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Solid State

Matter can be classified into different categories depending upon its physical and chemical properties. Matter is defined as something that occupies space, possesses mass, offers resistance and can be felt by our senses, for example, water, metals, plants, animals, etc.

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What is a Solid State?

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Matter exists in three physical states - Solid, Liquid and Gas. The existence of any state depends upon two main forces,
  1. Intermolecular forces : The force which binds the constituent particles and tries to keep them close together.
  2. Thermal energy : This is the energy which tries to keep the particles apart and makes their movement fast.
At low temperature, the thermal energy is low and intermolecular forces are strong, so the particles occupy fixed positions and can oscillate about their mean position. The compound exists in solid state.

A solid is defined as that form of matter which possesses rigidity and hence possesses a definite shape and a definite volume.

General Characteristic of Solids

As mentioned above, a solid has two main properties:
They have strong intermolecular forces and short inter nuclear distance due to close packing of constituent particles. Their constituent particles don’t possess translator motion but can oscillate only about their mean position.

Due to these two basic properties, the solid possesses the following characteristic properties.
  • They have a particular shape, mass and volume
  • They are rigid and incompressible
  • They have high density

Classification of Matter

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Matter can exist in three physical states which are solid, liquid and gas.

Liquid Gases
They have definite shape and volume. They have definite volume but no definite shape. Neither have definite shape nor volume.
Have many surfaces.
Have one surface. No surface.
Have a melting point above room temperature. Have melting point below room temperature. The boiling point is much below room temperature.
For example-sugar iron, wood etc For example-oil, mercury, water etc. For example-Hydrogen, oxygen, air etc.

The three states are inter convertible by changing the conditions of temperature and pressure as shown below-

Interchange In Three States Of Matter

Properties of Solids

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There are three main properties of solids which depend on their structure. The three main properties are:
  1. Electrical property
  2. Magnetic property
  3. Dielectric property

Electrical properties

Solids show a wide range of electrical conductivities from 10-20 to 107 ohm-1 m-1. On the basis of electrical conductivity the solid can be broadly classified into three types:

Metals (conductors)

The solids which have conductivities in order of 104 to 107 ohm-1 m-1. Metals are good conductors of electricity.


Solids which have very low conductivity in the range 10-20 to 10-10 ohm-1 m-1. For example wood, rubber, sulfur, phosphorus etc.


Their conductivity is in between conductor and insulator up to the order of 10-6 to 104 ohm-1 m-1.

Electrical Conductivity

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Solids can conduct electrical charge due to the motion of electrons and the positive holes (electronic conductivity) or because of the motion of ions (ionic conductivity). The reason for electrical conductivity of metals is the motion of electrons and it increases by increasing the number of participating electrons in the process of conduction.

Pure ionic solids, in which conduction occurs only through the ionic motion, are termed as insulators. The defects in crystal structure increases the conductance property of semiconductor and more so with insulator. The electrical conductivity of metals, insulators and semiconductors can be explained in terms of Band Theory.

Band theory

This is based on molecular orbital theory. The molecular orbitals are formed by overlapping of atomic orbitals and the number of molecular orbitals formed are equal to the number of atomic orbitals which take part in overlapping.

In the case of metals, the atomic orbitals are very close in energy so they form a large number of molecular orbitals which are very close in energy. This set of molecular orbitals is called band which is of two types.
  • Valence band :This is a band of lower energy
  • Conduction band :The band of higher energy

The energy difference separating these two bands is called band gap or energy gap. These energy bands are separated by space where no energy is allowed in and are termed as forbidden bands. The top of available electron energy level at low temperature is called Fermi level.
  • If the valence band is partially filled or it overlaps with higher energy or have unoccupied conduction band then the electrons can be excited from lower to higher energy level by supplying a very small amount of energy or applied electric field. Hence the metal conducts electricity and behaves as a conductor.
  • If the gap between the filled valence and unfilled conduction band is large and it's not possible for electrons to jump from the valence to conduction band, then the substance has extremely low conductivity and behaves as an insulator.
  • If the gap between the valence and conduction band is small and some electrons can jump from valence to conduction band, then the substance shows some amount of conductivity and behaves as a semiconductor.
Effect of temperature on conductivity
  1. In the case of metals, the conductivity decreases with increase in temperature because the positive ions of metals start vibrating and produce hindrance in the flow of electrons.
  2. There is no effect of temperature on the conductivity of an insulator.
  3. In the case of a semiconductor, it increases by increasing the temperature as more electrons can jump from valance to conduction band.

