chemistry for students

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CHEMICALBONDING

CHM 474

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Outlines…. Able to draw a Lewis structure fordimolecule

Able to predict the shape/geometrybased on VSEPR

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• Chemical bond : attractive force holding two or

more atoms together.• Covalent bond results from sharing electronsbetween the atoms. Usually found betweennonmetals.

• Ionic bond results from the transfer ofelectrons from a metal to a nonmetal.

Chemical Bonds, Lewis Symbols, and the OctetRule

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9.1

Valence electrons are the outer shell electrons of anatom. The valence electrons are the electrons that

participate in chemical bonding.

1A 1ns1

2A 2ns2 3A 3ns2np1

4A 4ns2np2

5A 5ns2np3 6A 6ns2np4 7A 7ns2np5

Group # of valence e - e - configuration

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The Octet Rule

• All noble gases except He has an s 2 p 6 configuration.

• Octet rule: atoms tend to gain, lose, or share

electrons until they are surrounded by 8valence electrons (4 electron pairs) .• Caution: there are many exceptions to the

octet rule.

Chemical Bonds, Lewis Symbols, and the OctetRule

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+ F1s22s 22p6

[Ne]

Li Li+ F -1s22s 1 1s22s 22p5

[He]Li Li+ + e-

e - + F F -

F -Li+ + Li+ F -

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Covalent Bonding…

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A covalent bond is a chemical bond in which two or

more electrons are shared by two atoms.

Why should two atoms share electrons?

F F

Lewis structure of F 2

lone pairslone pairs

lone pairslone pairssingle covalent bond

single covalent bond

F F+

7e - 7e -

F F

8e - 8e -

F F

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Continued…



8e -

H HO+ + OH H O HHor

2e - 2e -

Lewis structure of water

Double bond – two atoms share two pairs of electrons

single covalent bonds

O C O or O C O

8e - 8e - 8e - double

bonds

double bonds

Triple bon d – two atoms share three pairs of electrons

N N8e - 8e -

N N

triple bondtriplebond

or

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BondType

BondLength

(pm)

C- C 154

C C 133

C C 120

C- N 143

C

N 138C N 116

Lengths of Covalent Bonds

Bond Lengths

Triple bond < Double Bond < Single Bond 9.42/11/201414



Chemical BondsBond Type Single Double Triple# of e’s 2 4 6

Notation — = Bondorder

1 2 3

Bondstrength Increases from Single to Triple

Bond

length

Decreases from Single to Triple

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1. Draw skeletal structure of compound showing whatatoms are bonded to each other. Put leastelectronegative element in the center .

2. Count total number of valence e -. Add 1 for eachnegative charge. Subtract 1 for each positivecharge.

3. Complete an octet for all atoms except hydrogen

4. If structure contains too many electrons, formdouble and triple bonds on central atom as needed.

Writing Lewis Structures

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Write the Lewis structure of nitrogen trifluoride (NF 3).

F N F

F

Step 1 – N is less electronegative than F, put N in center

Step 2 – Count valence electrons N - 5 (2s 22p 3) and F - 7 (2s 22p 5)5 + (3 x 7) = 26 valence electrons

Step 3 – Draw single bonds between N and F atoms and completeoctets on N and F atoms.

Step 4 - Check, are # of e - in structure equal to number of valence e - ?

3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons

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Write the Lewis structure of the carbonate ion (CO 32-).

O C O

O

Step 1 – C is less electronegative than O, put C in center

Step 2 – Count valence electrons C - 4 (2s 22p 2) and O - 6 (2s 22p 4)-2 charge – 2e -

4 + (3 x 6) + 2 = 24 valence electrons

Step 3 – Draw single bonds between C and O atoms and completeoctet on C and O atoms.Step 4 - Check, are # of e - in structure equal to number of valence e - ?

3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons

Step 5 - Too many electrons, form double bond and re-check # of e - 2 single bonds (2x2) = 4

1 double bond = 48 lone pairs (8x2) = 16

Total = 24

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20

Lewis structure H 3PO4 Skeleton is:

Count total valence electrons:1 P = 53 H = 34 O = 24Total = 32 e - or 16 valence e - pairs.

