chemical bonding and molecular structure chapter 9
Download
Skip this Video
Download Presentation
Chemical Bonding and Molecular Structure (Chapter 9)

Loading in 2 Seconds...

play fullscreen
1 / 43

Chemical Bonding - PowerPoint PPT Presentation


  • 916 Views
  • Uploaded on

Chemical Bonding and Molecular Structure (Chapter 9). Ionic vs. covalent bonding Molecular orbitals and the covalent bond (Ch. 10) Valence electron Lewis dot structures octet vs. non-octet resonance structures formal charges VSEPR - predicting shapes of molecules

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 'Chemical Bonding ' - PamelaLan


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
chemical bonding and molecular structure chapter 9
Chemical Bonding and Molecular Structure (Chapter 9)
  • Ionic vs. covalent bonding
  • Molecular orbitals and the covalent bond (Ch. 10)
  • Valence electron Lewis dot structures
  • octet vs. non-octet
  • resonance structures
  • formal charges
  • VSEPR - predicting shapes of molecules
  • Bond properties
      • electronegativity
  • polarity, bond order, bond strength

Bonding and structure (2)

rules for making lewis dot structures

— 2 for # of PAIRS

Rules for making Lewis dot structures

1. Count no. of valence electrons

(- don’t forget to include the charge on molecular ions!)

2. Place a bond pair (BP) between connected atoms

3. Complete octets by using rest of e- as lone pairs (LP)

4. For atoms with <8 e-, make multiple bonds to complete octets

5. Assign formal charges : fc = Z - (#BP/2) - (#LP)

Indicate equivalent (RESONANCE) structures

6. Structures with smaller formal charges are preferred

- consider non-octet alternatives (esp. for 3rd, 4th row)

  • OCTET RULE:#Bond Pairs + #Lone Pairs = 4
  • (except for H and atoms of 3rd and higher periods)

#lone pairs at central atom in AXn = {(#e-) - 8*n}/2

Bonding and structure (2)

sulfur dioxide so 2

Rules 1-3 

O—S —O

+

+

Sulfur Dioxide, SO2

These equivalent structures

are called:

RESONANCE

STRUCTURES.

The proper Lewis structure

is a HYBRID of the two.

Each atom has OCTET . .

. . . BUT there is a +1 and -1 formal charge

Bonding and structure (2)

so 2 2

O = S = O

SO2 (2)

Alternate Lewis structure for SO2 uses 2 double bonds

Sulfur does not obey OCTET rule

BUT the formal charge = 0

This is better structure than O=S+-O-

since it reduces formal charge (rule 6).

3rd row S atom can have 5 or 6 electron pairs

NB: # of central atom lone pairs = (3*6 -8*2)/2 = 1 in both O=S+-O- and O=S=O structures

Bonding and structure (2)

thiocyanate ion scn

A. S=C=N

Calculated partial charges

B. S=C - N

C. S-C N

-0.16

-0.32

-0.52

Thiocyanate ion, (SCN)-

Which of three possible resonance structures

is most important?

ANSWER:

C > A > B

Bonding and structure (2)

molecular geometry

6_VSEPR.mov

MOLECULAR GEOMETRY

Molecule adopts the shape that minimizes the electron pair repulsions.

VSEPR

  • Valence Shell Electron Pair Repulsion theory.
  • Most important factor in determining geometry is relative repulsion between electron pairs.

Bonding and structure (2)

slide7

No. of e- Pairs

Around Central

Atom

linear

2

F—Be—F

180o

F

planar trigonal

3

B

F

F

120o

109o

H

4

tetrahedral

C

H

H

H

CAChe

image

Example

Geometry

Bonding and structure (2)

structure determination by vsepr

lone pair of electrons

in tetrahedral position

N

H

H

H

Structure Determination by VSEPR

Ammonia, NH3

There are 4 electron pairs at the corners of a tetrahedron.

The ELECTRON PAIR GEOMETRY is tetrahedral.

Bonding and structure (2)

vsepr ammonia

lone pair of electrons

in tetrahedral position

N

H

H

H

VSEPR - ammonia

Ammonia, NH3

Although the electron pair geometry is tetrahedral . . .

. . . the MOLECULAR GEOMETRY

— the positions of the atoms

— is PYRAMIDAL.

