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General ChemistryGeneral Chemistry Chem. 102 for EngineeringChem. 102 for Engineering

General ChemistryGeneral Chemistry Chem. 102 for EngineeringChem. 102 for Engineering

Tutorial 6

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Valence Electrons

2A2A 3A3A 4A4A 5A5A 6A6A 7A7A

8A8A

Number of valence electrons is equal to the Group number

12 13 14 15 16 17

18

1A1A1

Lewis Dot Symbols

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Step 2. Count valence electrons S = 6 3 x O = 3 x 6 = 18 Negative charge = 2

TOTAL = 6 + 18 + 2 = 26 e-

Step 1. Central atom = S

10 pairs of electrons are left.

Sulfite ion, SO32-

Step 3. Form sigma bonds

O O

O

S

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Remaining pairs become lone pairs, first on outside atoms

then on central atom.

Sulfite ion, SO32- (2)

Each atom is surrounded by an octet of electrons.

•• O O

O

S

••

••

•• ••

••

••

••

••

••

2-

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Example: Write the Lewis structure for CN- ion and calculate the formal charge for both atoms

• Number of valence electrons = 10• For C:

• There are 4 valence electrons (from periodic table).• In the Lewis structure there are 2 nonbonding electrons and

3 from the triple bond. There are 5 electrons from the Lewis structure.

• Formal charge: 4 - 5 = -1.

C N

• For N:•There are 5 valence electrons.•In the Lewis structure there are 2 nonbonding electrons and 3 from the triple bond. There are 5 electrons from the Lewis structure.•Formal charge = 5 - 5 = 0.

• We write:

C N0

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Some additional structures and observations•Isoelectronic molecules have the same number of valence electrons and thus the same Lewis Dot Structures

• NO+, N2, CO, CN- all have 10 electrons

• H3O+, NH3: 8 valence electrons

• CO2, OCN-, SCN-, N2O, NO2+, COS, CS2: 16 valence electrons

• CO32-, NO3

-: 26 valence electrons

• PO43-, SO4

2-, ClO4-: 32 valence electrons

[ N O ]+ N N C O C N[ ] -

N H

H

H OH

H

H +

C OO C ON-

CN S- +

O N O O C S S C S

N OO

O

-N OO

O

-

N OO

O

-

P

O

O

O

O

3-

etc.S

O

O

O O

2-

etc.

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Lewis Structures: Examples

a) CHa) CH22ClF ClF

H C F

Cl

H

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A resonance structure is one of two or more Lewis structures for a single molecule that cannot be represented accurately by only one Lewis structure.When one LS cannot represent the molecule and we have to use a set of equally good LS, this set is called the resonance structures.

O O O1+1-

OOO1+ 1-

O C O

O

1 1 O C O

O

1

1

OCO

O

1

1

What are the resonance structures of the carbonate (CO3

2-) ion?

Ozone O3

C – 4 e-

3O – 3x 6 e-

-ve charge 2 e-

24 e-

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Example: Write the possible Lewis structures for the sulfate ion (SO42-), and

decide which is more possible

Possible structure No. I

The number of valence electrons = 32e-

Each O atom has 6 unshared e- (3 lone pairs) and shared 2 electrons with S atom

If you have more than one possible Lewis structures, how do you tell which one is better? Formal charge principle

-1+2

-1

-1

-1

Calculate formal charges for S and O

10

0

Possible structure No. II

-1-1

0

0

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There are two fundamental assumptions about using formal charges to evaluate Lewis structure:

1. Atoms in molecules try to achieve formal charge as close to zero as possiblezero as possible.

2. Any negative formal charges are expected to reside on the most electronegativemost electronegative atoms.atoms.

3.Adjacent atoms should not carry formal cahrges of the same sign.formal cahrges of the same sign. Notice that,

• In the structure with only single bonds each oxygen has a formal

charge of –1, while the sulfur has a formal charge of +2.

• In contrast, in the structure with two double bonds and two single

bonds, the sulfur and the 2 oxygen atoms have a formal charge of zero.

• So this structure is preferred as it has lower formal charges.

Resonance structures

9 - 12

Sometimes we can have two or more equivalent Lewis structures for a molecule.

O S = O O = S O

-Satisfy the octet rule -have the same number of bonds

-have the same types of bonds -Have the same formal charges.

Which is right?

Resonance structuresResonance structuresSOSO22

9 - 13

No one of them is correct !

O S = O O = S O

O S O

This results in an average of 1.5 bonds between each S and O.

