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Today:
◦ Ionic Bonding vs. Covalent Bonding
◦ Bond Polarities:
 Nonpolar Covalent vs. Polar
Covalent vs. Ionic
 Electronegativity Differences
 Dipole Moments
◦ Molecular Polarities:
 Molecular Symmetry
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QUIZ 3 Next Week in Lab:
Covering Lectures 14-16
Next Meeting
◦ Please read Chapter 12, pp.
411-439 before Tuesday
Schedule an appointment or
stop by office hours to pick up
Midterm Exam 2
Concept Check:
Predicting Molecular Shapes based on Lewis Dot Structures
Based on the Lewis dot structure for phosphite below, what is
the molecular geometry about the central phosphorus atom?
A. Tetrahedral
B. Trigonal Planar
C. Trigonal Pyramidal
D. Bent
E. Linear
Summary of VSEPR Theory:
Commit these shapes & bond angles to memory
Electron Distribution within a Molecule:
Influencing how molecules react and interact with each other
Ionic vs. Covalent Bonding
Ionic Bonds: A transfer of electrons makes one atom negatively
charged (an anion) & one atom positively charged (a cation)
But why do different combinations of
elements produce different types of bonds?
Covalent Bonds: Two atoms sharing electrons with a mutual
attraction to the negative charge holding the nuclei together
Electronegativity: Measuring an element’s pull on electrons
Ionic Bonds form between elements with a large difference in electronegativities:
• …typically when METALS combine with NON-METALS
Covalent Bonds occur between elements with similar electronegativities:
• …when NON-METALS bond with other NON-METALS
Table of Electronegativities
A table of electronegativities will always be provided on exams if needed
A Continuum of Bonding
In reality, few bonds are purely ionic or purely covalent
The electronegativity difference (EN) between two
atoms will allow us determine whether two atoms in a
bond will display more ionic properties or more
covalent properties.
Bond Polarity
Electronegativity Differences generate “POLES” of charge separation
Bond Dipole Moment
Measuring the degree of CHARGE SEPARATION
Lower case delta symbols indicate a build up of partial charge at
an atom—NOT FULL IONIZED, though.
δ:
δ+
δ-
Bond Dipole Moment: A vector quantifying the
degree of charge separation across a bond. Points
in the direction of partial negative charge.
iClicker Participation Question:
Bond Polarities from Electronegativities
Which bond below would be the most polarized with the
largest dipole moment?
A. C—N
B. C—H
C. N—O
D. N—F
E. O—H
Bond Polarity influences Molecular Polarity:
…and molecular polarity influences how molecules interact with one another
POLAR MOLECULES:
1. Contain POLAR bonds
2. Have ASYMMETRIC SHAPES that cause the
bond dipole moments to add together
NONPOLAR
MOLECULE
POLAR MOLECULE
Without Polar Bonds a Molecule CANNOT be Polar:
Nonpolar bonds generate nonpolar molecules
ΔEN: 2.5-2.1 = 0.4
1. First consider bond polarities: If all
bonds are NONPOLAR, then the
molecule is also NONPOLAR.
• ONLY if polar bonds are present,
MIGHTthe molecule be polar.
Sometimes even Polar Bonds DO NOT Guarantee a Polar Molecule:
If a molecule is completely symmetric, the dipole moments cancel out and the
molecule overall is NONPOLAR
Even though B—F bonds are
strongly POLAR, overall BF3 is
NONPOLAR because it has a
1.
trigonal planar molecular
geometry
ΔEN: 4.0-2.0 = 2.0
First consider bond polarities.
If polar bonds are present, the
molecule MIGHT be polar.
2. Then consider MOLECULAR
GEOMETRY. If the molecule is
symmetric, it CANNOT be
polar.
Molecular Geometry Influences Molecular Polarity:
Symmetric Molecules are
NEVER polar, even if they
have polar bonds
REMEMBER: Even if a
molecule is asymmetric, it
CANNOT be polar if it only
has NONPOLAR bonds.
Asymmetric Molecules WITH POLAR
BONDS pull electrons unevenly
across the structure, this makes a
POLARIZED molecule
Symmetric vs. Asymmetric Molecules:
Symmetric Molecules are
NEVER polar, even if they
have polar bonds
Asymmetric Molecules WITH POLAR
BONDS pull electrons unevenly
across the structure, this makes a
POLARIZED molecule
Is Carbon Tetrachloride POLAR or NONPOLAR?
Tetrahedral
ΔEN:
3.0-2.5 = 0.5
1. First write the Lewis dot structure.
2. Use a table of electronegativities to calculate ΔEN for all bonds
present.
• If ΔEN is less than or equal to 0.4, the bonds are NONPOLAR.
STOP HERE. The molecule must also be NONPOLAR.
• If ΔEN is greater than 0.4, polar bonds are present & the
molecule MIGHT be polar (depending on molecular shape).
3. Next, use the dot structure to count the number of electron
regions & determine the MOLECULAR GEOMETRY.
• If the molecule is ASYMMETRIC, it is POLAR.
• If the molecule is SYMMETRIC, it is NONPOLAR.
Is Sulfur Dioxide POLAR or NONPOLAR?
ΔEN: 3.5-2.5 = 1.0
1. First write the Lewis dot structure.
2. Use a table of electronegativities to calculate ΔEN for all bonds
present.
• If ΔEN is less than or equal to 0.4, the bonds are NONPOLAR.
STOP HERE. The molecule must also be NONPOLAR.
• If ΔEN is greater than 0.4, polar bonds are present & the
molecule MIGHT be polar (depending on molecular shape).
3. Next, use the dot structure to count the number of electron
regions & determine the MOLECULAR GEOMETRY.
• If the molecule is SYMMETRIC, it is NONPOLAR.
• If the molecule is ASYMMETRIC, it is POLAR.
iClicker Participation Question:
Comparing Relative Molecular Polarities
Which molecule below is the MOST POLAR?
A. CH4
B. CHF3
C. CF4
D. CCl4
E. They are all NONPOLAR