Bonding and structure (2)

ax n e m notation
AXnEm notation
  • a good way to distinguish between
  • electron pair and molecular geometries
  • is theAXnEmnotation
  • where:
  • A - atom whose local geometry is of interest (typically the CENTRAL ATOM)
  • Xn - n atoms bonded to A
  • Em - m lone pair electrons at A
  • NH3 is AX3E system  pyramidal
  • (NB this notation not used by Kotz)

Bonding and structure (2)

vsepr water

••

H - O - H

••

VSEPR - water

Water, H2O

2. Count BP’s and LP’s = 4

3. The 4 electron pairs are at the corners of a tetrahedron.

1. Draw electron dot structure

The electron pair geometry is TETRAHEDRAL.

Bonding and structure (2)

vsepr water 2

••

H - O - H

••

VSEPR - water (2)

Although the electron pair geometry is TETRAHEDRAL . . .

. . . the molecular geometry is bent.

H2O - AX2E2 system - angular geometry

Bonding and structure (2)

vsepr formaldehyde

O

O

C

H

H

C

H

H

Formaldehyde, CH2O

VSEPR - formaldehyde

1. Draw electron dot structure

2. Count BP’s and LP’s:

At Carbon there are 4 BP but . . .

3. These are distributed in ONLY 3 regions.

Double bond electron pairs are in same region.

There are 3 regions of electron density

Electron repulsion places them at the corners of a planar triangle.

Both the electron pair geometry and the molecular geometry are PLANAR TRIGONAL  120o bond angles.

H2CO at the C atom is an AX3 species

Bonding and structure (2)

vsepr bond angles

6_CH3OH.mov

VSEPR - Bond Angles

Methanol, CH3OH

H

••

Define bond angles 1 and 2

Angle 1 = H-C-H = ?

Angle 2 = H-O-C = ?

Answer:

H—C—O—H

••

H

Angle 1

Angle 2

109o because both the C and O

atoms are surrounded by 4 electron pairs.

AXnEm designation ?

at C

at O

AX4 = tetrahedral

AX2E2 = bent

Bonding and structure (2)

vsepr bond angles 2

H

N

H—C—C

••

1

2

H

Acetonitrile, CH3CN

VSEPR - bond angles (2)

Angle 1 = ?

109o

Define bond angles 1 and 2

Angle 2 = ?

180o

Why ? :

The CH3 carbon is surrounded by 4 bond charges

The CN carbon is surrounded by 2 bond charges

AXnEm designation ?

at CH3 carbon

at CN carbon

AX4 = tetrahedral

AX2 = linear

Bonding and structure (2)

what about structures with central atoms that do not obey the octet rule
What about:STRUCTURES WITH CENTRAL ATOMS THAT DO NOT OBEY THE OCTET RULE ?

PF5

BF3

SF4

Bonding and structure (2)

geometry for non octet species also obey vsepr rules
Geometry for non-octet species also obey VSEPR rules

Consider boron trifluoride, BF3

The B atom is surrounded by only 3 electron pairs.

Bond angles are 120o

Molecular Geometry is

planar trigonal

BF3 is an AX3 species

Bonding and structure (2)

compounds with 5 or 6 pairs around the central atom

6_VSEPR.mov

Trigonal bipyramid

90°

F

5 electron pairs

F

120°

F

P

F

F

90°

Octahedron

F

6 electron pairs

F

F

F

S

F

90°

F

Compounds with 5 or 6 Pairs Around the Central Atom

AX5 system

AX6 system

Bonding and structure (2)

sulfur tetrafluoride sf 4

••

••

F

••

••

••

S

F

F

••

••

F

••

F

F

S

F

F

F

F

S

F

F

Sulfur Tetrafluoride, SF4

Number of valence e- = 34

No. of S lone pairs =

{17 - 4 b.p. - 3x4 l.p.(F)}

= 1 lone pair on S

There are 5 (BP + LP)

e- pairs around the S

THEREFORE:

electron pair geometry ?

= trigonal bipyramid

OR

AX4E system. Molecular geometry ?

Bonding and structure (2)

sulfur tetrafluoride sf 4 2

F

F

S

F

equatorial

F

axial

Sulfur Tetrafluoride, SF4 (2)

90°

120°

Lone pair is in the equatorial position because it requires more room than a bond pair.