Resonance structuresResonance structures

The correct structure

Hybrid structure

+1-1

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Alternate Lewis structure for SO2 uses 2 double bonds

O = S = O

Sulfur does not obey OCTET ruleBUT the formal charge = 0

This is better structure than O=S+-O- since it reduces formal charge. 3rd row S atom can have 5 or 6 electron pairs

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Carbon Dioxide, CO2

1 .Central atom = __C____

2 .Valence electrons = _16_ or _8_ pairs

3 .Form sigma bonds.

O OC This leaves __6__ pairs (12 e-).

4. Place lone pairs on outer atoms.

••O OC

•• ••

••••••

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••O OC

•• ••

••••••

Carbon Dioxide, CO2

4 .Place lone pairs on outer atoms.

••O OC

•• ••

••••••

The second bonding pair forms a pi () bond.

5. To give C an octet, form DOUBLE BONDS between C and O.

O C O••

• •••

• •

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04 - (1 / 2) (8) - 0 =

6 - (1 / 2) (4) - 4 = 0

Carbon Dioxide

At OXYGEN

O C O••

• •••

• •

At CARBON

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C atom charge is 0.

6 - (1 / 2) (6) - 2 = +1

6 - (1 / 2) (2) - 6 = -1

Carbon Dioxide

O C O••

• •••

• •

An alternate Lewis structure is:

AND the corresponding resonance form

+

O C O••

• •••

• •

+

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Carbon Dioxide, CO2

Which is the predominant resonance structure?

O C O••

• •••

• •

ORO C O•

•

• •••

• •

O C O••

• •••

• •

+

+

Answer ?Form with zero formal charges isBETTER [no +ve charge on O]

000

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A. S=C=N Thiocyanate ion, (SCN)-

Which of three possible structures is most important?

ANSWER:

AA

C. S-C N

B. S=C - N

Exercise: Write the three possible resonance structures for cyanate ion, NCO- and decide which form is most important?

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Boron Trichloride, BCl3

What if we form a B—Cl double bond to satisfy the B atom octet?

Cl••

Cl

B••

••

••

••

••

••Cl•

•

•

•

Exceptions to the Octet Rule (electron deficient atomsExceptions to the Octet Rule (electron deficient atoms))

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Boron Trifluoride BF3, BCl3 (2)

fc = 7 - 2 - 4 = +1 Chlorine

Cl••

••

Cl

Cl

B••

••

••

••

•

•

•

•

fc = 3 - 4 - 0 = -1 Boron

+1

• To have +1 charge on Cl, with its very high electron affinity is not good. -ve charges best placed on atoms with high EA.

• Similarly -1 charge on B is bad

• Ignore octet rule and consider boron as exception.

-1

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Exceptions to the Octet Rule (Expanded valence shells)

•Molecules having atoms with more than 8 electrons

•This can occur for molecules in which the central atom is in the third or later periodsGroup 4A

F

SiF

FF

F

-

Group 5A

F

SF

FF

F

F

Group 6A

F

ClF

F

F

BrF

F

F

F

Group 7A

F

Xe

F

XeF

F

F

F

Group 8A

S

F F

FF

Si

F

F

F

FF

F

2-

F

PF

FF

F

F

PF

FFF

F

-

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The total number of lone pairs in NCl3 is

A. 6B. 8C. 9D. 10E. 13

The Lewis structure for CS2 is:

None of the above

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The best Lewis structure for a chlorate ion, ClO3, should show

____single bond(s), ____ double bond(s), and ____ lone pair(s).

A. 2, 1, 10B. 3, 2, 10C. 1, 2, 8D. 3, 0, 10E. 2, 1, 9

The number of resonance structures for the sulfur dioxide molecule that satisfy the octet rule is (see previous)

A. 1.B. 2.C. 3.D. 4.E. None of these.

What is the formal charge on sulfur in the best Lewis structure for the SCN (thiocyanate) ion? (see previous)

A. +2 B. 2 C. +1 D. 1 E. 0

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Which of these statements is a useful guideline for the application of formal charges in neutral molecules?

A. Lewis structure in which there are zero formal charges is preferred.B. Lewis structures with large formal charges (e.g., +2,+3 and/or 2,3)

are preferred.C. The preferred Lewis structure is one in which positive formal charges

are on the most electronegative atoms.D. The preferred Lewis structure is one in which negative formal charge

is on the central atom.E. (A) and (D).

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Consider the following molecules.BF3

CHBr3 (C is the central atom)

Br2

XeCl2

COSF4

Select the molecule(s) that fits the statement "these molecules violate the octet rule"

A.I, II, IVB.I, III, IV, VIC.III, V, IVD.I, IV, VI E.I, II, IV, VI

A. 0 B. 1 C. 2 D. 3 E. 4

In the Lewis structure of ICl2, how many nonbonding pairs of electrons are

around the central iodine atom?