7 e - pairs needed to form s skeleton.

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21

Lewis structure H 3PO4 Add remaining e - pairs:

Left has a formal charge of +1 on P and -1 on one O,right has 5 e - pairs around P (hypervalence)

Analysis of phosphoric acid shows purely Tdphosphate groups, which requires something beyondeither simple Lewis model.

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Two possible skeletal structures of formaldehyde (CH 2O)

H C O H

HC O

H

An atom’s form al charge is the difference between thenumber of valence electrons in an isolated atom and thenumber of electrons assigned to that atom in a Lewisstructure.

formal chargeon an atom ina Lewisstructure

=1

2

total numberof bonding

electrons( )

total numberof valenceelectrons inthe free atom

-total numberof nonbonding

electrons

-

The sum of the formal charges of the atoms in a moleculeor ion must equal the charge on the molecule or ion.

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H C O HC – 4 e - O – 6 e -

2H – 2x1 e - 12 e -

2 single bonds (2x2) = 41 double bond = 4

2 lone pairs (2x2) = 4Total = 12

formal charge

on C= 4 - 2 - ½ x 6 = -1

formal chargeon O = 6 - 2 - ½ x 6 = +1

formal chargeon an atom in

a Lewisstructure

=1

2

total numberof bondingelectrons( )

total numberof valence

electrons inthe free atom -

total numberof nonbondingelectrons -

-1 +1

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Formal Charge and Lewis Structures1. For neutral molecules, a Lewis structure in which there

are no formal charges is preferable to one in whichformal charges are present.

2. Lewis structures with large formal charges are lessplausible than those with small formal charges.

3. Among Lewis structures having similar distributions offormal charges, the most plausible structure is the one inwhich negative formal charges are placed on the moreelectronegative atoms.

Which is the most likely Lewis structure for CH 2O?

H C O H

-1 +1 HC O

H

0 0

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A reson ance s t ruc ture is one of two or moreLewis structures for a single molecule that cannotbe represented accurately by only one Lewisstructure.

O O O+ -

OOO+-

O C O

O

- -O C O

O

-

-

OCO

O

-

-

What are the resonance structures of thecarbonate (CO 3

2-) ion?

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Exceptions to the Octet RuleThe Incomplete Octet

H HBe

Be –

2e-

2H – 2x1e - 4e -

BeH 2

BF 3

B – 3e -

3F – 3x7e - 24e - F B F

F

3 single bonds (3x2) = 69 lone pairs (9x2) = 18

Total = 24

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Exceptions to the Octet Rule



Exceptions to the Octet Rule

Odd-Electron MoleculesN – 5e - O – 6e -

11e - NO

N O

The Expanded Octet (central atom with principal quantum number n > 2)

SF 6 S – 6e -

6F – 42e - 48e -

S

F

F

F

FF

F

6 single bonds (6x2) = 1218 lone pairs (18x2) = 36

Total = 48

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VSEPR - Valence Shell Electron Pair Repulsion Theory

Electron-group repulsions and the five basic molecular shapes.

linear trigonal planar tetrahedral

trigonal bipyramidal octahedral

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Valence shell electron pair repul sion (VSEPR) model:

Predict the geometry of the molecule from the electrostaticrepulsions between the electron (bonding and nonbonding) pairs.

AB 2 2 0

Class

# of atomsbonded to

central atom

# lonepairs on

central atom

Arrangement of

electron pairs

Molecular

Geometry

linear linear

B B

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The single molecular shape of the linear electron-grouparrangement.

Examples:

CS 2, HCN, BeF 2

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VSEPR



AB 2 2 0 linear linearClass

# of atomsbonded to

central atom

# lonepairs on

central atom

Arrangement of

electron pairs

Molecular

Geometry

VSEPR

AB 3 3 0 trigonalplanar

trigonalplanar

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Class

# of atomsbonded to

central atom

# lonepairs on

central atom Arrangement of

electron pairsMolecularGeometry

VSEPR


trigonalplanar

2 1 AB 2Etrigonalplanar bent

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The two molecular shapes of the trigonal planar electron-group arrangement.

