Metallic bond
Definition: The force of attraction that binds a large number of metal atoms in a
metallic crystal is called the metallic bond.
Band Theory (Molecular orbital Theory):
According to MOT, two atomic orbitals combine to form 2 molecular orbitals.
Hence ‘n’ atomic orbitals overlap to give ‘n’ molecular orbitals. In metals, there is
a large number of closely packed atoms. The overlapping of all these atomic
orbitals produces several molecular orbitals. The energies of these MO’ s are so
close that they form a continuous energy band; hence the name Band Theory.
Example: Lithium metal (1s2 2s1)
When ‘n’ atoms of Li combine to form a metal crystal, the
following 3 bands are formed.
1s band: This is formed by ‘n’ number of
vacant 2p
band
1s atomic orbitals contain 2xn electrons. This
band is completely filled.
.
2s band: This band is formed by n number of 2s
orbitals,
2p orbitals
half filled 2s
band
2s orbitals
contains n electrons.This band is half filled.
2p band: This band is formed by 3x n atomic orbitals.
This band is vacant in lithium.
The 2s and 2p bands have nearly same energy, hence these
overlap each other. The overlapping portion is called
overlapping zone
completely
filled 1s
band
The energy bands formed by molecular orbitals can be distinguished into 3
types.
Prof. Agnes S. D’Souza, St. Philomena’s College, Mysore
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1. Conduction band 2.valence band 3.forbidden band or energy gap
1.Conduction band: The lowest unoccupied or partially filled energy band is called
conduction band.
2.Valence band: The highest occupied energy band (completely filled) is called valence
band. It is the non-conduction band.
3.Forbidden zone or forbidden gap: (Eg): The conduction band and valence band are
separated by an energy gap, which prevents the promotion of electrons from lower to a higher
band. This gap is called forbidden gap or forbidden zone.(Eg)
Electrical properties of metals insulators and semiconductor.
The electrical properties of metals, insulators and semiconductors depend on
the number of electrons present in the conduction band and the width of the
energy gap.
Vacant
Conduction band
Partially filled
conduction band
c

Conduction band
large energy gap
Conduction band
co
c
small Eg
Valence band
Valance band
Valence band
Valence band
Metals.
Insulators
semiconductors
Energy Bands
Metals: In metals (conductors) valence band is completely filled and the conduction band is
partially filled. Small energy is sufficient to promote the electrons to the vacant levels within
the same band. This movement of electrons in the conduction band gives rise to conduction. (In
alkaline earth metals the valence band and conduction band actually overlap resulting in a
partially filled upper –band)
The electrical conductivity of a metal decreases with increase in temperature.
This is because on increasing the temperature, thermal energy of atoms increases. Hence the
vibrational motion of atoms (kernels) increases, which hinders the free movement of electrons.
Prof. Agnes S. D’Souza, St. Philomena’s College, Mysore
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Insulators: Insulators have very low conductance or zero conductance at room temperature.
E.g. rubber, diamonds, wool etc.
In the insulators, the valence band is completely filled and conduction band is vacant. The V.B
and C.B are separated by large energy gap (Eg >6ev). No electron from valence band can cross
over to conduction band at room temperature, even if the electric field is applied. High energy
is required to promote the electron from V.B to C.B. crossing the large energy gap. This energy
is not available at room temperature. Hence they are non-conductors at room temperature.
Semiconductors: Semiconductors are materials, whose conductance lie in between that of
good conductors and insulators. Eg. Silicon, germanium, Gallium arsenide etc.
Semiconductors have filled valence band and empty conduction band, separated by a narrow
energy gap. The Eg values lie in the range of 2-3eV. Hence electrons can be promoted from
the V.B to C.B by the absorption of thermal energy at room temperature. As the temperature
increases more and more electrons are excited from V.B to C.B. Hence the conductance
increases with increase in temperature.
