8.4 Polar Bonds and Molecules
Objectives
 We will use electronegativity differences to
predict bond types
 Explain characteristics of water based on
hydrogen bonds
Electronegativity
 A periodic property of each element
 Defined as the attraction an atom has for the
electrons of another element.
 Fluorine has the highest electronegativity of
all elements at 4.0
 All the halogens have a strong attraction for
electrons to fill their octet
 Alkali metal have very low electronegativity
values and will give up electrons to any
element with higher electronegativity
 electronegativity video
Polar Bonds and Molecules
 Chemical bonds are classified based on
Electronegativity Differences
Range Probable type of bond Example
0.0-0.4 Nonpolar covalent
H-H (0)
0.4-1.0 Moderate polar covalent H-Cl (0.9)
1.0-2.0 Very polar covalent
H-F (1.9)
>2.0
Ionic
Na+ Cl- (2.1)
*What kind of bond is a S-O bond?
 What type of bond is an S-O bond?
 What type of bond is a C-N bond?
Solubility-Dissolving of salt
Polar Molecules
Water is a very polar Covalent
molecule
Hydrogen Bonds
 These are weak attractions between the positive
(hydrogen) end of one molecule and the negative
end of another.
 These are NOT the covalent bond linking H-O in
the molecule
As water freezes, hydrogen bonds
hold the molecules in place, forming
an open ring structure
Properties of water are due to its
polarity
 Cohesion-molecules
are attracted to one
another
 Solvent PropertiesWater is the universal
solvent, and can
dissolve other polar or
ionic compounds
 Because it is polar, it
can’t dissolve nonpolar substances
(oils, fats)
Why is Ice less dense than water?
 When water molecules align, an “open” structure
is created that is less densely packed than when
liquid molecules can move around.
Hydrogen Bonding in Water ice
http://www.its.caltech.edu/~atomic/snowcrystals/faqs/faqs.htm
animation
Why do snow crystals have 6 sides?
Electron Micrographs of snow
Snow close up
avalanche
8.3 Bonding Theories
 Molecular Orbitals- Like the
atomic orbitals, s,p,d,f
 Molecular orbitals are the
orbitals of valence electrons
that are shared when
forming a bond.
 Sigma bonds- symetrical
around bond axis
 Pi bonds- form bond above
or below axis
8.4 VSEPR Theory
Valence Shell Electron Pair Repulsion
 The repulsion
between electron
pairs causes
molecular shapes
to adjust so that
electron pair stay
as far apart as
possible.
Molecular Shapes
 Linear
 Trigonal Planar
 Tetrahedral
 Trigonal biPyramidal
VSEPR Theory
 Octahedral
p.