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1!LRSVDS! CH110 Hyrbrid Orbitals!

Chapter 9 part 2: Polarity in Molecules,

Valence Bond Theory!

Read: ! !BLB 9.3–5!

HW: ! !BLB 9.33, 35, 38 !

! !Packet 9:8-11!

Know: !

bond angles and geometry!

polarity of molecules!

Which Skill Check Test Bonus Deadline is Approaching?? __________________________________

When is EXAM 2???

_______________________________


Polarity in Molecules!

•" Just because a

molecule possesses

polar bonds does not

mean the molecule as

a whole will be polar.!

Dipole Moment (µ)

•" How can we determine the overall dipole moment for the molecule?!


Dipole Moment!For a diatomic molecule: µ = Q r

| r | units = debye (D) •!!!• = 3.33 x10-30 C-m +Q -Q

For two charges (+1 and -1) separated by 1 Å

µ = 4.79 D per Å

Where Q = 1.6 x10-19 C = charge of an electron

For a polyatomic molecule: µ depends on:

1.

2. !


Which molecules are

polar?!


Dipole Trends in H-X!

HX bondlength Å

µ(exp)D

µ(ionic)D

!EN % ionic

HF 0.92 1.82 4.41 1.9 41%

HCl 1.27 1.08 6.08 0.9 18%

HBr 1.41 0.82 6.75 0.7 12%

HI 1.61 0.44 7.71 0.4 6%

If the H-X bond were ionic, µ(ionic) = Q r

If the H-Cl bond were ionic (r = 1.27 Å ) µ = 4.79 D /Å x 1.27 Å = 6.08 D

However, experimental data show: µ = 1.08 D

Compare the two values: µ(exp) /µ(ionic) x 100 = 1.08/6.08 x 100 = 18%

The bond is only 18% ionic (82% covalent; polar covalent)


Oxidation Numbers, Formal

Charges, and Partial Charges!

+1 -1 0 0 +" !" H-Cl H-Cl H-Cl " = 0.18

oxidation formal partial charges numbers charges (experimental)

Dipole moment (µ) Measures:


Polarity of Molecules!

A molecule is polar if there is a NET charge separation between two "ends"

of the molecule: molecule has a negative "end" and a positive "end".

Requirements to have a net dipole: 1."

2.

_________________________________

To determine the polarity of a molecule that has more than 2 atoms:

1."

2."

3."

4." !


Examples!

CO2 O=C=O electronegativity: C = 2.5 O = 3.5

EPG: MG: bond dipoles? Net dipole moment? _________________________________

CF4

electronegativity: C = 2.5 F = 4.0

EPG: MG:

Bond dipoles? Net dipole moment?!

F

CF

F

F


Examples!

H2O electronegativity: H = 2.1 O = 3.5

EPG: MG: bond dipoles? Net dipole moment? _________________________________

CH3Cl

electronegativity: H = 2.1 C = 2.5 Cl = 3.0

EPG: MG:

Bond dipoles? Net dipole moment?

OH

H

C

Cl

H

HH


Examples!

NH3

electronegativity: H = 2.1 N = 3.0

EPG: MG: Net dipole moment?

NH

HH

NF

FF

NH3 vs. NF3

µ"= 1.47 D µ = 0.24 D

[electronegativity: H = 2.1 N = 3.0 F = 4.0]

lone-pairs also have small dipoles partial cancellation of bond dipoles in NF3


Chapter 9 part 3: Valence Bond Theory!

Read: ! !BLB 9.4–6!

HW: ! !BLB 9:43, 47 !

! !Packet 9:13 !

! !!

Know: !

molecular orbitals!

orbital hybridization!

VBT!


How are bonds made?

Lewis structures: location and number of bonding and lone-pair electrons

VSEPR: spatial distribution of electrons, shape of molecule

How have we described electron distribution in atoms?

What is the electron distribution in molecules?

Molecular Orbitals

Two models:

Valence Bond Theory

Valence orbitals on one atom overlap

with valence orbitals on another atom: this overlap is a covalent bond.

Molecular Orbital Theory a better model that uses wave theory, BUT not

covered in Chem 110 ! [studied in Chem 112 and

organic chemistry] !


Overlap and Bonding!

•" Covalent bonds formed through the

sharing of electrons by adjacent

atoms.!

•" Only occurs when orbitals on the

two atoms overlap.!


Overlap and Bonding!

Two forces operating: •" increased overlap of atomic orbitals

(better sharing) brings atoms together

• closer distance between nuclei increases positive-positive charge repulsion

balance of forces ! bond length (0.74 Å for H2)!


How can atomic orbitals

overlap to form observed

geometries?!


Bonding in CH4 Carbon ground-state configuration:

Using only open (unpaired) subshell electrons:

Expect:"CH2" molecule 90o bond angles.

What if we “Promote” 2 electrons?

If all subshell electrons participated in bonding, this "CH4" molecule would have:

1. 2. 3. 4.

However, "" Real molecule is tetrahedral "" 109.5o bond angles, "" all bonds are equal length and strength

H

C H


Bonding Orbitals in Be!

•" Consider beryllium:!

–" No singly-occupied

orbitals!

–" Can’t form Bonds?!

•" Absorb Energy!–" Now form 2 bonds?!

–" Bonds equal?!


Hybrid Orbitals!•" Mixing the s and p orbitals yields two

degenerate orbitals that are hybrids of the two orbitals.!

–" These sp hybrid orbitals have ____ lobes like a p orbital.!

–" One of the lobes is larger and more rounded as is the s orbital.!

!The sp orbitals are higher in energy than the 1s orbital but lower than the 2p.!


Hybrid Orbitals on Be!

•" These two degenerate sp orbitals would align themselves 180° from each other.!

•" This is consistent with the observed geometry of beryllium compounds: linear.!


Hybrid orbitals on B!

Using a similar model for boron leads

to… !

…________ degenerate sp2 orbitals.!


Hybrid Orbitals on C!

With carbon we get…!

…_____ degenerate sp3 orbitals.!


Naming Hybrid Orbitals !

NOTE: 1. start with four atomic orbitals

s px py pz

2. end up with four hybrid orbitals

4 sp3

# orbitals name of orbital

Each hybrid orbital is composed of:


Hybrid Orbitals Using d Orbitals!


Hybrid Orbitals!


Deduce the Hybridization from

Molecular Shape!

NH H

H

N

H

H

H

Formula !Structure ! Hybridization!

OH

H

O

H

H

B

F

F

F

F

BF

F

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