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Where are we going?



How are electrons SHARED between elements to form ionic
compounds or molecules?
What is the SHAPE and POLARITY of a molecule?
 How does bonding within a molecule influence physical
properties like boiling point, melting point, solubility, surface
tension, elasticity or viscosity?
How can theories of bonding be applied to complex problems in
biology, medicine, engineering or the environment?
The MACROSCOPIC world we are familiar
with is governed by interactions at the
atomic & molecular scale
Summary of Quantum Mechanics:

Electrons exist in orbitals around nuclei as MATTER WAVES:
◦ The orbitals are filled in layers or SHELLS with different shapes
& energies

Electrons PAIR UP in atomic orbitals…

The INNER SHELLS are composed of CORE ELECTRONS. These do
not participate in chemical reactions.

The OUTER SHELL is composed of VALENCE ELECTRONS. These
are more exposed & participate in chemical reactions.
◦ The number of valence electrons corresponds to the group
number (i.e. 1A for Na, 2A for Mg, etc.)

The VALENCE ELECTRONS are considered to be those in the outer
shell of s & p orbitals. These are more exposed & most readily
participate in chemical reactions.
◦ The number of valence electrons corresponds to the group
number (i.e. 1A for Na, 2A for Mg, etc.)
Counting Valence Electrons:
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Lewis Dot Structures of Ionic Compounds:
Valence electrons are written as dots around an atom PAIRING
UP to reach a complete octet (8 e- total)
+
+
+
+
+
• Write the Lewis Dot Structure for aluminum oxide
Diatomic Molecules:
Covalent Bonding in Nonmetals
• “Co-valent” Bonding: atoms sharing valence electrons to reach a
fill outer shell.
• Occurs most often in bonding between NONMETALS
Lewis Dot Structures:
Valence electrons represented as dots
that PAIR UP to reach a complete octet
(or duet if Hydrogen)
Shared Electron Pairs are written as
single lines
+
Diatomic Molecules:
Understanding why certain elements pair up
OCTECT RULE:
Each atom shares enough electrons with its neighboring atoms to
arrive at a noble gas configuration with 8 ELECTRONS
• Write the Lewis Dot Structures for diatomic chlorine gas, oxygen
gas, & nitrogen gas
Electron orbitals overlap on adjacent atoms to share
electrons and reach a STABLE noble gas configuration
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Bonding vs. NON-Bonding Electron Pairs:
+
+
• Bonding Electrons: 2 electrons SHARED between atoms
written as a single line
• Lone Pair Electrons: reside exclusively on one atom,
NOT shared between atoms
iClicker Participation Question:
Predicting the Formula of Covalent Molecules with Lewis Theory
A. PCl
Using only the concepts of Lewis Bonding Theory:
Predict the likely covalent molecule that would form
between Phosphorus & Chlorine.
B. PCl2
C. PCl3
Hint: They may combine in any ratio as
needed so that each atom arrives at a
COMPLETE OCTET (through mutually
sharing electrons with neighboring atoms).
D. P3Cl
E. P2Cl3
Lewis Dot Structures for More Complex Molecules:
H
H
C
H
H
• Write the Lewis Dot Structure for Carbon Dioxide.
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Lewis Dot Structures for Neutral Molecules:
A Systematic Approach to Predict Bonding Arrangements
6.
If needed, move lone pairs from outer atoms to bond with the
central atom to form an octet on the central atom.
 This will make double or triple bonds.
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
iClicker Participation Question:
Writing the Lewis Dot Structure for Polyatomic Ions
How many TOTAL electrons would be involved in bonding
in the polyatomic anion, NITRATE: NO3- ?
A. 11 electrons
B. 12 electrons
C. 22 electrons
D. 23 electrons
E. 24 electrons
Polyatomic Ions are just
CHARGED molecules
 For each negative (-) charge:
add one electron.
 For each positive (+) charge:
subtract one electron.
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Lewis Dot Structures for Polyatomic Ions:


6.
For each negative (-) charge: add one electron.
For each positive (+) charge: subtract one electron.
If needed, move lone pairs from outer atoms to bond with the
central atom to form an octet on the central atom.
 This will make double or triple bonds.
iClicker Participation Question:
Recognizing CORRECT & INCORRECT Lewis Dot Structures
Select the Lewis Dot Structures below that are VALID.
A.
B.
C.
D. ALL of these Lewis Dot Structures are VALID
E. NONE of these Lewis Dot Structures are VALID
Valence Shell Electron Pair Repulsion (VSEPR) Theory:
Molecules and polyatomic ions take on particular shapes based on
the number of electron regions located around a central atom:
• Valence Shell (VS…): Consider ONLY the valence electrons:
• Lone pair electrons & bonding electrons are all treated the
same:
• A single bond counts the same as a double or triple bond
• Lone pairs are considered the same as bonding electrons
• Electron Pair Replusions (…EPR): electrons are negative charges
that naturally REPEL from other electrons. These repulsive forces
between electrons determines molecular shape.
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LINEAR Molecular Geometry
Electron Dot
Structure
Total # of
ELECTRON
Regions
# of
BONDING
Regions
# of
LONE PAIRS
2
2
0
The Total Number of Electron
Regions determines the Electron
Geometry:
180°
2 REGIONS = LINEAR GEOMETRY
Linear
LINEAR Molecular Geometry
Electron Dot
Structure
Total # of
ELECTRON
Regions
# of
BONDING
Regions
# of
LONE PAIRS
2
2
0
180°
Linear
TRIGONAL PLANAR Molecular Geometry
Electron Dot
Structure
Total # of
ELECTRON
Regions
# of
BONDING
Regions
# of
LONE PAIRS
3
3
0
120°
Trigonal planar
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TRIGONAL PLANAR Electron Geometry
BENT MOLECULAR GEOMETRY
Electron Dot
Structure
Total # of
ELECTRON
Regions
# of
BONDING
Regions
# of
LONE PAIRS
3
2
1
The Total # of Electron Regions
determines the Electron
Geometry:
3 REGIONS = TRIGONAL
PLANAR Electron Geometry
TRIGONAL PLANAR Electron Geometry
TRIGONAL PLANAR MOLECULAR
GEOMETRY
When lone pairs are present on the central atom, the
molecular geometry is DIFFERENT from the electron
geometry.
• Molecular Geometry only considers the positions
of the atoms
TRIGONAL PLANAR Electron Geometry
BENT MOLECULAR GEOMETRY
4 REGIONS = TETRAHEDRAL Electron Geometry
Electron Dot
Structure
Total # of
ELECTRON
Regions
# of
BONDING
Regions
# of
LONE PAIRS
4
4
0
109.5°
Tetrahedral
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4 REGIONS = TETRAHEDRAL Electron Geometry
ONE LONE PAIR = TRIGONAL PYRAMIDAL MOLECULAR GEOMETRY
Electron Dot
Structure
Total # of
ELECTRON
Regions
# of
BONDING
Regions
# of
LONE PAIRS
4
3
1
109.5°
Tetrahedral
Trigonal Pyramid
4 REGIONS = TETRAHEDRAL Electron Geometry
TWO LONE PAIRS = BENT MOLECULAR GEOMETRY
Electron Dot
Structure
Total # of
ELECTRON
Regions
# of
BONDING
Regions
# of
LONE PAIRS
4
2
2
109.5°
Tetrahedral
Bent
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iClicker Participation Question:
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
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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.
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Bond Polarity
Electronegativity Differences generate “POLES” of charge separation
Bond Dipole Moment
δ:
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-
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
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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
ΔEN: 4.0-2.0 = 2.0
strongly POLAR, overall BF3 is
NONPOLAR because it has a
1. First consider bond polarities.
trigonal planar molecular
If polar bonds are present, the
geometry
molecule MIGHT be polar.
2. Then consider MOLECULAR
GEOMETRY. If the molecule is
symmetric, it CANNOT be
polar.
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Molecular Geometry Influences Molecular Polarity:
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
REMEMBER: Even if a
molecule is asymmetric, it
CANNOT be polar if it only
has NONPOLAR bonds.
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.
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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
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