polarity in molecules - pennsylvania state...

CH110 Hyrbrid Orbitals 1LRSVDS

Chapter 9 part 2: Polarity in Molecules,

Valence Bond Theory

Read: BLB 9.3–5

HW: BLB 9.33, 35, 38, 43, 47;

Packet 9:8-16

Know:

polarity of molecules

molecular orbitals

orbital hybridization

Which Skill Check Test Bonus Deadlineis 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 wedetermine theoverall dipolemoment for themolecule?


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 was 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 # = 0.18oxidation formal partial chargesnumbers 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 moleculethat 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 dipolespartial cancellation of bond dipoles in NF3


Chapter 9 part 3: Valence Bond Theory

Read: BLB 9.4–5

HW: BLB 9:43, 47;

Packet 9:12-16

Know:

molecular orbitals

orbital hybridization


How are bonds made?

Lewis structures: location and number ofbonding and lone-pair electrons

VSEPR: spatial distribution of electrons, shape ofmolecule

How have we described electron distribution inatoms?

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 Theorya 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 increasespositive-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" molecule90o 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 ofthe two orbitals.

– These sp hybrid orbitals have ____ lobeslike a p orbital.

– One of the lobes is larger and morerounded as is the s orbital.

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


Hybrid Orbitals on Be

• These two degenerate sp orbitals wouldalign themselves 180° from each other.

• This is consistent with the observedgeometry 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 hybridorbitals

4 sp3

# orbitals name of orbital

Each hybrid orbital iscomposed 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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