Concept of METALLIC SOLIDS 2022
The metal atoms in a metallic crystal are held together by a special type of bond called metallic bonds.
Theories of Metallic Bonding
To explain metallic banding, the following theories have been proposed:
1. Electron gas theory 2. Valence bond theory 3. Molecular orbital theory
1. Electron Gas Theory
It was proposed by Drude and Improved by Loren (1923).
According to this theory, all the atoms of the metallic crystal lose their valence electrons. These electrons are from an electron pool or electron gas in which positively charged nuclei are present at definite positions at a measurable distance.
The electrons of the sea are not attached to a particular nucleus. These electrons are delocalized over the entire crystals, therefore, these are called Free electrons
The positively charged nuclei are held together by the free electrons throughout the lattice. The force, which binds a metal cation to a number of electrons around is called a Metallic bond.
2. Valence bond theory
It was proposed by L. Pauling.
According to this theory, metallic bonds are actually covalent bonds. However, these covalent bonds are highly delocalized and extended over the whole crystal.
3. Molecular Orbital Theory (or Band Theory)
According to this theory, valence orbitals of metals overlap with each other to form delocalized orbitals. These are called molecular orbitals. Molecular orbitals are spread over the whole crystal. The electrons in filled orbitals are localized.
Actually, when a large number of valence orbitals of atoms overlap, they produce a large number of molecular orbitals. These molecular orbitals have very close energy states and thus form a band of energy states. Hence, this theory is also called the Band theory.
The energy difference between two bands determines the properties of metallic crystals.
Properties of Metallic Solids (Explanation of properties on the basis of Electron Gas Theory)
1. Electrical Conductivity
Metals are good conductors of electricity. When a metal is connected between two poles of a battery, the mobile electrons begin to move towards the positive pole and the electrons enter into metal from the negative pole. In this way, metals conduct electricity as shown in the figure.
The electrical conductivity of metals decreases with an increase in temperature.
It is because, an increase in temperature, increases the vibrational motion of positive nuclei in metallic solids. These nuclei produce hindrance in the motion of free electrons. Hence, electrical conductivity decreases.
2. Thermal Conductivity
These are good conductors of heat.
During this free electrons take up heat from one end and transfer it throughout the crystal during their motion and collision with other electrons. Thus free electrons cause thermal conduction in metals.
3. Lustrous Surface
Metals have lustrous surfaces.
The freshly cut surface shining (lustrous surface). It is because when light strikes the free electrons on the surface, they are excited. When these excited electrons come to their original position, they emit light. Thus light appears to be reflected by the metal surface and it appears shining.
4. Malleable and Ductile
Metals are malleable and ductile.
When a force is applied on metals, their layer slip over each other and their shape is changed. Hence, these can be converted into sheets (malleable) or wires (ductile) without breaking.
STRUCTURE OF METALS
In metals, free electrons are present while positively charged nuclei act as spheres. spheres packed together to form metallic crystals.
Close packing of spheres is done in the following ways.
Consider three balls joined in one plane as shown in the figure. There is present a hole between these balls. The hole is called interstices or crevices.
If a fourth ball is placed on this hole, a tetrahedral structure is obtained as shown in the figure.
Now consider the packing of eleven spheres as shown in the figure. In this layer, holes will be developed.
Now the second layer of spheres is placed directly above the holes of the first layer.
By this arrangement, all the holes of the bottom layer are not covered by the spheres of the top layer.
Hence, two types of holes will be produced. . .
(a) Holes created between spheres of the second layer. Through this layer spheres of the bottom layer can be seen. These are marked A.
(b) Holes that are not completely covered by the top layer. Through this layer, the ground can be seen. These are marked B. Now a third layer can be placed in two ways.
(a) (Hexagonal Close-Packed Structure)
If the third layer is placed directly above the holes marked A, then the spheres of the third layer will be directly above the spheres of the first layer. This arrangement produces an ABAB or 1212 pattern.
This structure is called a hexagonal close-packed structure.
(b) (Cubic close-packed Structure)
If the third layer is placed directly above the holes marked B, then the spheres of all the three layers have a different patterns.
This arrangement produces an ABCABC or 123123 patterns.
This structure is called a face-centered cubic lattice.