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Lecture 13Professor Hicks

Inorganic Chemistry (CHE151)

Lewis symbols aka Lewis Structures of atoms

• Visual representation of electron configuration

• 1 electron = dot 2 paired electrons = line

• Nucleus + core electrons = element symbol

C

Element symbol =

nucleus + core electrons

Valence electron =

C2 valence

electrons

All three represent

a carbon atom G. N Lewis=

or

Electron

Configuration

C = 1s22s22p2

Lewis symbol notation

4 valence electrons

2

Octet rule

N

O

F

• Main group atoms ions

• Gain/lose electrons noble gas electron configuration

• Noble gases all have 8 valence electrons

• Ions of main group elements have an octet of valence

electrons obey Octet Rule

N

O

F

[He] 2s22p6

all their ions

all have the 8 valence e-’s

electron configuration

8 = octet of

valence electrons

Lewis symbols

of three atoms

O O2- N N3- F F-

Lewis Structures

• Lewis symbols• Atoms and monatomic ions- Atoms +/- electrons ions - Octet rule

• Lewis structures• Molecules and polyatomic ions- Show bonding atom to atom

(the skeletal structure of the molecule/ion)- Atoms molecules/polyatomic ions- Extend concept of Octet Rule from atoms

forming monatomic ions

C H

H

H

H

C

H

H

H

H

3

Lewis structures

Can be used to predict

1) If a compound / polyatomic ion will exist

2) Its solubility in oil or water (non-polar or

polar liquids)

3) Three-dimensional shape

4) Intermolecular forces (tendency to stick

to other molecules)

Molecular compounds

and polyatomic ions

• More numerous than ionic compounds

• Multiple compounds with same elements

- CO, CO2, NO, NO2, N2O, N2O4,

- ClO- , ClO2- , ClO3

- , ClO4-

• No crossing over rule to determine formulas

• If a valid Lewis structure can be drawn the compound/ion probably exists

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Steps for Drawing Lewis Structures

1 Calculate the total the number of valence electrons in the

molecule or polyatomic ion.

2 Draw the skeletal structure for the molecule/polyatomic ion.

3 Give octets to all atoms in the skeletal structure (except

hydrogen) by placing lone pairs on them.

4 Count the total number of valence electrons in this structure.

5 If the Lewis structure contains the correct number of valence

electrons it is complete and has no resonance structures that

obey the octet rule.

6 If the structure contains more valence electrons than the

molecule or polyatomic ion has then reduce the number of

valence electrons in the structure by erasing lone pairs on

adjacent atoms and replacing them with a bonding pair

between those atoms.

Bonding in molecular compounds

and polyatomic ions• Based on attraction of nuclei for electrons

• Electrons act like glue to hold nuclei together

nucleus

z+e-e-

e-e-nucleus

z+

repulsion

attractionattraction

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Two kinds of electron pairs

O

H H

O

H H

Bonding electron pair

Non-bonding or

Lone electron pair

Hydrogen

• Does not obey octet rule

• Noble gas it becomes like is [He] = 1s2

• Obeys duet rule

• Like H- ion

• Only 1 bond H at end like a cap

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O

F

F

O

H H

O

O

C H

H

H

H

O

C

C

O

N H

C O

NH

C

O

O

N

O

N

O

O

Circle the octets on all the atoms that have octets.

B

F

F

F

O

F

F

O

H H

O

O

C H

H

H

H

O

C

C

O

N H

C O

NH

C

O

O

N

O

N

O

O

Circle the octets on all the atoms that have octets.

B

F

F

F

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Draw Lewis structures that obey

the Octet/Duet rule for

• H2

• H2S

• Cl2

• HCN (skeletal structure H-C-N)

Resonance structures

• More than one possible way to arrange

electrons = resonance structures

• Atoms connected in the same way

• not different substances

• Molecules/ion is one structure that is a weighted

average of all resonance structures

Example: Draw the Lewis structure and

resonance structures for CO2.

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C

O

O

C O

O

C

O O

Draw a Lewis structure and its resonance

structures for CO2 (skeletal structure O-C-O)

• Double ended arrows are placed between

resonance structures

• Resonance structures are shown in brackets

Isomers

• Different compounds with same formula

• Atoms arranged differently

• NOT resonance structures

cyanic acid isocyanic acid

C

O

N H C O

NH

same atoms arranged differently

H attached to N H attached to O

one structure cannot be changed into the other by moving electrons/double bonds

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Draw Lewis structures and

resonance structures for

• O3

• SO2 (skeletal structure O-S-O)

• BO33- (skeletal structure has B in center)

• CO32- (skeletal structure has C in center)