Class

Shape

Examples:SO 3, BF 3, NO 3

-, CO 32-

Examples:

SO 2, O 3, PbCl 2, SnBr 2

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# of atomsbonded to

central atom

# lonepairs on

central atom

Arrangement of

electron pairs

Molecular

Geometry

VSEPR


trigonalplanar

AB 4 4 0 tetrahedral tetrahedral

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Class

# of atomsbonded to

central atom

# lonepairs on

central atom Arrangement of

electron pairsMolecularGeometry

VSEPR


AB3E 3 1 tetrahedral

trigonal

pyramidal

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Class

# of atomsbonded to

central atom

# lonepairs on

central atom

Arrangement of

electron pairs

Molecular

Geometry

VSEPR


AB 3E 3 1 tetrahedral trigonal

pyramidal AB 2E2 2 2 tetrahedral bent

H

O

H

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The three molecular shapes of the tetrahedral electron-grouparrangement.

NH3

PF3

ClO3 H3O+

H2O

OF2

SCl2

Examples:

CH4, SiCl4,SO 4

2-, ClO4-

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bonding-pair vs. bonding

pair repulsion

lone-pair vs. lone pair

repulsion

lone-pair vs. bonding

pair repulsion> >

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VSEPR




# of atomsbonded to

central atom

# lonepairs on

central atom

Arrangement of

electron pairs

Molecular

Geometry

VSEPR


trigonalplanar


AB 5 5 0 trigonalbipyramidal

trigonalbipyramidal

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VSEPR



Class

# of atomsbonded to

central atom

# lonepairs on

central atom

Arrangement of

electron pairs

Molecular

Geometry

VSEPR


trigonalbipyramidal

AB4E 4 1 trigonal

bipyramidal

distorted

tetrahedron

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Class

# of atomsbonded to

central atom

# lonepairs on

central atom

Arrangement of

electron pairs

Molecular

Geometry

VSEPR


trigonalbipyramidal

AB4E 4 1 trigonal

bipyramidal

distorted

tetrahedron

AB 3E2 3 2 trigonalbipyramidal

T-shaped

ClF

F

F

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Class

# of atomsbonded to

central atom

# lonepairs on

central atom

Arrangement of

electron pairs

Molecular

Geometry

VSEPR


trigonalbipyramidal

AB4E 4 1 trigonal

bipyramidal

distorted

tetrahedron

AB 3E2 3 2 trigonalbipyramidal

T-shaped

AB 2E3 2 3trigonal

bipyramidal linearI

I

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The four molecular shapes of the trigonal bipyramidal electron-group arrangement.

SF4

XeO2F2 IF 4

+

IO 2F2-

ClF3

BrF3

XeF2

I 3-

IF 2-

PF5

AsF5

SOF 4

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Class

# of atomsbonded to

central atom

# lonepairs on

central atom

Arrangement of

electron pairs

Molecular

Geometry

VSEPR

AB 2 2 0 linear linear


trigonalplanar



trigonalbipyramidal

AB 6 6 0 octahedraloctahedral

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VSEPR



Class

# of atomsbonded to

central atom

# lonepairs on

central atom

Arrangement of

electron pairs

Molecular

Geometry

VSEPR


AB 5E 5 1 octahedral squarepyramidal

Br

F F

FF

F

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Class

# of atomsbonded to

central atom

# lonepairs on

central atom

Arrangement of

electron pairs

Molecular

Geometry

VSEPR


AB 5E 5 1 octahedral squarepyramidal

AB 4E2 4 2 octahedral squareplanar

Xe

F F

FF

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The three molecular shapes of the octahedral electron-grouparrangement.

SF6 IOF 5

BrF5

TeF 5-

XeOF4

XeF4

ICl4-

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Predicting Molecular Geometry



Predicting Molecular Geometry1. Draw Lewis structure for molecule.

2. Count number of lone pairs on the central atom and

number of atoms bonded to the central atom.3. Use VSEPR to predict the geometry of the molecule.

What are the molecular geometries of SO 2 and SF 4?

SO O

AB 2E

bent

S

F

F

F F

AB 4E

distortedtetrahedron

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