For NCERT and First year B.Sc. Students studying the course of Chemistry

1. F. Y. B.Sc.
Sem-I
Paper Code : CECH-101
Unit-II (a)
Chemical Bonding

2. MO approach to metallic bond and its
Application
• The theory is known as MO theory or
Band theory.

3. MO for Metallic Bond or
Band Theory
• To understand the MO for metallic bond, the
example of Lithium is taken here.
• The electronic arrangement of Lithium atom
is 1S2 2S1 2P0 the Li2 molecule exist in the
vapour state and bonding occurs due to 2S
orbital.
• There are three empty 2P orbitals in the
valence shell. The presence of empty orbitals
reflects the metallic properties of it.

4. • Now, for Lithium, number of Atomic
Orbitals(AOs) are more then that of valence
electrons. So even if all electrons are used for
bond formation, it can not achieve stable
electronic configuration like inert gas.
• Compounds of these types are called ‘electron
deficient’.
• Then, how the empty orbitals can be utilized in
Lithium?
• Either empty AOs of metals accepts the lone
pairs from other atoms or ligands and make
coordinate bonds or make cluster compounds
by sharing of electrons with neighbor atoms.

5. • The MO of Lithium molecule have discussed
earlier. There is 1 valence electron for each
atom in Li2 molecule, that forms two MOs σ2S
and σ2S*.But only σ2S makes bond formation
possible.
• Suppose that three Li atoms joined to from Li3.
Three 2S AOs combined to form
3 MOs – one bonding, one non bonding and one
anti bonding.
• Non bonding MOs has energy between BMO
and ABMO.
• In Li3 the one BMO occupies two valence
electrons and one non bonding occupies one
valence electrons.

6. • In Li4, four 2S AOs combined 4 MOs- two
BMO and two ABMO.
• Two non bonding MOs are also there in
between two BMO and two ABMO, that
reduces the energy gap between them.
• The 4 valence electrons are occupied by 2
lower energy BMOs.
• As the number of electrons in the cluster
increases, the energy gap between the
orbitals decreases, when there are large
number of atoms, the energy levels of
orbitals are so close that they form a
continuum.

8. • Since there is only 1 valence electron per atom
of Lithium and MOs are two per molecule, there
are only half filled MOs and half are unfilled.
• Since the energy levels are so closed that it
requires only minute amount of energy to jump
the electron to unfilled MOs.
• Now in MO, the electron have high degree of
mobility, which is responsible for thermal
conductivity and electric conductivity.
• Conduction occurs because of two things:
(a)MO extend over the whole crystal.
(b) There is no energy gap between filled and
unfilled MOs.

9. • Let we take the example of Beryllium. It has 2
valence electrons which fill the 2S valence band
of MOs.
• In Beryllium atom the energy gap between 2S
and 2P is 160 kJ/mol.
• As 2S AOs form a band of MOs, 2P also forms
a band of MOs.
• The upper part of 2S band overlaps the lower
part of 2P band. Because of this overlap some
of the 2P band is occupied and some of 2S
band part is empty. There is possible and easy
for electrons to perturb to unoccupied levels in
the conduction band where they can easily
move throughout the crystal. Beryllium therefore
behaves as a metal.

11. MO approach to Conductors,
Insulators & Semiconductors
 Conductors:
• In Electric Conductors (metals), either the
valence band is partly full, or the valence
and conduction bands overlap.
• Therefore no significant gap between filled
and unfilled MOs, and perturbation can
occur readily.

12. Insulators:
• In insulators (non- metals), the valence band
is full, so perturbation within the band is
impossible. So there is appreciable
difference in energy (called the band gap)
between the valence band and the next
empty band.
• Electrons cannot therefore be promoted to
an empty level in insulators.

13. Semiconductors:
• Intrinsic semi conductors are basically
insulators, where the energy gap between
adjacent band is sufficiently small for thermal
energy to be able to promote a small number
of electrons from the full valence band to the
empty conduction band.
• The promoted electrons and unpaired
electrons in valence band can conduct
electricity.

14. • As temperature increases, conductivity of
semiconductors increases, because number
of electrons in conduction band increases as
temperature increases.
• The extrinsic semiconductors (n -type and
p- type) are produced by doping the suitable
impurity to intrinsic semiconductors.

15.  Superconductors:
• Metals are good conductors of electricity, and their
conductivity increases as the temperature is
lowered. In 1911 the Dutch scientist H.K. Onnes
discovered that metals such as Hg and Pb became
superconductors at temperatures near absolute
zero.
• A superconductor has zero or almost zero electrical
resistance.
• It can therefore carry an electric current without
losing energy, and in principle the current can flow
for ever.
• There is a critical temperature Tc at which the
resistance drops sharply and super conduction
occurs.

16. • Later, Meissner and Ochsenfeld found that
some superconducting materials will not permit
a magnetic field to penetrate in their bulk.
• This is now called the Meissner effect, and
gives rise to 'levitation'.
• Levitation occurs when objects float on air. This
can be achieved by the mutual repulsion
between a permanent magnet and a
superconductor.
• A superconductor also expels all internal
magnetic fields (arising from unpaired
electrons), so superconductors are
diamagnetic.