By - Dr. Vikas Baboo,
M.Sc., Ph.D (Organic Chemistry)
University of Lucknow
THEORIES OF BONDING IN METALS
Following theories are given to explain bonding in metals –
(1) The free electron theory or Electron sea model
(2) Valance bond theory or Resonance theory
(3) Molecular orbital theory or Band theory
THE FREE ELECTRON THEORY OR ELECTRON SEA MODEL
This theory is given by Drude and Lorentz (Drude-Lorentz theory). According to this theory –
All the atoms in a metal have several vacant orbitals in their outer shell. The metals have to tendency to
lose some valance shell electrons to form positive ions are called kernels. These kernels occupy closely
packed fixed positions and the valance electrons free to move throughout the volume of metals. Those
electrons are called mobile or delocalized electrons.
Thus metal can be compared to an assembly of kernels immersed in the sea of electrons. Thus
this model is referred as electron sea model.
This simultaneous force of attraction between the kernels and mobile electrons which hold the
metal atoms together is called metallic bond.
PHYSICAL PROPERTIES OF METALS AND THEIR EXPLANATION –
The following properties can be explained on the basis of electron sea model –
(1) Thermal Conductivity – On heating a part of metal, the electrons absorb energy from the source and
transfer their energy by collision with electrons present in cooler part till the temperature of all parts
kernels. Note – On heating the conductivity of metals decreases.
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(2) Electrical Conductivity – The electrical conductivity takes place due to free electrons present around the
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become same.
By - Dr. Vikas Baboo,
M.Sc., Ph.D (Organic Chemistry)
University of Lucknow
(3) Malleability and Ductility – When impact is applied on a metal surface, the kernels present in one
portion get displaced with respect to other portion. The inter kernel distance remains the same and the
crystal structure does not change but shape of metal changes.
(4) Metallic Luster – When light falls on the metal surface, the mobile electrons start vibrating. These
vibrating electrons emit electromagnetic radiations in the form of light. Thus, the appeared light emitted
from the metal surface is known as metallic luster.
(5) Elasticity – This property is due to the returning of the mobile electrons in their original position when
the stress applied is removed. A small kink is sometimes left because kernels do not exactly come in
their original position.
(6) Hardness – Due to strength of metallic bond, all the metals are very hard. The strength of metallic bond
is directly proportional to the number of valance electrons. Alkali metals are soft because of one valance
electron and large kernel size.
VALANCE BOND THEORY OR RSONANCE THEORY
According to Pauling, the metallic bond has partially covalent character and involving in
resonance.
Example – The crystal lattice of Li atom revels that Li atom is surrounded by 8 Li atoms and 6 next Li
atoms. The resonance in Li-Li bond is similar to the resonance in benzene. Li-Li resonance is shown as –
Although, this theory able to explain some of the characteristics of metals. This theory does not explain
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electrical conductivity, thermal conductivity and metallic luster etc. properties of metals.
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By - Dr. Vikas Baboo,
M.Sc., Ph.D (Organic Chemistry)
University of Lucknow
MOLECULAR ORBITAL THEORY OR BAND THEORY
According to Molecular Orbital Theory, in a metal the outer atomic orbitals of large number of
metal atoms overlap to form large number of molecular orbitals, which are delocalized over the metal
atoms and crowded together to form bands.
Example – Sodium metal has the electronic configuration –
Na (11) : 1s2 2s2 2p6 3s1
When two sodium atoms approaches each other, their 3s orbitals overlap to form molecular orbitals i.e.
one bonding and another antibonding. When three orbitals overlap, they form one bonding, one
antibonding and one non-bonding and so on. These MOs merge into one another to form the bands.
The bands containing the outer shell electrons are called valance bands and the vacant or
partially filled bands are called conduction bands.
In metals, conduction band is close or overlap to valance band. Therefore, they are good
conductors. e.g. Iron etc.
In non-metals, the energy gap between valance band and conduction band is very large. Hence
they are non conductors. e.g. Carbon etc.
In semi conductors, the energy gap in between those of metals and non-metals. At room
temperature, these are not good conductors but at high temperature they are good conductors. e.g.
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Germanium etc.
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By - Dr. Vikas Baboo,
M.Sc., Ph.D (Organic Chemistry)
University of Lucknow
Semiconductors are also prepared by doping an insulator with a suitable impurity. Such types of
insulators are called extrinsic semiconductors. These are two types –
(a) n – type semiconductors- When the impurities of group 15th added to semiconductors. e.g. – P,
As
(b) p – type semiconductors - When the impurities of group 13th added to semiconductors. e.g. B,
Al
The impurity lies between valance band and conduction band in insulators, behave as a bridge for
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electrons.
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By - Dr. Vikas Baboo,
M.Sc., Ph.D (Organic Chemistry)
University of Lucknow
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