1. Bond Polarity Electronegativity
Attraction an atom has for a shared pair of electrons.
higher e-neg atom  -
lower e-neg atom +
Bond polarity
1. Extent to which it is polar
2. Determined largely by the relative electronegativities of the bonded atoms
3. Electronegativity () — ability of an atom in a molecule or ion to attract electrons to itself
4. Direct correlation between electronegativity and bond polarity
a. A bond is nonpolar if the bonded atoms have equal electronegativities
b. If electronegativities of the bonded atoms are not equal, bond is polarized toward the more electronegative atom
c. A bond in which the electronegativity of B (B) is greater than the electronegativity of A (A) and is indicated with the partial negative charge on the more electronegative atom
õ õ-
(less electronegative) A --- B (more electronegative)
d. To estimate the ionic character of a bond (the magnitude of the charge separation in a polar covalent bond), calculate the difference in electronegativity between the two atoms
Dipole moments
1. Produced by the asymmetrical charge distribution in a polar substance
2. Abbreviated by µ
3. Defined as the product of the partial charge Q on the bonded atoms and the distance r between the partial charges
µ = Qr
Q measured in coulombs (C)
r measured in meters (m)
4. Unit for dipole moment is the debye (D)
1D = x 10-30C•m
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2. Bond Polarity Nonpolar Covalent Bond electrons are shared equally
symmetrical electron density
usually identical atoms

3. + - Bond Polarity Polar Covalent Bond electrons are shared unequally
asymmetrical e- density
results in partial charges (dipole) + -
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4. Bond Polarity Nonpolar Polar Ionic
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5. + - Dipole Moment H Cl Direction of the polar bond in a molecule.
Arrow points toward the more electronegative atom.
H Cl + -
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6. Determining Molecular Polarity
Depends on:
dipole moments
molecular shape H Cl + – + –
We have seen that molecules can have a separation of charge
This happens in both ionic and polar bonds (the greater the EN, the greater the dipoles)
Molecules are attracted to each other in a compound by these +ve and -ve forces + – + – + –
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7. Determining Molecular Polarity
Nonpolar Molecules
Dipole moments are symmetrical and cancel out.
BF3 F B
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8. Determining Molecular Polarity
Polar Molecules
Dipole moments are asymmetrical and don’t cancel .
H2O H O
net
dipole
moment
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9. Determining Molecular Polarity
Therefore, polar molecules have...
asymmetrical shape (lone pairs) or
asymmetrical atoms
CHCl3 H Cl
net
dipole
moment
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10. Dipole Moment Nonpolar Polar C O O O H H .. Bond dipoles
In H2O the bond dipoles are also equal in
magnitude but do not exactly oppose each
other. The molecule has a nonzero overall
dipole moment. C O O ..
Overall dipole moment = 0 O
Bond dipoles
Nonpolar H H
In complex molecules that contain polar covalent bonds, the three-dimensional geometry and the compound’s symmetry determine if there is a net dipole moment
• Mathematically, dipole moments are vectors; they possess both a magnitude and a direction
• Dipole moment of a molecule is the vector sum of the dipole moments of the individual bonds in the molecule
• If the individual bond dipole moments cancel one another, there is no net dipole moment
• Molecular structures that are highly symmetrical (tetrahedral and square planar AB4, trigonal bipyramidal AB5, and octahedral AB6) have no net dipole moment; individual bond dipole moments completely cancel out
• In molecules and ions that have V-shaped, trigonal pyramidal, seesaw, T-shaped, and square pyramidal geometries, the bond dipole moments cannot cancel one another and they have a nonzero dipole moment
The overall dipole moment of a molecule
is the sum of its bond dipoles. In CO2 the
bond dipoles are equal in magnitude but
exactly opposite each other. The overall
dipole moment is zero.
Overall dipole moment
Polar
Brown, LeMay, Bursten, Chemistry The Central Science, 2000, page 315

11. VSEPR Theory Valence Shell Electron Pair Repulsion Theory
Electron pairs orient themselves in order to minimize repulsive forces.
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12. Lone pairs repel more strongly than bonding pairs!!!
VSEPR Theory
Types of e- Pairs
Bonding pairs - form bonds
Lone pairs - nonbonding electrons
Lone pairs repel more strongly than bonding pairs!!!
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13. N H .. .. C H O .. H H .. O CH4, methane NH3, ammonia H2O, water O
lone pair
electrons
Oxygen contains two pairs of electrons that don’t bond at all. These electron pairs are referred to as unshared electron pairs, lone pairs or unbonded pairs. O O
O3, ozone

