4. What type of forces bind components of matter to each other ?

Binding Forces

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.
I. Forces within molecules
Ionic Bonds Covalent Bonds Metallic Bonds
Are bonds formed between two Are bonds formed between two Are bonds formed due to sharing of
oppositely charged ions due to atoms (same or different) due to free valence electrons between a group
complete transfer of valence sharing of electrons. of positively charged metal ions.
electrons from one atom to another. Example: H 2, CCl4 Example: Gold, silver, copper
Example: NaCl

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II. Forces between molecules

An intermolecular force (or secondary force) is the force that mediates interaction between
molecules. These forces must exist between molecules so that such molecules can form aggregates in gases,
liquids, and solids.

The intermolecular forces may involve weak attractive forces due to the electron distribution in the
molecules
molecules or involve specific intermolecular bonds – such as hydrogen bonds.

The common types of attractive intermolecular forces are categorized into the following classes:
Keesom forces
Van der Waals forces Debye forces
London forces
Hydrogen bonding
Note: Before discussing
Ion–dipole forces intermolecular forces, you
need to understand the
Ion–induced dipole forces concept of a dipole.
(see supplementary material at slides
at the end of lecture if required )
 Intermolecular Forces

Van der Waals Forces are relatively weak attractive forces that arise because
of fluctuation in the electron density of all molecules and these cause small
dipoles within the molecules. The electric polarization will attract one molecule
to another.

When two atoms come within 5 nanometers of each other, there will be a
transient shift in electron density to one side of the nucleus, thus causing
polarity and a slight attraction between the atoms.
I. Dipole–Dipole Forces (Keesom forces )
• It occur between two polar molecules (have asymmetric electron distribution and are permanent dipoles)

• Dipolar molecules frequently tend to align themselves with their neighbors to lower the energy of the
assembled molecules so that the negative pole of one molecule (- ) points toward the positive pole (+ ) of
the next. Thus, a large groups of molecules may be associated through weak attractions.

Energy of attraction: 1-7 Kcal/mole

Examples
I. Dipole–Dipole Forces (Cont.)
The alignment could be end-to-end, resulting in a stronger
interaction than a side-by-side alignment.

End-to end alignment

Side-by-alignment
Attractive Forces
(Dipole-Dipole interactions)
 II. Dipole–Induced dipole Forces (Debye forces)
Permanent dipoles (polar molecules) are capable of inducing a temporary dipole in non-polar molecules, and
thus produce a net attractive force between the two particles.
This effect explains the ability of neutral molecules such as hydrocarbons, and even some of the noble gas
elements to form stable hydrate compounds with water.

This attraction is usually rather weak (about half of dipole-dipole forces), but in a few cases it can lead to the
formation of loosely-bound compounds.
Energy of attraction: 1-3 Kcal/mole
 III. Induced Dipole–Induced Dipole Forces (London forces, or dispersion force)
London forces are “the only kind” of intermolecular interaction forces seen in non-polar molecules, and
is the weakest of all intermolecular forces.
Energy of attraction: 0.5-1 Kcal/mole
Non polar molecules can induce polarity in one another.
This is because there is no guarantee that the distribution of negative
charge around the center of an atom will be perfectly symmetrical at every
instant.
On a very short time scale, every atom therefore has a weak, fluctuating
dipole moment that is continually disappearing and reappearing in another
direction.
Although these extremely short-lived fluctuations quickly average out to zero,
they can still induce new dipoles in a neighboring atom or molecule, which
helps keep up the original dipole and gives rise to a the weak London force.
Intermolecular Forces
Hydrogen Bonding
Hydrogen bonding is a specific type of dipole-dipole interaction which occurs when a hydrogen
atom bonded to an electronegative atom (Z) such as (O, N, or F) is attracted to a lone pair of electrons
on an atom in another molecule.

Hydrogen bonding is not a covalent bond within the molecule. They are shown with dotted lines
connecting a lone pair with the hydrogen.

Hydrogen bonding is very common in nature starting from water and going to complex biological systems
like DNA and proteins.
The hydrogen bond is stronger than Van der Waals interaction, but weaker than covalent or ionic bonds.

Energy of attraction: 2-8 Kcal/mole

Hydrogen Bonding
Examples on Hydrogen Bonding
Simple hydrogen bonding between two diatomic molecules
+ H-F - --- + H-F -
As in hydrogen fluoride in the vapor state which exist as a
hydrogen bonded polymer (F-H….)n where can be as large as 6.

Hydrogen bonding between carbonyl and amide group

Hydrogen bonds can also exist between alcohol molecules, carboxylic acids, aldehydes, esters, and
polypeptides.
Hydrogen Bonding in water
Hydrogen bond exists in ice and in liquid water; it accounts for many of the
unusual properties of water including its high dielectric constant, abnormally
low vapor pressure, and high boiling point.
Ice has the very unusual property that its solid state is less dense than its
liquid state. As a result, ice floats in liquid water. The expansion of water
when freezing also explains why unprotected pipes in houses break if they are
allowed to freeze.
Ice is well ordered 3-dimensional array of regular tetrahedra with oxygen in
the center and H atom at each corner (H-atoms are not exactly midway
between the oxygens)

Water as liquid
About one sixth of the hydrogen bonds of ice are
broken when ice convert to liquid state and all are
Water as solid (ice) destroyed when it vaporize
Hydrogen Bonding

Examples on Hydrogen Bonding

Hydrogen bonding between two carboxylic molecules

The hydrogen bonds of formic acid and acetic acid are

sufficiently strong to yield dimers (two molecules attached
together), which can exist even in the vapor state.

Intramolecular hydrogen bonding within one carboxylic molecule

It is noticed that intra- as well as intermolecular hydrogen bonds

may occur

(as in salicylic acid).

Intermolecular Forces

A dipole that is close to a positive or negative ion will

orient itself so that the end whose partial charge is
opposite to the ion charge will point toward the ion.

This kind of interaction is very important in aqueous

solutions of ionic substances.
Energy of attraction: 1-7 Kcal/mole

Example:
Solubility of NaCl in water.

In a solution of sodium chloride, H 2O is a highly polar

molecule, and NaCl breaks up to ions of Na+ and Cl-
The Na+ ions will be enveloped by a shell of water
molecules with their oxygen-ends pointing toward these
ions, while H 2O molecules surrounding the Cl– ions will
have their hydrogen ends directed inward.
Intermolecular Forces

An ion-induced dipole attraction is a weak attraction that results when the

approach of an ion induces a dipole in an atom or in a non-polar molecule
by disturbing the arrangement of electrons in the non polar species.

Energy of attraction: 6 Kcal/mole

KI + I2 + KI3
Bond energies serve as a measure of the strength of bonds

Beware:
The strength between
intermolecular bonds is a lot
weaker than the strength of
intramolecular
 Supplementary information:

Dipole is a body or system (in our case a molecule) having a pair of

equal and opposite electric charges separated by a small distance.
This is caused by the location of a few more electrons on one side of the nucleus
more than on the other (i.e., it is formed as a result of unbalanced distribution of
electrons in asymmetrical molecules).

In chemistry, dipole usually refers to the separation of charges within a

molecule between two covalently bonded atoms or atoms that share an
ionic bond.

Example: Hydrogen Flouride Water molecule (H2 O) is a

Flourine is more electronegative than hydrogen dipole.
Electrons are predominantly around the F atom, which The oxygen side of the molecule carries a net
creates a permanent negative charge. negative charge, while the side with the two
Also, the H is electron deficient, so it has a permanent hydrogen atoms has a net positive electrical
positive charge, which is separated by the bond length. charge.