Semiconductors are of two types. 1. Intrinsic semiconductors
2. Extrinsic semiconductors
1. Intrinsic semiconductors: Semiconductors in their extremely pure form are called
intrinsic semiconductors. Eg. Si, Ge
2. Extrinsic semiconductors: Extrinsic semi conductors are the materials whose semiconducting
properties are due to the addition of an extremely small amount of impurity atoms. The
impurity atoms are called dopants. The impurity atoms present is usually one atom per 109
atoms of the host. These are of two types.
(a) n-type extrinsic semiconductors: These are obtained by adding ( doping ) a small number of
pentavalent atoms like P, AS, Sb etc. to pure elements like Si or Ge. Each Si atom forms 4
covalent bonds with 4 neighbouring Si atoms using its four valence electrons. Its 5th electron is
left unused. Thus there is an extra electron at each Si atom. These loosely bound extra
electrons can be excited easily from the balance band to the conduction band when an electric
field or thermal energy is applied. Such semi conductors ‘doped’ with pentavalent impurity
atom are called n-type semiconductors because negatively charged electrons are responsible
for conduction.
Prof. Agnes S. D’Souza, St. Philomena’s College, Mysore
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vacant 2p
band
2p orbitals
half filled 2s
band
2s orbitals
completely
filled 1s
band
Prof. Agnes S. D’Souza, St. Philomena’s College, Mysore
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The excess loosely bound electrons occupy delocalized level called donor level just below the
conduction band of Ge crystal. Electrons can be easily excited from donor band to conduction
band. As the number of electrons in the conduction band increases, the conductance
also increases.
Note: Fermi level is defined as an energy level, below which all the energy states
are filled
Empty C. band
—
e
_
e
_
e
_
e
Empty
C. Band
conduction band
_
e- e
-------------------------------------------------
Donor impurity level
---------------------------------------
fermilevel
Eg
Eg
ENERGY
Filled V. band
Energy level: n-type semiconductor
O OO O O O O
Filled V.Band
bbbbvvvvvalenm
p-type semiconductor
V>bv.vaadsvavvvb
(b) p-type extrinsic semiconductors: These are obtained
by adding trivalent
bbbbababand
atoms like B, Al, Ga, In to the parent Si or Ge. Each boron atom added forms 3
covalent bonds with 3 silicon atoms, while it is bonded to the fourth Si atom by
one – electron bond. This electron deficiency creates a positive hole or vacancy at
the site where the electron is missing. These holes occupy an energy level called
acceptor level close to the filled valence band of silicon. The hole formed may be
filled up by an electron from neighbouring atoms, thereby creating a positive hole
there. In this way, the hole moves through the crystal. The movement of holes is
thus regarded as the movement of positive electric charge. The holes move in a
direction opposite to the motion of electrons. Since the carriers are positively
charged holes, this type of semiconductors is called p-type semiconductors.
Prof. Agnes S. D’Souza, St. Philomena’s College, Mysore
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Superconductors: Super conductors are the materials which offer no resistance to the flow of
electric current through them. Super conductivity was discovered by Kammerlingh Onnes in
1913. Most of the metals become superconducting at 4K.
The critical temperature of the superconductors is the maximum temperature below which a
material exhibits superconductivity. Eg. YBa2Cu3O9 (93K)
Tl2Ba2Ca2Cu3O10 (125K)
Uses 1. used in microwave communications as filters and antennas 2. In the levitation
transportation 3. In medicine, to detect weak magnetic signals from heart and brain.
Hydrogen Bonding
Definition: The electrostatic force of attraction between H-atom and an
electronegative atom present in a molecule of the same substance, or present in
a molecule of different substance or present within the same molecule, is known
as a hydrogen bond.
𝛿+
𝛿_
𝛿+
𝛿__
𝛿+
𝛿__
1. H – F…….H - F……..H –F
Hydrogen bonding is represented by dotted line. The strength of hydrogen bond is about 1040kJ /mol while that of covalent bond is 400kJ/mol. Therefore hydrogen bond is much weaker
than covalent bond.
The condition for the formation of a hydrogen bond:
1. Presence of highly electronegative atom: The molecule having hydrogen bonding should
have a highly electronegative atom like N, O or F directly linked to the hydrogen atom.
