SO2


0 00

000 0

+1-1 0

+1

-1

Click here to view

L +L + L +

Formal charge Sulfur dioxide

Formal charge for O


FC = 6 – (4 +2) = 0

Formal charge for O


FC = 6 – (4+2) = 0

Formal charge for S


FC = 6 – (2+4) = 0

✓Lowest formal charge is preferredDifferent Lewis structures for SO2



✓

All resonance structure contribute to electronic structure. Real structure is combination of them.Lowest formal charge (stable) contribute more than less stable structure.Sum of formal charges must be zero for neutral or equal to charge on ion.

0 00

000 0

+1-1 0

+1

-1

Click here to view

L +L + L +


Resonance structure cyanate


Cyanate ion



Which structure is acceptable ?

-NCO


Resonance structure cyanate


Cyanate ion




-1 0 0 0 0 -1 +1 0 -2

Contribute the MOST

Negative formal charge located on more electronegative O atom is more stable than one located on a less electronegative N atom


✓

-100✓

Contribute the least

High formal charge/unstable

✓

χχ

Contribute less

Negative formal charge on less electronegative N atom

-NCO

Click here to view


Xenon trioxide

Resonance •Describing delocalization of electrons within a molecule/polyatomic ion where bonding cannot be express by ONE single Lewis structure•Delocalization of π bond – π electrons spread over more than 2 nuclei•π electrons are shared•π electrons spread – more stable


3XeOClick here to view

Different resonance structure for XeO3

formal charge for O formal charge for Xe formal charge for O

L + L + L +


Xenon trioxide

Resonance •Describing delocalization of electrons within a molecule/polyatomic ion where bonding cannot be express by ONE single Lewis structure•Delocalization of π bond – π electrons spread over more than 2 nuclei•π electrons are shared•π electrons spread – more stable


3XeOClick here to view

Different resonance structure for XeO3

✓


formal charge for O


FC = 6 – (4 +2) = 0

formal charge for Xe formal charge for O

V- Valence electron Xe = 8L- Lone pair electron Xe = 2B- Bonding electron Xe = 12


FC = 8 – (2 +6) = 0 FC = 6 – (4 +2) = 0

0 0

0

0

L + L + L +


Resonance structures sulfur trioxide


Sulfur Trioxide 3SO


S 120



How about structure 4 ?

resonance hybrid


Resonance structures sulfur trioxide


• All SO3 bonds are identical in length and strength• Hybrid of 3 resonance structures• NO S-O (single) or S=O (double) bonds found• Only S ----- O bond• Intermediate in character bet single and double bond


Sulfur Trioxide 3SO


S 120




How about structure 4 ?

resonance hybrid

Formal charge Sulfur Trioxide

Formal charge for O


FC = 6 – (4 +2) = 0

Formal charge for O


FC = 6 – (4+2) = 0

Formal charge for S


FC = 6 – (0+6) = 0


Which is acceptable? 0

00

0

0

L +L + L +

Formal charge Sulfur Trioxide

Formal charge for O


FC = 6 – (4 +2) = 0

Formal charge for O


FC = 6 – (4+2) = 0

Formal charge for S


FC = 6 – (0+6) = 0

✓Lowest formal charge is preferred




✓All resonance structure contribute to electronic structure. Real structure is combination of them.Lowest formal charge (stable), contribute more than less stable structure.Sum of formal charges must be zero for neutral or equal to charge on ion.

0

00

0

0 0

0

0

0

-1

-1

+2

0

-1

-1

+2

0

+2

-1-1

L +L + L +


Resonance structures methanoate


• All CO bonds are identical in length and strength

• Hybrid of 2 resonance structures• NO C-O (single) or C=O (double) bonds

found• Only C ----- O bond• Intermediate character bet single and

double bond

• Bond Order =

211

Methanoate ionHCOO


Click here to view

resonance hybrid

Click here to view

Resonance structures ethanoate

Ethanoate ionCOOCH3


resonance hybrid

HH

CH3

FORMAL CHARGE (FC)




✓Formal charge methanoate ion

formal charge for O

L +


L + L + L +

FORMAL CHARGE (FC)




✓Formal charge methanoate ion

formal charge for O


L +

FC = 6 – (6 +1) = -1

formal charge for O


FC = 6 – (4+2) = 0

formal charge for C


FC = 4 – (0+4) = 0


✓0

0

-1

0

L + L + L +


Resonance structures thiocyanate


Thiocyanate ionSCN



Which is acceptablestructure?

