Intermolecular
Forces of Attraction
Prepared by: Jerome A. Bigael, Leyte Progressive High School
Intermolecular forces:
This force holds the
molecules together
which can be
attractive or
repulsive.
Inter-Ionic Forces
On the other
hand, inter-ionic
forces are forces
that hold ions
together.
These intermolecular
forces are accountable for
the properties of
substances. Moreover,
intermolecular forces also
explain why substances
exist as solids, liquids, or
gases at room
temperature.
Intermolecular and Inter-ionic Forces of Attraction
Type of Interaction
Interacting Particles
Dispersion
All molecules (especially
nonpolar molecules
Polar molecules
Ions and polar molecules
Ions (cations and anions)
N, O, or F bonded with H atom
Dipole-dipole
Ion-dipole
Ion-ion
Hydrogen bond
4 Types of Intermolecular Forces
1.
2.
3.
4.
(involving covalent molecules)
Dipole-dipole
Ion-dipole
Dispersion forces
Hydrogen bond
The first three types are known as “Van der
Waals forces”. On the other hand, hydrogen
bond is a special type of dipole-dipole
interaction.
Johannes Diderik van Der Waals
 was born on November 23,
1837 in Leyden, The
Netherlands.
 In his 1873 thesis, van der
Waals noted the non-ideality
of real gases and attributed it
to the existence
of intermolecular interactions.
Dipole-dipole forces
Dipole-dipole forces are
attractive forces existing
between polar molecules
(molecules that exhibit dipole
moment), such as HCl.
Because there is
unequal sharing of
electron between H and
Cl atoms, partial positive
and partial negative
poles exist and are
referred to as a dipole.
The more electronegative atom becomes
partially negative and has most electron
density, whereas the least electronegative
becomes partially positive and has a lesser
electron density.
Because poles of
opposite charges attract,
the partial negative end
(i.e.,Cl atom) will be
attracted to the partial
positive end (i.e., H atom)
of another HCl molecule.
This force is weaker than
ionic and hydrogen
bonds.
Sum Up!
All polar molecules have a partial negative
end and partial positive end which are
attracted to each other.
The resulting forces are called dipole-dipole
forces. Dipole-dipole forces are much weaker
than ionic or covalent bond.
Ion-Dipole Forces
Exist in the attraction between a charged particle
called ion (which can be a positively charged
cation or a negatively charged anion) and a polar.
This exhibited in hydration, or the interaction of
water molecules with the cations and anions of
ionic compounds dissolved in water.
When NaCl is dissolved in water (H2O), NaCl will dissociate
into sodium cations (Na+) and chlorine anions (Cl-).
Because water is a polar molecule and has a partial negative
and partial positive poles, the Na+ will form an ion-dipole
interaction with the partial negative part of water molecule
(O), while Cl- will form an ion-dipole interaction with the
partial positive part of the water molecule (H+).
Sum Up!
The ion force exists in the attraction between an ion and
a dipole molecule.
An ion is a charged atom because it has gained or lost
one or more electrons. It can be either positively charged
cation or negatively charged anion.
The partial charges at the ends of the dipole molecules
make an ion-dipole interaction.
Dispersion Forces or London Forces
Are the weakest attractive force that are
formed due to the temporary dipoles induced
in non-polar molecules.
This force is also called induced-dipoleinduced-dipole attraction.
Take for example, if there is an ion or a polar
molecule near an atom or non polar
molecule, there will be a distortion of the
nonpolar molecule due to the force exerted
by the ion or the polar molecule.
This distortion will result in temporary dipoles
in the nonpolar molecule which is called
induced dipole.
Induced dipole is the separation of the positive
and negative charges in a nonpolar molecule
due to its nearness of an ion or polar molecule.
On the other hand, when the induced dipole
is due to the the interaction between an ion
and non-polar molecule, the interaction is
called ion-induced dipole interaction.
When the induced dipole is due to the
interaction between a polar and nonpolar
molecule, the interaction is called dipoleinduced dipole interaction.
Sum Up!
The London dispersion force is the weakest
intermolecular force because it has only a temporary
attractive force.
Dispersion force exist when the electrons in two
adjacent atoms attract and induce temporary dipoles.
This forces is sometimes called induced-dipoleinduced-dipole attraction.
Hydrogen Bond
A special type of dipole-dipole interaction
between the hydrogen atom in a polar
molecule and the highly electronegative
atoms, N, O,and F in another molecule.
The water molecule
is a very good
example of a
substance that
exhibits hydrogen
bonding.
Hydrogen Bonding in H2O
The electronegative atoms, like OXYGEN,
cause the bond to be polar.
These atoms become slightly negatively
charged and the hydrogen atom becomes
slightly positively charged. Thus, a dipole
exists.
Sum Up!
The hydrogen bond is an attraction
between a partial positive hydrogen
in a polar molecule and a slightly
negative atom in another molecule.