CHEMICAL BONDING
Set 1
Cocaine
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Credits
• Thank you to
Mr. Neil Rapp
who provided
the bulk of this
powerpoint on
his website
www.chemistry
geek.com
Chemical Bonding
Problems and questions —
How is a molecule or
polyatomic ion held
together?
Why are atoms distributed at
strange angles?
Why are molecules not flat?
Can we predict the structure?
How is structure related to
chemical and physical
properties?
Review of Chemical Bonds
Most bonds are
somewhere in between
ionic and covalent.
• There are 3 forms of bonding:
• ionic—complete transfer of 1 or
more electrons from one atom to
another (one loses, the other
gains) forming oppositely
charged ions that attract one
another
• covalent—some valence
electrons shared between atoms
• metallic – holds atoms of a metal
together
Why do atoms bond in these ways?
• A bond will form if the energy of the resulting
molecule is lower than that of the separated
atoms.
• Bond energy: the energy required to break a
bond
• Bond length: the distance where the energy is
minimal
•
Zumdahl, Steven, and Susan Zumdahl. Chemistry. Boston: Houghton Mifflin, 2003.
The type of bond can usually be calculated by
finding the difference in electronegativity of the two
atoms that are going together.
Electronegativity
Difference
• If the difference in electronegativities is
between:
– 1.7 to 4.0: Ionic
–
0.45 to 1.7: Polar Covalent (uneven sharing of
electrons)
– 0.0 to 0.45: Non-Polar Covalent (even sharing of
electrons)
Example: NaCl
Na = 0.8, Cl = 3.0
Difference is 2.2, so
this is an ionic bond!
Learning Check: Relative Bond
Polarities
• Order the following bonds according to
increasing polarity: H-H, O-H, Cl-H, S-H,
and F-H.
Ionic Bonds
All those ionic compounds were made
from ionic bonds. We’ve been through
this in great detail already. Positive
cations and the negative anions are
attracted to one another (remember
the Paula Abdul Principle of
Chemistry: Opposites Attract!)
Therefore, ionic compounds are usually
between metals and nonmetals (opposite
ends of the periodic table).
Ions: Electron Configurations
and Sizes
• Atoms in stable compounds usually have a noble gas
electron configuration
– When two nonmetals react to form a covalent bond,
they share electrons in a way that completes the
valence electron configurations of both atoms. That is,
both nonmetals attain noble gas electron
configurations.
– When a nonmetal and a metal react to form a binary
ionic compound, the ions form so that the valence
electron configuration of the nonmetal achieves the
electron configuration of the next noble gas atom and
the valence orbital of the metal are emptied. In this
way, both ions achieve noble gas electron
configurations.
– Zumdahl, Steven, and Susan Zumdahl. Chemistry. Boston: Houghton Mifflin, 2003.
Predicting Formulas of Ionic
Compounds
• It should be emphasized that when chemists use the term
ionic compound, they are usually referring to the solid state
of the compound
– In the solid state of an ionic compound, the ions are relatively
close together and many ions are simultaneously interacting
– In the gas phase of an ionic substance the ions would be
relatively far apart and would not contain large groups of ions
• Solid ionic compounds contain a large collection of positive
and negative ions packed together in a way that minimizes
the (-)(-) and (+)(+) repulsions and maximizes the (+)(-)
attractions.
•
Zumdahl, Steven, and Susan Zumdahl. Chemistry. Boston: Houghton Mifflin, 2003.
Homework Questions
1) Explain the difference between the following pairs of terms.
– A. electronegativity and electron affinity
– B. covalent bond and polar covalent bond
– C. polar covalent bond and ionic bond
2) Some of the important properties of ionic compounds are as
follows:
– Low electrical conductivity as solids and high conductivity in
solution or when molten
– Relatively high melting and boiling points
– Brittleness
– Solubility in polar solvents (e.g., water)
How does the concept of ionic bonding discussed in previous
units account for these properties?
Homework Questions Cont’d
• 3) Predict which bond in each of the
following groups will be the most
polar.
– A. C-F, Si-F, Ge-F
– B. P-Cl, or S-Cl
– C. S-F, S-Cl, S-Br
– D. Ti-Cl, Si-Cl, Ge-Cl