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What kind of forces hold the atoms together in a solid?
We can classify solids according to the
nature of bonding and energies of
Interactions Between Atoms:
 Ionic bonding
 NaCl
 Covalent bonding
 Comparison of ionic and covalent bonding
 Metallic bonding
 Van der waals bonding
 Hydrogen bonding
1

General Considerations
 There must be an attractive force
 An apparent candidate is the Coulomb Force
q q
1 2
2
1
0 4
r
F

 
 Here r is a distance between atoms (ions) forming a solid
 What stops atoms (ions) from getting closer than they
do?
 When ions are very close to each other, other forces arise.
These are the so-called short-range repulsive forces, due
to rearrangement of electrons as nuclei approach
 Equilibrium distance, r0, is point at which energy is at
a minimum, forces are balanced
2

Energies of Interactions Between Atoms
 The energy of the crystal is lower than that of
the free atoms by an amount equal to the
energy required to pull the crystal apart into a
set of free atoms. This is called the binding
(cohesive) energy of the crystal.
 NaCl is more stable than a collection of free Na
and Cl.
 Ge crystal is more stable than a collection of free
Ge.
Cl Na NaCl
4

Types of Bonding Mechanisms
It is conventional to classify the bonds between
atoms into different types as
 Ionic
 Covalent
 Metallic
 Van der Waals
 Hydrogen
All bonding is a consequence of the electrostatic
interaction between the nuclei and electrons.
5

IONIC BONDING
 Ionic bonding is the electrostatic force of
attraction between positively and negatively
charged ions (between non-metals and metals).
 All ionic compounds are crystalline solids at room
temperature.
 NaCl is a typical example of ionic bonding.
6

Metallic elements have only up to the valence electrons
in their outer shell.
When losing their electrons they become positive ions.
Electronegative elements tend to acquire additional
electrons to become negative ions or anions.
Na Cl
7

 When the Na+ and Cl- ions approach each other
closely enough so that the orbits of the electron
in the ions begin to overlap with each other,
then the electron begins to repel each other by
virtue of the repulsive electrostatic coulomb
force. Of course the closer together the ions
are, the greater the repulsive force.
 Pauli exclusion principle has an important role
in repulsive force. To prevent a violation of the
exclusion principle, the potential energy of the
system increases very rapidly.
8

COVALENT BONDING
 Covalent bonding takes place between atoms with
small differences in electronegativity which are close
to each other in the periodic table (between non-metals
and non-metals).
 The covalent bonding is formed when the atoms share
the outer shell electrons (i.e., s and p electrons).
 Noble gas electron configuration can be attained.
9

 Each electron in a shared pair is attracted to both
nuclei involved in the bond. The approach,
electron overlap, and attraction can be visualized
as shown in the following figure representing the
nuclei and electrons in a hydrogen molecule.
e
e
10

Comparison of Ionic and
Covalent Bonding
11

METALLIC BONDING
 Metallic bonding is found in
metal elements. This is the
electrostatic force of attraction
between positively charged
ions and delocalized outer
electrons.
 The metallic bond is weaker
than the ionic and the covalent
bonds.
 A metal may be described as a
low-density cloud of free
electrons.
 Therefore, metals have high
electrical and thermal
conductivity.
+
+
+
+
+
+
+
+
+
12

VAN DER WAALS BONDING
 These are weak bonds with a typical strength of
0.2 eV/atom.
 Van Der Waals bonds occur between neutral
atoms and molecules.
 Weak forces of attraction result from the natural
fluctuations in the electron density of all molecules
that cause small temporary dipoles to appear
within the molecules.
 It is these temporary dipoles that attract one
molecule to another. They are called van der
Waals' forces.
13

 The shape of a molecule influences its ability to form
temporary dipoles. Long thin molecules can pack closer to
each other than molecules that are more spherical.
 The bigger the 'surface area' of a molecule, the greater the
van der Waal's forces will be and the higher the melting
and boiling points of the compound will be.
 Van der Waal's forces are of the order of 1% of the
strength of a covalent bond.
Homonuclear molecules,
such as iodine, develop
temporary dipoles due to
natural fluctuations of electron
density within the molecule
Heteronuclear molecules,
such as H-Cl have permanent
dipoles that attract the opposite
pole in other molecules.
14

 These forces are due to the electrostatic
attraction between the nucleus of one atom and
the electrons of the other.
 Van der waals interaction occurs generally between
atoms which have noble gas configuration.
van der waals
bonding
15

HYDROGEN BONDING
 A hydrogen atom, having one electron, can be
covalently bonded to only one atom. However, the
hydrogen atom can involve itself in an additional
electrostatic bond with a second atom of highly
electronegative character such as fluorine or oxygen.
This second bond permits a hydrogen bond between
two atoms or strucures.
 The strength of hydrogen bonding varies from 0.1 to
0.5 ev/atom.
 Hydrogen bonds connect water
molecules in ordinary ice.
Hydrogen bonding is also very
important in proteins and
nucleic acids and therefore in
life processes.
16

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Phys 4710 lec 2

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