Properties of Solids:
In a solid, the constituent particles (atoms, ions, molecules) are in a definite geometrical arrangement. Thus, there is a close-relationship between the properties of a solid and its composition structure. Solids mainly show electrical, ma and dielectric properties. These are discussed below:
Electrical properties: –
Solids can he classified into following three types on the basis of their electrical conductivity.
(i) Conductors
(ii) Insulators
(iii) Semi-conductors.
Conductors: –
The solids which allow the electric current to pass through them are called conductors. The property of conductivity throws light on the internal structure and bonding in solids. There are two types of conductors.
Metallic conductors:
Metallic conductors are those which allow the electric current to pass through them without undergoing any chemical change in them. The conductance is them in due to the movement of electrons under the influence of an applied electric potential. Their conductivity is very high and is of the order of 106 to 108 ohm–cm–. For example, silver, copper, aluminium etc.
Electrolytic conductors:
Electrolytic conductors are those which allow the electric current to pass through them by undergoing chemical changes in them. The conductance in them is due to the movement of ions or other charged particles under the influence of an applied electric potential. Electrolytes (ionic solids) NaCl, KNO3 etc. do not conduct electricity in their solid state because of strong electrostatic force of attraction between their ions. These conduct electricity only in their molten state or in the form of their aqueous solutions. In these states, the ions of the electrolyte become free and move towards opposite electrodes under the influence of an applied electric potential.
Ionic solids which possess some defects also conduct electricity to a small extent because of the migration of ions into the vacancies or interstitial site present in such solids.
Semi-conductors:
The solids whose conductivity lies between those of typical metallic conductors and insulators are called semiconductors. The semi-conductors have conductivity in the range of 102 to10-9 ohm-1 cm-1. The conductivity of semi-conductors is due to the presence of impurities and defects.
Insulators:
The solids which do not allow the passage of electric current through them are called insulators. For example, wood, sulphur, phosphorus, rubber, etc.
Types of Semi-Conductors
n-Type Semi-conductors (n-stands for negative):
n-Type semiconductors are obtained due to metal excess defect or by adding trace amounts of group 15 elements (P, As) to extremely pure silicon or germanium by doping. When an element of group 15 (say As) is added to germanium (group 14) crystal, some of the atoms of germanium are replaced by arsenic. In such cases, four electrons of impurity element are used in forming bonds to Ge while the fifth electron remains unused. This extra electron can serve to conduct electricity as in case of metals. Consequently, germanium containing impurities of arsenic i.e. arsenic-doped germanium begins to exhibit fairly high electrical conductivity. Silicon doped with a group 15 element (P, As, Sb, Bi) is called an n-type semi -conductor, ‘n’ standing for negative since electrons are used for conduction of electricity.
p-Type Semi-conductor (p stands for positive):
p-Type semi-conductors are obtained due to metal deficiency defect or by doping with impurity atoms containing less electron (i.e. atoms of group 13) than the parent insulator to the lattice of an insulator. On adding a group 13 element such as B, Ga or In to germanium, some germanium atoms are replaced by impurity atoms. For example, on adding indium to germanium, indium atoms are not able to fully satisfy the tetravalency of Ge atom because they have one electron less than that in case of Ge. Hence some of the sites normally occupied by electrons will be left unoccupied. This gives rise to electron vacancies, commonly known as positive holes, because the net charge at these sites is positive. On the application of electric field, electrons from adjacent sites move into the positive holes resulting in the formation of new positive holes. The migration of positive holes thus continues and current is carried throughout the crystal. Thus doping of germanium crystal with indium (Ge with B) increases the conductivity of germanium. Since the current in this case is carried due to the migration of positive holes, this type of conduction is called p-type semi-conduction
Magnetic Properties of Solids:
The magnetic properties of solids are also related to the electronic structures. Materials can be divided into the following types depending upon their response to magnetic field:
Diamagnetic materials:
The substances which are weakly repelled by the magnetic field are known as diamagnetic substances. For example, NaCl, benzene, etc. are diamagnetic substances. Diamagnetism arises when all the electrons are paired.
Paramagnetic materials:
The substances which have permanent magnetic dipoles and are attracted by the magnetic field are known as paramagnetic substances. These consist of atoms, ions or molecules having one or more unpaired electrons. The common examples are TiO, Ti2O3, VO2, CuO, etc. They lose their magnetism in the absence of magnetic field.
Ferromagnetic substances:
The substances which are strongly attracted by the magnetic field and show permanent magnetism even when the magnetic field is removed are known as ferromagnetic substances. Once such a material is magnetised, it remains magnetised permanently. Iron is the most common example. Other examples are cobalt, nickel, CrO2, etc. at room temperature. These substances are very important in technology. For example, CrO2 is used to make magnetic tapes for use in cassette recorders.
The ferromagnetism arises due to spontaneous alignment of magnetic moments due to the presence of unpaired electrons. Depending upon the alignment of magnetic moments in ferromagnetic substances are divided into three types:
(a) When there is spontaneous alignment of magnetic moments in the same direction, we get ferromagnet
(b) If the alignment of magnetic moments is in a compensatory way so as to give zero net magnetic moment because of cancellation of the individual magnetic moments then we get anti-ferromagnetism in the material. The common example is MnO
(c) When the magnetic moments are aligned in parallel and anti-parallel directions in unequal numbers resulting in net magnetic moment, we get ferri-magnetism. For example, Fe3O4 and ferrites of formula M2+Fe2O4; M = Mg, Cu, Zn, etc. show fern-magnetism.