Bonding:
General Concepts
Part 2
Dr. Deborah Walker
Announcements
• From building atoms to building molecules!
• By the end of today, you should be able to:
–
–
–
–
–
Identify a bond as covalent, polar covalent, or ionic.
Explain bonding (what, when, why, how).
Predict which bond is more strongly polar (has strongest dipole).
Identify polar bonds and identify the partial charges.
Determine which ionic compound has the stronger lattice energy.
MON
TUES
WED
THURS
FRI
Deb
12-1:30pm
WEL 4.142
Deb
12-1:30pm
WEL 4.142
Deb
12-1:30pm
WEL 4.142
Seyma
11-12noon
Review
• Ionic bonds involve the exchange of electrons from one
atom to another, or
• Ionic bonds involve a cation bonding to an anion.
• Covalent bonds involve the sharing of electrons between
two atoms.
• In all cases, bonds form because they form a lower
energy state.
Knowledge Check
• Which of the following involves a covalent bond?
–
–
–
–
A. Cl2
B. BeF2
C. Fe2O3
D. None of these.
Puzzler
• Which of the following bonds exhibits the most covalent
character?
–
–
–
–
–
A. BF3
B. CH4
C. Al2O3
D. HCl
E. I don’t know.
The Catch…
• Continuum
ΔEN=0
Nonpolar
covalent
ΔEN=1.5
Polar
covalent
Ionic
• So, electronegativity is key to identifying bond type.
Electronegativity
• What is electronegativity?
Electronegativity
Knowledge Check
• For the H-F bond, which element has the greater
electronegativity?
– A. H
– B. F
– C. They have the same electronegativity because H could
be in column 1A or in column 7A.
– D. I have no clue.
Possibilities
• 1. X and Y have identical electronegativities.
(ex. X and Y are the same element).
Electrons are equally shared.
This is a nonpolar covalent bond.
• 2. X and Y have dissimilar electronegativities.
(ex. Y is more electronegative)
Y has greater control of the electrons.
The bond is polar covalent.
• 3. X and Y have very different
electronegativities.
(ex. Y is much more electronegative)
Y has full control of the electrons.
Y is negative; X is positive
The bond is ionic.
X
Y
d+
d-
+
X
Y
X
Y
(Note: Metallic bond not included in above discussion as it is not on the continuum.)
Continuum Bond Types
X
Y
d+
d-
+
X
Y
X
Y
The Catch…
• Continuum
ΔEN=0
Nonpolar
covalent
ΔEN=1.5
Polar
covalent
Ionic
• So, electronegativity is key to identifying bond type.
•
•
•
•
Ex. H2
Ex. F2
Ex. HF
Ex. NaF
Generic Example
• Consider two atoms, X and Y, bound together by two
electrons.
• What are the possible bond types?
– (Note: we’re ignoring metallic bonds for now)
Practice (see packet)
• Identify the dEN for the following bonds and classify each
as having mostly covalent, mostly polar covalent, or
mostly ionic character. Identify partial charges when
applicable.
– H-H
– Cl-Cl
– H-Cl
– C-H
Your Turn (see packet)
• Now more about ionic bonds…
• How do we tell which ionic bond is stronger?
– (Note: A stronger bond indicates a more stable bond)
Ionic Compounds
• Stabilize by forming crystalline solids
• Alternating cations and anions in a repeating pattern
Lattice Energy
• Lattice energy is the change in energy that occurs
when the separated gaseous ions are packed
together to form an ionic solid.
M  (g)  X  (g) 
 MX ( s)  energy
• Question: Which is more stable, M+ & X- or MX?
• We can determine lattice energy for an ionic
compound via a series of steps.
Determining Lattice Energy
• Step 1: Metal to gas
• Step 2: Ionize metal
• Step 3: Break diatomic gas
• Step 4: Ionize
• Step 5: Combine ions
Lattice Energy
Lattice Energy
Lattice Energy = k Q1 Q2
r
r = average distance between cations and anions
k = a constant for this crystal structure
Q1, Q2 = charges on the ions
Figure
13.10:
The driving
force
for making MgO
is the bigger
release of
energy (big
negative LE
value)
NOTE: Making Freleases energy
but making O2- requires
energy!
Knowledge Check
• Lattice Energy will be large for:
A. small r (close spacing - permitted by small ions)
B. large r (large spacing required by larger ions)
Knowledge Check
• Lattice Energy will be large for:
A. large Q (highly charged ions (+2,-3 for example)
B. small Q (low charged ions (+1, -1 for example)
Knowledge Check
• Which would have the larger lattice energy?
A. NaCl
B. KCl
Knowledge Check
• Which would have the larger lattice energy?
A. Calcium phosphate
B. Calcium chloride
Summary Lattice Energy
• The difference in energy between ions
packed together in a solid and ions
widely separated in a gas.
• High lattice energy indicates the ions
are held together tightly in the solid.
• Lattice energy is large when charge is
large and distance between ions is
small.
• High lattice energy leads to high melt
point for ionic compounds, ex. LiF &
Ca3(PO4)2.
Summary So Far
• Multiple bond types exist. Bonds form to lower energy
for the atoms involved.
• Bond type is estimated by calculating the difference in
electronegativity.
• Partial charges are assigned to atoms in polar covalent
bonds.
• Full charges are assigned to ions in ionic bonds.
(below if time or for next time)
Knowledge Check
1. Which of the following bonds would you expect to be
classified as polar covalent?
KF
CO O2
A. KF, with the F end having a partial negative charge.
B. KF, with the K end having a partial negative charge.
C. CO, with the O end having a partial negative charge.
D. CO, with the C end having a partial negative charge.
E. O2, with neither end having a partial negative charge.
Knowledge Check
Which bond would you expect to be MOST polar? (i.e.,
Which would have the largest partial positive and partial
negative charges?)
O-F N-F C-F
A. O-F
B. N-F
C. C-F