Isoelectronic structures

• Same arrangement of same # valence

electrons on different atoms

O

C

N

N

N

C

-

S

H H

O

H H

Se

H H

substitute

elements in

same group

substitute

elements from

different groupsthese three are

isoelectronicthese three are

isoelectronic

same # e-’s

C has 1 less proton

structure has

negative charge

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Normal Valence

• The number of bonds an atom forms in order to

achieve an octet is called its Valence

• The Normal Valence of an atom is the number

of bonds it most often forms to obey the octet

rule

• Normal valence is a periodic property

Normal valence = 4 3 2 1

Formal charge

• Charge atoms would have if they got

divorced (equitably)

• Each bond split 1 electron to each atom

• Formal charges add up to total charge

If the octet rule is obeyed

1) Each bond over normal valence +1 in FC

2) Each bond under normal valence -1 in FC

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Assigning formal charge

1) Draw circles around each atom

- split bonding pairs in half

- give lone pairs to the atom they are on

2) Evaluate the charges of the circles as if

they were Lewis symbols

N

O

CH

C O

NH

H atom

FC = 0

O with 5 valence

e-’s = O+1

FC= +1

C atom

FC = 0

N with 6 valence

e-’s = N-1

FC= -1

9.42

Write Lewis structures for these ions: (a) O22- , (b)

C22- , (c) NO+, (d) NH4

+ . Show formal charges.

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Formal charge

• Resonance structures with lower formal

charges dominate the observed structure

C

S

N

C S

N

C

S N

+1 0-2 0 0

-1-1 0 0

Best structure b/c

1) Minimal charges and

2) Charges consistent with

electronegativity of atoms

Formal charge

• Resonance structures with lower formal

charges dominate the observed structure

C

F

O

C F

O

C

F O

+2 0-1 +1 0

00 0 +1

Best structure b/c

1) Minimal charges and

2) Charges consistent with

electronegativity of atoms

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Exceptions to Octet rule

• Group three elements sometimes are

stable with 6 electrons (sextet)

• Molecules with odd numbers of electrons

cannot have octets

- Some (especially neutral species) are

stable enough to exist with less than an

octet

9.59

The BCl3 molecule has an incomplete octet around

B. Draw three resonance structures of the molecule

in which the octet rule is satisfied for both the B and

the Cl atoms. Also, draw one where the B has a

sextet. Show formal charges on all structures.

Based upon the formal charges which is the best

structure?

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Exceptions to the octet rule

• Octet = 8 = full s + p orbitals

• Elements with d orbitals can have more

than octet

• 18 electrons = full s + p + d orbitals of

same n value

• Many elements can lower FC’s by

exceeding octet

• More than 8 electrons = expanded octet

Rarely less than octet

never exceed octet

Rarely less than octet

sometimes exceed octet

Octet rule/expanded octets

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Draw a Lewis structure for H2SO4 that

obeys the octet rule and assign all formal

charges. The skeletal structure has all O

bound to S and the H’s each bound to

different O’s.

Scramble these three maybe

9.43 The following Lewis structures for (a) HCN, (b)

C2H2, (c) SnO2, (d) BF3, (e) HOF, (f) HCOF, and (g) NF3

are incorrect. Explain what is wrong with each one and

give a correct structure for the molecule. (Relative

positions of atoms are shown correctly.)

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9.51

Write three resonance structures for hydrazoic acid,

HN3. The atomic arrangement is HNNN. Show

formal charges.

Based upon the FC predict which resonance

structure or structures dominates the observed

structure.

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9.53

Draw three reasonable resonance structures for the

OCN− ion. Show formal charges.

9.63

Write Lewis structures for SeF4 and SeF6. Is the

octet rule satisfied for Se?

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Draw Lewis Structures and any resonance

structures for NCCN

Draw Lewis Structures and any resonance

structures for N-N-NO2

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Draw Lewis Structures and any resonance

structures for NO2

Draw Lewis Structures and any resonance

structures for IO2

Bases upon this structure predict the

charge of an ion formed from IO2.

Steps for Drawing Lewis Structures

1 Calculate the total the number of valence electrons in the molecule

or polyatomic ion.

2 Draw the skeletal structure for the molecule/polyatomic ion.

3 Give octets to all atoms in the skeletal structure (except hydrogen)

by placing lone pairs on them.

4 Count the total number of valence electrons in this structure.

5 If the Lewis structure contains the correct number of valence

electrons it is complete and has no resonance structures that obey the

octet rule.

6 If the structure contains more valence electrons than the molecule or

polyatomic ion has then reduce the number of valence electrons in

the structure by erasing lone pairs on adjacent atoms and replacing

them with a bonding pair between those atoms.

7 If the molecule or polyatomic ion contains less valence

electrons then add electrons to the structure on any atom that

can exceed the octet rule.