14. Molecular Shapes Three atoms (AB2) Four atoms (AB3) B A Linear (180o)
Bent
Trigonal planar (120o)
Trigonal pyramidal
T-shaped B A
linear
trigonal planar B A
Five atoms (AB4)
tetrahedral
Tetrahedral (109.47o)
Square planar
Seesaw A Be Ba
Molecular formula
– Gives the elemental composition of molecules
Structural formula
Shows which atoms are bonded to one another and the approximate arrangement in space
Enables chemists to create a three-dimensional model that provides information about the physical and chemical properties of the compound
A single bond, in which a single pair of electrons are shared, is represented by a single line (–)
A double bond, in which two pairs of electrons are shared, is indicated by two lines (=)
A triple bond, in which three pairs of electrons are shared, is indicated by three lines (≡)
Six atoms (AB5)
Trigonal bipyramidal (BeABe, 120o) & (BeABa, 90o)
Square pyramidal B A
Seven atoms (AB6)
Trigonal
bipyramidal
Octahedral
Bailar, Moeller, Kleinberg, Guss, Castellion, Metz, Chemistry, 1984, page 313.

15. Bonding and Shape of Molecules
Number
of Bonds
Number of
Unshared Pairs
Covalent
Structure
Shape
Examples 2 3 4 1 2
-Be-
Linear
Trigonal planar
Tetrahedral
Pyramidal
Bent
BeCl2
BF3
CH4, SiCl4
NH3, PCl3
H2O, H2S, SCl2 B C N : O :

16. …the blending of orbitals
Hybridization
…the blending of orbitals
Valence bond theory is based on two assumptions:
1. The strength of a covalent bond is proportional to the amount of overlap between atomic orbitals; the greater
the overlap, the more stable the bond.
2. An atom can use different combinations of atomic orbitals to maximize the overlap of orbitals used by bonded atoms.
Two overlapping orbitals form what is known as a hybrid or molecular orbital. Just as in a s,p,d, or f orbital the electrons can be anywhere in the orbital
(even though the electron has started out in one atom, at times, it may be closer to the other nucleus). Each hybrid orbital has a specific shape.
You need to know that hybrid orbitals exist and that they are formed from overlapping orbitals

17. First, the formation of BeH2 using pure s and p orbitals.
Be = 1s22s2 H Be
BeH2 H s p
No overlap = no bond!
atomic orbitals
atomic orbitals
The formation of BeH2 using hybridized orbitals. Be H s p
atomic orbitals Be H
hybrid orbitals Be s p Be
BeH2 sp p
All hybridized bonds have equal strength and have orbitals with identical energies.

18. sp hybrid orbitals shown together
Ground-state Be atom 1s 2s 2p
Be atom with one electron “promoted” sp
hybrid orbitals
Energy 1s sp 2p
Be atom of BeH2 orbital diagram px py pz
A more sophisticated treatment of bonding is a quantum mechanical description of bonding, in which bonding electrons are viewed as being localized between the nuclei of the bonded atoms
• The overlap of bonding orbitals is increased through a process called hybridization, which results in the formation of stronger bonds According to quantum mechanics, bonds form between atoms because their atomic orbitals overlap, with each region of overlap accommodating a maximum of two electrons with opposite spin, in accordance with the Pauli principle
• Electron density between the nuclei is increased because of orbital overlap and results in a localized electron-pair bond
• Localized bonding model is called the valence bond theory and uses an atomic orbital approach to predict the stability of the bond
n = 1
n = 2 s
two sp hybrid orbitals
s orbital
p orbital
hybridize H Be
sp hybrid orbitals shown together
(large lobes only)

19. sp2 hybrid orbitals shown together
Ground-state B atom 2s 2p 2s 2p
B atom with one electron “promoted”
sp2
hybrid orbitals
Energy
sp2 2p px py pz s
B atom of BH3 orbital diagram
p orbitals H B
three sps hybrid orbitals
sp2 hybrid orbitals shown together
(large lobes only)
hybridize
s orbital