-1 0 0 0 0 -1 +1 0 -2


Resonance structures thiocyanate


Thiocyanate ionSCN




Which is acceptablestructure?

-1 0 0 0 0 -1 +1 0 -2

Contribute the MOST

Negative formal charge located on more electronegative N atom is more stable than one located on a less electronegative S atom


✓

-100✓

Contribute the least

High formal charge/unstable

✓

χχ

Contribute the less

Negative formal charge on less electronegative S atom

Formal charge thiocyanate ion

Formal charge for S Formal charge for C Formal charge for N

V- Valence electron S = 6L- Lone pair electron S = 4B- Bonding electron S = 4

V- Valence electron C = 4L- Lone pair electron C = 0B- Bonding electron C = 8


FC = 6 – (4+2) = 0 FC = 4 – (0+4) = 0 FC = 5 – (4+2) = -1

00 -1

Different Lewis structures for SCN-

0 -10


0 0-1 +1 0 -2

L + L +L +

Formal charge thiocyanate ion

Formal charge for S Formal charge for C Formal charge for N

V- Valence electron S = 6L- Lone pair electron S = 4B- Bonding electron S = 4

V- Valence electron C = 4L- Lone pair electron C = 0B- Bonding electron C = 8


FC = 6 – (4+2) = 0 FC = 4 – (0+4) = 0 FC = 5 – (4+2) = -1

00 -1

Different Lewis structures for SCN-

0 -10


✓


Lowest formal charge is preferred ✓


0 0-1 +1 0 -2


L + L +L +


Resonance structures ozone


resonance hybrid

• All bonds O-O are identical in length and strength• Hybrid of 2 resonance structures• NO O-O (single) or O=O (double) bonds found• Only O ----- O bond• Intermediate in character bet single and double bond


Ozone3O

Click here on video ozone


• Pale blue gas, polar, dimagnetic• Oxidizing agent• Potent respiratory hazard and pollutant at ground level• Beneficial prevent UV B/C from reaching Earth surface• Highest ozone level in stratosphere,(10 km and 50 km)

UV radiation

Ozone at stratosphere

strongest radiation

3O

FORMAL CHARGE (FC)




✓Formal charge ozone

formal charge for O

L +

formal charge for Oformal charge for O

L + L + L +

FORMAL CHARGE (FC)




✓Formal charge ozone

formal charge for O


L +

FC = 6 – (4 +2) = 0

formal charge for O


FC = 6 – (6+1) = -1

formal charge for O

V- Valence electron O = 6L - Lone pair electron O = 2B - Bonding electron O = 6

FC = 6 – (2+3) = +1


✓0+1

-1✓

L + L + L +

Ozone Good and Bad Good Side Bad Side

Ozone in Strastophere• blocks UV B + C

Ozone in Troposphere act as• Greenhouse gas

Ozone in ground level act as •Pollutant •Photochemical

Click here on ozone depletion chemicals (phaseout)

Why ozone able to absorb UV B and UV C?

Breakdown of ozone – High UV radiation – Skin cancer - DNA mutation

Ozone depletion

UV Exposure

Acknowledgements

Thanks to source of pictures and video used in this presentation

Thanks to Creative Commons for excellent contribution on licenseshttp://spaceplace.nasa.gov/greenhouse/en/http://www.ozonedepletion.info/education/part3/ozonesources.htmlhttp://creativecommons.org/licenses/

Prepared by Lawrence Kok

Check out more video tutorials from my site and hope you enjoy this tutorialhttp://lawrencekok.blogspot.com

Education

IB Chemistry on Resonance, Delocalization and Formal Charges