Metals exhibit certain characteristic properties which are as follows:
(i) High thermal and electrical conductivity.
(ii) High melting and boiling points.
(iii) High density and mechanical strength.
(iv) High ductility and malleability.
(v) Metallic lustre.
(vi ) Alloy formation.
(vii) Emission phenomenon.
(viii) Power to replace hydrogen from acids etc.
(ix) Crystalline solid.
(x) Electropositive character.
(xi) Opaque to light.
The above said properties of metal cannot be explained on the basis of normal valencies.
Hence a concept of metallic bond is given which may be defined as:
(i) The force that binds a metal ion to a number of electrons within its sphere of influence is known as metallic bonding.
(ii) It may also be defined as a bond which is formed between electropositive elements. Electropositive metal atom + Electropositive meta atom
= Metallic bond
(iii) It may also be defined as the attraction force which holds the atoms of two or more metals together in an alloy or attraction force which binds the atom of pure metal together in a crystal.
It is found that the force of attraction between metal ions and valency electrons is very strong. This strong force of attraction is responsible for a compact solid structure of metal as discussed above.
The following theories have been advanced to explain the different properties of metals:
(A) Electron-sea theory
The first idea about this bond was given by Drude (1900) who proposed metals as containing a number of free electrons moving in spaces among atoms like the molecules of the ideal kinetic theory of gases. Lorentz (1923) believed that solid metals consist of lattices of liquid cation spheres with free electrons moving in the interstices. Thus this theory is also known as Drude Lorentz theory.
According to this theory, valency electrons are weakly bound to the kernel (or core) in metals. The valency electrons are completely delocalized and are frequently exchanged between the atoms. In other words, the valency electrons do not belong to one atom in particular and they move freely from one kernel to another. Hence the atoms assume a positive charge (cation) and the valency electrons form an ‘Electron Sea or pool” which binds together the cationic centres, hence a metallic lattice is thought of as containing positive ions together by a mobile gas or sea. The force that binds a metal ion to the mobile within its sphere of influence is known as metallic bond.
This theory explains different metallic properties as:
(i) Metallic lustre: This property is am to the fact, that when light falls on the surface of metal, the electrons get excited and on coming back to ground state, the energy is emitted in the form of visible light of all wavelength. Therefore the metals have metallic lustre on of To and Fro oscillations of mobile electrons.
(ii) Electricity conductivity: Since in the absence of an external electric field the motion of free electrons is equally probable in all directions, there is no net flow of any electric current in any direction. Under the influence of an external potential difference across a metal piece the free electron start moving in one particular direction, i.e., towards positive and constituting electric current through the metal.
(iii) Thermal conductivity: When a part of a metal is heated, the electrons of that part get large amount of energy and start to move in the lattice in zig-zag motion. Thus the heat energy is transferred to all corners of the metal. On increasing temperature conductivity of metal decreases as slippery kernels start vibrating and mobile electrons are pushed away.
(iv) Malleability and Ductility: In these properties one layer of atoms slids over another without a series disturbance at any stage on applying the force. Now, since the atoms are arrange in a particular fashion. When the form is applied one layer of atoms may slide over the next and we can get fine sheet or wire of the metal.
(V) Elasticity: Elasticity can be defined as a property by virtue of which a substance can resist a deforming force or a property by virtue of which a given substance can recover its original form soon after the removal of deforming force. It is considered that elasticity of metals is due to the ease with which metal is bent move from one lattice site to another. When a wire is bent, the pattern of ions and electron is broken and a new pattern is set up in the substance giving rise to new planes and edges.
(v) Hardness, B.P., Thermal Stability: All these properties are directly proportional to strength of metallic-bond.
- Ir has maximum density.
- W has maximum m.p. among metals
- Hg is a liquid metal with lowest m.p. among metals
Limitations: Although electrons-sea theory explains many properties of the metals. But some questions of quantitative aspect are still unsolved such as why does the resistance to electrical conductance of some metals increase with rise of temperature while in some cases it decreases exponentially; why do some metals exhibit almost no conductance in the solid state and must be classified as insulators.
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