Ionic and Covalent Bonding

1
Ionic and Covalent Bonding

• Chapter 9

2
Ion Sizes

• Cations Shrink
  
• Anions Increase

Ba
Ba2
Ba
Cl
Cl-
3
Periodicity and Electron Configurations

• Which of the following is the smallest ion? and
  WHY?
• S-, Fe2, As3-, Ca2, Be

4
Ions Want to Form a Closed Shell Too!

• S- Ne3s23p4 1 electron Ne3s23p5
• This is equivalent to Cl
• Fe2 Ar4s23d6 - 2 electrons
  Ar4s23d4 Ar4s13d5
• This is equivalent to Cr
• Ca2 Ar4s2 - 2 electrons Ar
• Be He2s2 - 1 electron He2s1

5
Lewis Structures A Crude Predictive Model

• By concentrating on the valence electrons we
  can predict something about the reactions of
  elements.
• Using electron-dot structures we can envision how
  electrons are shared in poly-atomic molecules.
• Most elements want to have a noble gas
  configuration. -- OCTET RULE

6
Lets Go Back to Our Examples

• H2
• 2 H. HH

7
OR

• What about NaCl?

Cl
Na
8
OR

• What about NaCl?

Cl
Na
9
Polarity in a Molecular Bond

• An unequal sharing of electrons
• IONIC bonding

Cl
Na
10
Polarity in a Molecular Bond
Cl-
Na
11
Another Example - Cl2

• A non-polar bond
• COVALENT - sharing of electrons

Cl
Cl
12
Electronegativity

• Linus Pauling - Two Nobel Prizes
• The ability of an atom in a molecule to attract
  shared electrons to itself

EN is Electron Affinity D is the difference in
Electronegativity Fluorine is Defined as 4.0
13
Electronegativity and Bond Type
14
Concept Check

• Rank the following diatomics from most covalent
  to most ionic.
• SH, HCl, HF, HH, OH
• H-HgtS-HgtCl-HgtO-HgtF-H 0.0 0.4 0.9
  1.4 1.9

15
What About O2?

• The number of electrons doesnt add up.
• HOW CAN WE FORM TWO OCTETS?

2
12 electrons / 2 6 pairs
O
16
What About O2? The Solution

• Double Bonds

O
O
17
Writing Lewis Structures

• Sum the valence electrons from all the atoms.
• Use a pair of electrons to form a bond between
  each pair of bound atoms.
• Arrange the remaining electrons to satisfy the
  octet rule (note duet rule for H).

18
Lewis Formulas

• Write the Lewis formula for P2.

19
Lewis Formulas

• Write the Lewis formula for P2.

Total valence electrons 2 x 5 10.
The skeleton is P-P. Distribute the remaining
electrons symmetrically
20
Lewis Formulas

• Write the Lewis formula for P2.

Total valence electrons 2 x 5 10.
The skeleton is P-P. Distribute the remaining
electrons symmetrically
..
..
P____ P
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Neither P atom has an octet. Make one lone pair
from each P a bonding pair. P
P
22
PCl5
23
PCl5
P 5 Cl 5 x 7 40 valence electrons
Cl
Cl
P
Cl
Cl
Cl
24
Beryllium Chloride

• BeCl2

25
Beryllium Chloride

• BeCl2

Be
Cl
Cl
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Basic Exceptions

• B and Be often have less than the octet.
• a complete 2s shell is stable
• 3rd row and below can exceed the octet.
• d-orbitals are available
• If you must exceed the octet, place extra
  electrons on the central atom with a charge.

27
Oxidation States

• We can refer to the oxidation state of each atom
  within a molecule.

The oxidation state is equal to the charge on an
ion if a noble configuration is assumed for each
atom.
28
Got it?

• NaCl
• Na oxidation state 1
• Cl oxidation state -1
• NH3
• N oxidation state -3
• H oxidation state 1

29
What about O3?

• 3 x 6 valence electrons 18

30
What about O3?

• 3 x 6 valence electrons 18

O
O
O
31
Resonance

• Resonance is a representation of the fact that
  electrons are not stationary in a molecule.

O
O
O
32
Resonance

• Resonance is a representation of the fact that
  electrons are not stationary in a molecule.

O
O
O
Or
O
O
O
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What Do We Predict About the Bond Structure in O3?

• Based on our resonance model
• Bond lengths should be the same
• Molecule should be bent
• Only one kind of bond energy

34
Formal Charges

• Subtract the number of formal valence electrons
  assigned to an atom in a molecule from the number
  of electrons normally around the atom.
• Formal Charge
• Valence electrons - 1/2 of electrons in
  bonds - electrons in lone pairs

35
Example of Formal Charge

• SO42-

2-
O
S
O
O
O
Formal Charge on S 6 - (1/2 x 12)0 Formal
Charge on O 6 - (6 1/2x2) -1
OR 6
- (41/2x4) 0
36
If More Than One Lewis Structure Exists

• Atoms in molecules try to achieve formal charge
  0 first.
• Negative formal charges are expected to reside on
  the most electronegative atoms.

37
A Problem to Consider

• Consider the Lewis structures that may exist for
  XeO3. Use the concept of formal charges to
  predict the structure.

38
Formal Charge and XeO3

• Assign the most effective Lewis dot structure
  based on formal charges.

(3)
Xe
O
O
O
(-1)
(-1)
(-1)
39
Formal Charge and XeO3

• Assign the most effective Lewis dot structure
  based on formal charges.

(0)
Xe
O
O
O
(0)
(0)
(0)
40
Bond Length, Order and Energy

• Use bond energies to estimate ?H for the
  following gas-phase reaction. H H
  H H
  H H H H

-
C
C
C
C
Br
H
H
Br

This is called an addition reaction, because a
compound is added across the double bond.
41
An Addition Reaction
H
H
H
Br
C
C

H
H
H
H
H
C
C
Br
H
H
...
42
Enthalpy of ReactionFrom Bond Energies (BE)

• In the reaction, a CC double bond is converted
  to a C-C single bond. An H-Br bond is broken,
  and one C-H bond and one C-Br bond are
  formed. ?H BE(CC) BE(H-Br) -
  BE(C-C) - BE(C-H) -BE(C-Br) (602 362 -
  346 - 411 - 285) kJ -78 kJ