This document defines and differentiates the main types of intermolecular forces: dipole-dipole forces, London dispersion forces, ion-dipole forces, and hydrogen bonding. It provides examples of polar and nonpolar molecules, and describes the characteristics of each type of intermolecular force, including their relative strengths and the molecules between which they typically occur.
2. Section Objectives
1. Define intermolecular force.
2. Describe and differentiate the types of
intermolecular forces.
3. Review
Polar
Polar molecules occur when two atoms do not share
electrons equally in a covalent bond.
Examples of polar molecules include:
water - H2O
ammonia - NH3
sulfur dioxide - SO2
hydrogen sulfide - H2S
ethanol - C2H6O
4. In a polar covalent
bond, one atom
spends more time
with the electrons
than the other.
5. Nonpolar
When molecules share electrons equally in a covalent
bond there is no net electrical charge across the molecule.
Examples of nonpolar molecules include:
• any of the noble gasses: He, Ne, Ar, Kr, Xe (These are
atoms, not technically molecules.)
• any of the homonuclear diatomic elements: H2, N2, O2, Cl2
(These are truly nonpolar molecules.)
• carbon dioxide - CO2
• benzene - C6H6
7. Dipole
- a molecule in which a concentration of
positive electric charge is separated
from a concentration of negative
charge.
8.
9. Intermolecular Forces
• Forces of attraction between molecules
are called intermolecular forces (IMF).
• These are forces that act between
individual molecules that hold them close
together as in a solid or a liquid.
10. Intermolecular forces can be grouped for
convenience into Dipole-dipole forces, Ion-
dipole forces, Dispersion forces and
Hydrogen bonds.
11. The attractions between molecules are not nearly as strong
as the intramolecular attractions that hold compounds
together
They are, however, strong enough to control physical
properties such as boiling and melting points, vapor
pressures, and viscosities.
12.
13. London dispersion forces (LDF)
- Is the weakest intermolecular force. The
London dispersion force is a temporary
attractive force that results when the electrons in two
adjacent atoms occupy positions that make the atoms form
temporary dipoles. This force is sometimes called an
induced dipole-induced dipole attraction.
14.
15. London dispersion forces are weak
intermolecular forces and are considered van
der Waals forces. Temporary dipoles can
occur in molecules when the
electrons that constantly orbit the nucleus
occupy a similar location by chance
16. • Dispersion (London) Forces exist between
non polar molecules as a consequence of
random electron positions (temporary dipole)
• Much weaker than real dipole
• Larger molecules are more likely to exhibit
London forces
17. • This is why non-polars have lower melting
and boiling points
18.
19.
20. Dipole-dipole forces
are attractive forces between the positive
end of one polar molecule and the negative
end of another polar molecule. Dipole-dipole
forces have strengths that range from 5 kJ to
20 kJ per mole. The partially positive end of a
polar molecule is attracted to the partially
negative end of another.
21. Dipole-dipole forces
- force that occurs between two polar
molecules. They occur when the slightly
positive end of one polar molecule is attracted
to the slightly negative end of another polar
molecule.
22.
23. - result when two dipolar molecules interact
with each other through space. When this
occurs, the partially negative portion of one
of the polar molecules is attracted to the
partially positive portion of the second polar
molecule
24.
25.
26. - these forces exist when polar molecules are
attracted to ions. The positive pole is attracted
to a negative ion (anion), while the negative
pole is attracted to a positive ion (cation).
27. - are generated between polar water molecules
and a sodium ion. The oxygen atom in the
water molecule has a slight negative charge
and is attracted to the positive sodium ion.
These intermolecular ion-dipole forces are
much weaker than covalent or ionic bonds.
28.
29.
30.
31. Hydrogen bonding
Hydrogen bonding is a special type of
molecular attraction between the hydrogen
atom in a polar bond and nonbonding electron
pair on a nearby small electronegative ion or
atom (usually F, O or N).
32. Hydrogen bonding arises in part from the
high electronegativity of nitrogen, oxygen,
and fluorine.
Also, when hydrogen is bonded to one of
those very electronegative elements, the
hydrogen nucleus is exposed.