Magnetic Property

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The magnetic properties are explained in terms of magnetic moments which arise due to the spinning motion and the orbital motion of electrons. As the electron is a charged particle, its orbital motion generates the magnetic moment of the magnetic field along the rotational axis and spinning motion produces along the spin axis respectively.

Thus, the magnetic moment of any electron is mainly because of orbital motion and the axis spinning around the nucleus.
The magnetic moment is a vector quantity and net magnetic moment of any electron could be represented by an arrow.

magnetic moment of electrons

Any substance can contain a number of magnetic dipoles. So, the substances, which are divided into different categories based on their nature in the external magnetic field, are explained below:
    1. Diamagnetism
    2. Ferromagnetism
    3. Paramagnetism
    4. Antiferromagnetism
    5. Ferrimagnetism


      Diamagnetic substances are substances which are weakly repelled by magnetic field. They have all the electrons paired. For example-H2O, C6H6, TiO2, NaCl, etc.


      These are the substances which are strongly attracted by magnetic field. They show permanent magnetism even when there is no magnetic field. For example iron, nickel, gadolinium, CrO2, etc.
      In the solid state the metal ions grouped together into small regions called Domain. Each domain acts like a tiny magnet. In an unmagnetized sample of ferromagnetic substance, the domains are randomly oriented and their magnetic moments cancel each other.
      But when magnetic field is applied, all domains get oriented in one direction and produce a strong magnetic field.

      This direction of domains remains even in the absence of magnetic field. Ferromagnetic is a case of a large amount of paramagnetism.


      The substances which are attracted by a magnetic field are called paramagnetic substances. This is due to the presence of unpaired electrons but they lose their magnetism in the absence of magnetic field.
      For example-Cr+3, TiO, VO2, CuO, O2, Cu+2, Fe+3 etc.

      Alignment of Magnetic Moment of Domains


      They are expected to have ferromagnetism or paramagnetism on the basis of magnetic moment of the domains but they have net magnetic moment zero.
      This is due to the equal number of domain presence in opposite directions.
      For example, MnO2, MnO, etc.


      These types of substances are expected to have large magnetic moment on the basis of their magnetic moment but they possess a small amount of magnetism.
      This is due to the unequal number of domains in opposite directions which results in small number of magnetic moment. For example, Magnetite, ferrite etc.

      Dielectric Property

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      These are the substances which do not allow electricity to pass through them because electrons are strongly bounded by individual atoms and not by net flow of electric charge. When an electric field is applied, induced charges are produced and as nuclei are attracted to one side and the electron cloud to the other side, this leads to polarization.

      The permanent dipoles can be presented in the crystal. So, the alignment of these dipoles may be compensated and the net dipole moment is zero or is non-compensatory and produces some net dipole moment. This net dipole moment gives some characteristic properties to solids which are as below-

      Piezoelectricity (or pressure electricity)

      In some crystals, the ordered orientation of dipoles may give some net dipole moment. When crystals undergo mechanical stress leading to the deformation, there is a production of electricity due to the displacement of ions. This form of electricity is better known as piezoelectricity and the crystals that undergo this are termed as piezoelectric. The mechanical strain resulted due to the atomic displacement when electric field is applied is sometimes known as inverse piezoelectric effect.

      The crystals utilization is best used as pick – ups in record players where they help in producing electrical signals by pressure application. For Example, titivates of barium and lead, lead zircon ate (PbZrO3), ammonium dihydro gen phosphate (NH4H2PO4) and quartz which are utilized in instruments like microphones, sonar detectors and ultrasonic generators.


      Some piezoelectric crystals help in producing small electric current on heating and thus the electricity which is produced is categorized as pyroelectricity.


      In some piezoelectric crystals, there are always some permanently polarized dipoles even in the absence of the electric field. On applying the electric field, the polarization direction will change; it is called as ferroelectricity and the crystals as ferroelectric crystal.
      For example, Barium titanate (BaTiO3) sodium potassium tartarate (Rochelle salt) and potassium dihydrogen phosphate (KH2PO4).

      Anti Ferroelectricity

      In some crystals, the orientations of dipoles are in opposite direction so the net dipole moment is zero. Such types of crystal are called anti Ferroelectric. For example-Lead zircon ate (PbZrO3).
      More topics in Solid State
      Types of Solids Thermodynamic Equilibrium
      Lattice Energy Ligand Field Theory
      Isomerism Alloys
      Magnetic Materials Metals
      Liquid Nitrogen Properties of Liquids
      Viscosity Properties of Solids
      Optical Properties of Solids
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