Molecular geometry of SF4 is “see-saw”

Q: What is molecular geometry of SO2 ?

Bonding and structure (2)

bonding with hybrid atomic orbitals

6_CH4.mov

Bonding with Hybrid Atomic Orbitals

- Carbon prefers to make 4 bonds as in CH4

4 C atom orbitals hybridize to form four

equivalent sp3 hybrid atomic orbitals.

But atomic carbon has an s2p2 configuration

Why can it make more than 2 bonds ?

Bonding and structure (2)

orbital hybridization
Orbital Hybridization

BONDS SHAPE HYBRID REMAIN e.g.

s2p2 

2 linear {2 x sp & 2 p’s} C2H2

3 trigonal {3 x sp2 & 1 p} C2H4 planar

4 tetrahedral {4 xsp3 } CH4

Bonding and structure (2)

multiple bonds s and p bonding in c 2 h 4

­

­

­

­

­¯

­

­

p

2s

2p

3 sp2

orbital

hybrid

orbitals

H

H

sp2

120°

C

C

H

H

Multiple Bondss and p Bonding in C2H4
  • The extra p orbital electron on each C atom overlaps the p orbital on the neighboring atom to form the p bond.

C atom orbitals are COMBINED

(= re-hybridized) to form orbitals

better suited for BONDING

  • The 3 sp2 hybrid orbitals
  • are used to make the C-C
  • and two C-H  bonds

6_C2H4-sg.mov

6_C2H4-pi.mov

6_C2H4.mov

Bonding and structure (2)

consequences of multiple bonding

233

E (kJ/mol)

27

-180 0 180

C-C=C angle (o)

Consequences of Multiple Bonding

Restricted rotation around C=C bond in

1-butene = CH2=CH-CH2-CH3.

See Butene.Map in ENER_MAP in CAChe models.

P. 475 - Photo-rotation

about double bonds

lets us see !!

Bonding and structure (2)

bond properties
Bond Properties

- bond order

- bond length

- bond strength

- bond polarity

- MOLECULAR polarity

  • What is the effect of bonding and structure on molecular properties ?

Buckyball in HIV-protease, see page 107

Bonding and structure (2)

bond order

triple, BO = 3

H

H

and 2 p

1

s

H

C

C

C

N

double, BO = 2

single

and 1 p

1

s

BO = 1

1

s

Bond Order
  • the number of bonds between a pair of atoms.

CH2CHCN

Acrylonitrile

Bonding and structure (2)

bond order 2

Total #

of e

-

pairs used for a type of bond

Bond

order

=

Total #

of bonds of that type

3 (e - pairs in N-O bonds)

=

Bond order in NO2-

2 (N - O bonds)

Bond Order (2)

Fractional bond orders occur in molecules with resonance structures.

Consider NO2-

N-O bond order in NO2- = 1.5

Bonding and structure (2)

bond order and bond length

110 pm

745 kJ

123 pm

414 kJ

Formaldehye

Bond Order and Bond Length

Bond order is related to two important bond properties:

(a) bond strength

as given by DE

(b) Bond length

- the distance between the nuclei of two bonded atoms.

Bonding and structure (2)

bond length
Bond Length

Molecule R(H-X)

H- F 104 pm

H- Cl 131 pm

H- I 165 pm

- depends on size of bonded atoms:

- depends on bond order.

Molecule R(C-O)

CH3C- OH 141 pm

O=C=O 132 pm

C O 119 pm

Bonding and structure (2)

bond strength
Bond Strength
  • Bond Dissociation energy (DE) - energy required to break a bond in gas phase.
  • See Table 9.5

BOND STRENGTH (kJ/mol) LENGTH (pm)

H—H 436 74

C—C 347 154

C=C 611 134

CºC 837 121

NºN 946 110

The GREATER the number of bonds (bond order)

the HIGHER the bond strength and the SHORTER the bond.

Bonding and structure (2)

bond strength 2
Bond Strength (2)

Bond Order Length Strength

HO—OH 1 149 pm 210 kJ/mol

O=O 2 121 498 kJ/mol

1.5 128 ?

303 kJ/mol

O3 (g)  3 O(g)

HOW TO CALCULATE ?

