Ion – Ion Interactions
Ion-ion interactions occur between ions of opposite
charges, resulting in solids with ionic bonds. These
interactions are most significant in the solid state.
Ion-ion forces are very strong, and result in ionic
solids with high boiling points and melting points.
When dissolved in water, ions are shielded from one
another by water molecules, making ion-ion
interactions less dominant.
In the solid state, ions interact by forming crystalline
lattices in which oppositely charged ions arrange
themselves in a regular fashion. Melting points are
high because a great deal of energy is required to
break apart the crystal.
Ion-Ion Forces
Ion-Dipole Forces
Dipole – Dipole Interactions
A dipole is most often found in polar molecules,
in which the electrons are unevenly shared.
This uneven sharing gives one side of the
molecule a partially positive charge and the
other side a partially negative charge. Dipole –
dipole interactions occur when the partially
positively charged part of a molecule interacts
with the partially negatively charged part of the
neighboring molecule. This type of attraction to
exist between molecules of hydrogen chloride,
HCl. Dipole-dipole interactions are the strongest
intermolecular force of attraction.
Hydrogen Bonding
This is a special kind of dipole-dipole interaction that occurs
specifically between a hydrogen atom bonded to either an oxygen,
nitrogen, or fluorine atom. The partially positive end of hydrogen is
attracted to the partially negative end of the oxygen, nitrogen, or
fluorine of another molecule. Hydrogen bonding is a relatively strong
force of attraction between molecules, and considerable energy is
required to break hydrogen bonds. This explains the exceptionally
high boiling points and melting points of compounds like water, H2O,
and hydrogen fluoride, HF. Hydrogen bonding plays an important role
in biology; for example, hydrogen bonds are responsible for holding
nucleotide bases together in DNA and RNA.
Ion – Dipole Interactions
An ion-dipole interaction involves the attraction between a fully charged entity and a
polar molecule. Both negative ions (anions) and positive ions (cations) can participate in
this type of bonding. A cation will be attracted to the partial negative portion of the polar
molecule, while an anion will interact with the partial positive region.
Polar molecules.
The interaction of sodium and chloride ions with water is one example of an ion-dipole
interaction:
Interactions of ions with polar water molecules.
The positive sodium ions are attracted to the partial negative portion of the water
molecule, while the negative chloride ions interact with the partial positive hydrogen
portion. Because one of the bonding partners has only a partial positive or negative
charge, these forces are somewhat weaker than ion-ion interactions.
Dispersion Forces
Somewhat more challenging to visualize are dispersion forces.
These interactions are defined as attractive forces that arise as a
result of temporary dipoles (charge separations) induced/created
in atoms or molecules.
Two things need to be kept in mind with regard to these forces:
they are relatively weak, and they do not require any permanent
polarity. Induced dipoles are created by a momentary shift of
electrons to one side of a nonpolar molecule. This allows some
nonpolar molecules to exist as solids.
The more electrons a molecule has, the stronger the London
dispersion forces are. For example, bromine, Br2, has more
electrons than chlorine, Cl2, so bromine will have stronger
dispersion forces than chlorine, resulting in a higher boiling point
for bromine, 59o C, compared to chlorine, –35 oC. It also explains
why Chlorine is a gas at room temperature, and Bromine is a
liquid.