3.11.1: Intermolecular
Forces General Info
So Far we have talked about
intramolecular forces
or forces within a molecule
• 1. ionic bonds
• 2. covalent bonds
• 3. metallic bonds
• There are also forces that hold molecules
together…intermolecular forces
• Ex. Intranet is SCHS computers linked to other
SCHS computers
• Internet SCHS computers linked to computers
around the world
Intermolecular Forces
 Forces that attract molecules to
other molecules. These include:
 Hydrogen bonding
 Dipole-dipole attraction
 London dispersion forces
Relative Magnitudes of Forces
The types of bonding forces vary in their
strength as measured by average bond
energy.
Strongest
Ionic bonds (over 100 kcal)
(Based on Coulombic attraction: Greater ion charge, smaller atomic size)
Covalent bonds (over 100 kcal)
IMF
Hydrogen bonding (12-16 kcal )
Dipole-dipole interactions (2-0.5 kcal)
Weakest London forces (less than 1 kcal)
Background Intermolecular Forces (IMF)
• IMF occur between molecular substances
(covalent bonds) or nonpolar atoms
• Intermolecular forces – attractions between
molecules – not bonds
– Gases – farthest apart molecules and
weakest IMF
– Liquids – less than solids, but more than
gases in both space and IMF
• Strength compared to ionic or covalent bonds –
IMF are weaker than covalent and ionic bonds
• Boiling point & other properties – higher boiling
point indicates a stronger IMF and vice versa
Hydrogen Bonds- Strongest IMF
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Hydrogen and ONLY F, O, or N (strongest electronegativity).
H MUST be bonded to F,O, or N
F, O , N need unshared electron pairs also
The polar H is attracted to the electrons in F, O, or N.
This attraction is VERY strong. Why ice floats, and pond
skimmer skim.
Dipole – Dipole Force- 2nd strongest IMF
• Two molecules, each with
oppositely charged ends.
• The molecules line up so that each
end is facing the opposite charge.
• The 2 neutral molecules are very
close in proximity.
• The strength depends on the
strength of polarity and having
similar size and similar mass of the
two neutral molecules
London Dispersion Forces (LDF)- weakest
• Nonpolar atoms or molecules
• This forms when the molecule temporarily acts like a
dipole (all the electrons are “Stuck” on one side of the
molecule). When the two atoms/molecules come close
together, their opposite charges attract.
• Polarizability – ability to become polar. When this is
greater, the dispersion force is stronger
Factors Affecting London Forces
• The shape of the molecule
affects the strength of dispersion
forces: long, skinny molecules
(like n-pentane tend to have
stronger dispersion forces than
short, fat ones (like neopentane).
• This is due to the increased
surface area in n-pentane.
Factors Affecting London Forces
• The strength of dispersion forces tends to
increase with increased molecular weight.
• Larger atoms have larger electron clouds, which
are easier to polarize.