2. Presence of atom with small size: The highly electronegative atom should be of small size.
Prof. Agnes S. D’Souza, St. Philomena’s College, Mysore
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Types of hydrogen bonding:
1. Inter-molecular hydrogen bonding: The electro static attraction between the hydrogen atom
of a molecule and the electronegative atom of another molecule of the same substance is
called inter-molecular hydrogen bonding. Water, hydrogen fluoride and ammonia show this
type of bonding.
Example:
𝛿+
𝛿_
𝛿 + 𝛿__
1.
H – F…….H - F……..H –F
𝛿+
𝛿__
2) Intramolecular hydrogen bonding: The electrostatic force of attraction between the
hydrogen atom and electronegative atom of the same molecule is called intramolecular
hydrogen bonding.
Example: o-nitrophenol, o-chlorophenol,
salicylaldehyde.H
Note: p-nitro phenol and p-chloro phenol do not show
any intra molecular hydrogen bonding due to the
distance between the 2 groups. On the other hand, they
show usual inter molecular hydrogen bonding as shown
below.
Give reason: Ortho derivatives are more volatile than
the para derivatives. Or the boiling point of para
derivative is higher than ortho derivatives.
Answer: As a result of intermolecular hydrogen bonding para molecules undergo
association which results in an increase in the boiling point. In ortho, on account of intra
molecular hydrogen bonding, no such association is possible.
Consequences of hydrogen bonding :
1. High melting and boiling point: The compounds having hydrogen bonding show abnormally
high melting and boiling points.
2.The physical state of H2O and H2S: Hydrogen bonding is responsible for the association of
molecules in water. hence it exists as a liquid. Since sulphur atom is less electronegative than
oxygen atom t her is no hydrogen bonding in hydrogen sulphide; it exists as a gas.
Prof. Agnes S. D’Souza, St. Philomena’s College, Mysore
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2. Association: The molecules of carboxylic acids exist as dimers because of
the hydrogen bonding. The molecular masses of such compounds are found
to be double than those calculated from their simple formulae. For example, the
molecular mass of acetic acid is found to be 120.
3 Solubility: The solubility of organic compounds in water is
due to the formation of H-bonds between H2O molecules
and organic compounds. For example, lower alcohols are
soluble in water because of the hydrogen bonding between
water and alcohol molecules as shown below,
4. Crystal structure: As a result of hydrogen bonding, chain structure, sheet structure and threedimensional net work are formed in the crystals structures.
5. Water shows some unique properties because of hydrogen bonding.
Anomalous properties of water:
a. High boiling point of water. Water has a high boiling point.This is because a large number of
water molecules are associated through hydrogen bonding. This gives water (H2O)n the
polymerized clusters. In order to break these H-bonds, high energy is required. So the boiling
point of water is high.
b. Water is a gas where as hydrogen sulphide is a gas: Though both the water and H2S are the
hydrides of the elements of the same group, water is liquid. This is because sulphur atom does
not possess the desired electronegativity to indulge in hydrogen bonding. H2S remain
unassociated
c. The solubility of certain covalent compounds in water: H-bonding accounts for the
solubility of certain covalent compounds in water.E.g., lower alcohols are soluble in water
because these molecules form H- bonding in water.
d.The density of ice is less than water: As ice is solid, the molecules are rigidly held in space.
Each oxygen atom is tetrahedrally surrounded by four hydrogen atoms, two by covalent bonds
and other 2 by H-bonds. The H-bonds are longer (weaker) bonds than covalent bonds. This
arrangement gives rise to open-cage-structure with a lot of holes or open space between the
atoms. These vacant spaces are responsible for its lesser density.
e. Water has a maximum density at 40C: As water is heated from
00C, hydrogen bonds continue to break and the molecules come
closer and closer. This leads to contraction and thereby increases its
density. Up to 40C, this contraction will be more than the expansion
due to heating effect.
Prof. Agnes S. D’Souza, St. Philomena’s College, Mysore
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Prof. Agnes S. D’Souza, St. Philomena’s College, Mysore
Page 9