Title: Ionic and Covalent Bonding
1Ionic and Covalent Bonding
- Chapter 9
- Ebbing Textbook
2Ion Sizes
- Cations Shrink
- Anions Increase
Ba
Ba2
Ba
Cl
Cl-
3Periodicity and Electron Configurations
- Which of the following is the smallest ion? and
WHY? - S-, Fe2 Ca2, Be
4Ions 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
5Lewis 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. - Every element wants to have a nobel gas
configuration. -- OCTET RULE
6Lets Go Back to Our Examples
7OR
Cl
Na
8OR
Cl
Na
9Polarity in a Molecular Bond
- An unequal sharing of electrons
- IONIC bonding
Cl
Na
10Polarity in a Molecular Bond
Cl-
Na
11Another Example - Cl2
- A non-polar bond
- COVALENT - equal sharing of electrons
Cl-
Cl-
12Electronegativity
- 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
13Electronegativity and Bond Type
14Concept 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
15What About O2?
- The number of electrons doesnt add up.
- HOW CAN WE FORM TWO OCTETS?
2
12 electrons / 2 6 pairs
O
16What About O2? The Solution
O
O
17Writing Lewis Structures
- Determine the total number of valence electrons
in the species by adding together the numbers of
valence electrons of each atom. - Remember Column number gives the number of
valence electrons for the atom.
18Writing Lewis Structures
- If the species is an anion (has a negative
charge) add the charge number to the overall
number of electrons. - If the species is an cation (has a positive
charge) subtract the charge number from the
overall number of electrons.
19Writing Lewis Structures
- Place the atoms in their relative positions with
element around the central atom as far apart as
possible. - Draw a line representing a single bond,
containing two electrons, between the atoms.
20Writing Lewis Structures
- Distribute the remaining electrons evenly in
pairs on the outer atoms so that each atom has
eight electrons. (Remember that hydrogen is an
exception and only has two electrons) - Subtract the number of electrons used from the
total and place the remaining electrons on the
central atom. Pair them if possible.
21Writing Lewis Structures
- If the central atom has fewer than eight
electrons, move a sufficient number of non-bonded
pairs of electrons from the outer atoms,other
than hydrogen and halogens, to between the atoms
such that the central atom has at least eight
electrons.
22Lewis Formulas
- Write the Lewis formula for P2.
23Lewis Formulas
- Write the Lewis formula for P2.
Total valence electrons 2 x 5 10.
The skeleton is P-P. Distribute the remaining
electrons symmetrically
24Lewis 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
25Neither P atom has an octet. Make one lone pair
from each P a bonding pair. P
P
26PCl5
27PCl5
P 5 Cl 5 x 7 40 valence electrons
Cl
Cl
P
Cl
Cl
Cl
28Beryllium Chloride
29Beryllium Chloride
Be
Cl
Cl
30Basic Exceptions
- B and Be often have less than the octet.
- a complete 2s shell is stable
- 3rd row and higher can exceed the octet.
- d-orbitals are available
- If you must exceed the octet, place extra
electrons on the central atom with a charge.
31Oxidation States
- When refering to a molecule, we can refer to the
oxidation state of each atom.
The oxidation state is equal to the charge on an
ion if a noble configuration is assumed for each
atom.
32Got it?
- NaCl
- Na oxidation state 1
- Cl oxidation state -1
- NH3
- N oxidation state -3
- H oxidation state 1
33What about O3?
- 3 x 6 valence electrons 18
34What about O3?
- 3 x 6 valence electrons 18
O
O
O
35Resonance
- Resonance is a representation of the fact that
electrons are not stationary in a molecule.
O
O
O
O
O
O
36Resonance
- Resonance is a representation of the fact that
electrons are not stationary in a molecule.
O
O
O
O
O
O
Or
O
O
O
37What 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
38Formal Charges
- If more than one Lewis structure is possible and
it is not certain which should be the central
atom (sometimes there is not a central atom),
formal charge is used to derive the most likely
molecular structure. - Formal charge is a theoretical assessment and
does not represent an actual charge on the
species.
39Formal 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 - local molecular electrons
40Formal Charges
- The number of valence electrons is the same as
the group number of the element. - Local molecular electrons 1/2 the number of
shared electrons the number of unshared
electrons.
41Formal Charges
- Calculate the formal charge for each element in
all possible structures. The structure closest
to a formal charge of zero is the most likely
structure. - Also, within a structure, the most
electronegative element usually has the most
negative formal charge.
42Example of Formal Charge
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
43If 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.
44A Problem to Consider
- Consider the Lewis structures that may exist for
XeO3. Use the concept of formal charges to
predict the structure.
45Formal Charge and XeO3
- Assign the most effective Lewis dot structure
based on formal charges.
(3)
Xe
O
O
O
(-1)
(-1)
(-1)
46Formal Charge and XeO3
- Assign the most effective Lewis dot structure
based on formal charges.
(0)
Xe
O
O
O
(0)
(0)
(0)
47Bond Length, Order and Energy
- Use bond energies to estimate ?H for the
following gas-phase reaction. H H
H H
H H H H
Br
H
C
-
C
H
Br
C
C
This is called an addition reaction, because a
compound is added across the double bond.
48An Addition Reaction
H
H
H
Br
C
C
H
H
H
H
H
C
C
Br
H
H
...
49Enthalpy 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