Hrxn = {3xHf(O) - Hf(O3)} = {3x249.2 - 142.7} = 605 kJ/mol

2 O-O bonds in O3  DE (O3) = 605/2 = 302.5 kJ/mol

Bonding and structure (2)

bond polarity
Bond Polarity

HCl is POLAR because it has a positive end and a negative end (partly ionic).

Polarity arises because Cl has a greater share of the bonding electrons than H.

Calculated charge by CAChe:

H (red) is +ve (+0.20 e-)

Cl (yellow) is -ve (-0.20 e-).

(See PARTCHRG folder in MODELS.)

Bonding and structure (2)

bond polarity 2
Bond Polarity (2)
  • Due to the bond polarity, the H—Cl bond energy is GREATER than expected for a “pure” covalent bond.

BOND ENERGY

“pure” bond 339 kJ/mol calculated

real bond 432 kJ/mol measured

Difference 92 kJ/mol.

This difference is the contribution of IONIC bonding

It is proportional to the difference in

ELECTRONEGATIVITY, c.

Bonding and structure (2)

electronegativity c
Electronegativity, c

c is a measure of the ability of an atom in a molecule to attract electrons to itself.

Concept proposed by

Linus Pauling (1901-94)

Nobel prizes:

Chemistry (54), Peace (63)

See p. 425; 008vd3.mov (CD)

Bonding and structure (2)

slide35

Electronegativity, c

Figure 9.7

  • F has maximum c.
  • Atom with lowest c is the center atom in most molecules.
  • Relative values of c determines BOND POLARITY (and point of attack on a molecule).

Bonding and structure (2)

bond polarity36
Bond Polarity

Which bond is more polar ? (has larger bond DIPOLE)

O—H O—F

c H

2.1

O F

3.5 4.0

c(A) - c(B)3.5 - 2.1

Dc 1.4

3.5 - 4.0

0.5

(O-H) > (O-F)

Therefore OH is more polar than OF

Also note that polarity is “reversed.”

Bonding and structure (2)

molecular polarity
Molecular Polarity
  • Molecules—such as HCl and H2O— can be POLAR (or dipolar).
  • They have a DIPOLE MOMENT.
  • Polar molecules turn to align their dipole with an electric field.

Bonding and structure (2)

predicting molecular polarity

Symmetric molecules

Predicting molecular polarity

A molecule will be polar ONLY if

a) it contains polar bonds

AND

b) the molecule is NOT “symmetric”

Bonding and structure (2)

molecular polarity h 2 o
Molecular Polarity: H2O

Water is polar because:

a) O-H bond is polar

b) water is non-symmetric

The dipole associated with polar H2O

is the basis for absorption of microwaves

used in cooking with a microwave oven

Bonding and structure (2)

carbon dioxide

-0.73 +1.46 -0.73

Carbon Dioxide
  • CO2 is NOT polar even though the CO bonds are polar.
  • Because CO2 is symmetrical the BOND polarity cancels

The positive C atom is why water attaches to CO2

CO2 + H2O  H2CO3

Bonding and structure (2)

slide41

HBF2 is polar

BF3 is NOT polar

Molecular Polarity in

NON-symmetric molecules

B—F, B—H bonds polar

molecule is NOT symmetric

Atom Chg. 

B +ve 2.0

H +ve 2.1

F -ve 4.0

B +ve

F -ve

B—F bonds are polar

molecule is symmetric

Bonding and structure (2)

fluorine substituted ethylene c 2 h 2 f 2
Fluorine-substituted Ethylene: C2H2F2

C—F bonds are MUCH more polar than C—H bonds.

(C-F) = 1.5, (C-H) = 0.4

CIS isomer

  • both C—F bonds on same side

 molecule is POLAR.

TRANS isomer

  • both C—F bonds on opposite side

 molecule is NOT POLAR.

Bonding and structure (2)

chemical bonding and molecular structure chapter 943
Chemical Bonding and Molecular Structure (Chapter 9)
  • Ionic vs. covalent bonding
  • Molecular orbitals and the covalent bond (Ch. 10)
  • Valence electron Lewis dot structures
  • octet vs. non-octet
  • resonance structures
  • formal charges
  • VSEPR - predicting shapes of molecules
  • Bond properties
      • electronegativity
  • polarity, bond order, bond strength

Bonding and structure (2)

ad