explain band theory & magnetic properties of solids in detail ,?

Dear Student,

Solids exhibit a wide range of conductivities, based upon which they are classified as conductors, semi-conductors and insulators. These conductivity usually depends upon the number of valence electrons available per atom.
Atomic orbitals of atoms form molecular orbitals, and it is these molecular orbitals which behaves like a Band or an energy states.
 
If the valence band is partially filled or overlapped by the higher unoccupied band i.e. conduction band, then in such a case the electrons can flow easily and can hence be called as conducting. On the contrary, if the gap between a valence band and conduction band is large enough that the electrons cannot jump between them, then they behave like an Insulator.


There is an another class of solids which are semi-conductors, wherein the gap between the valence band and the conduction band is small and the electrons can jump from the valence band to the conduction band and can show some conductivity. Their conductivity is a function of temperature as more electrons can be excited with rise in temperatures.


Please post that query of magnetic properties of solids  in a different thread . 


 

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Depending upon the response to magnetic fields, substances (solids) can be divided into following categories-
1. Diamagnetic substances - These are weakly repelled by the magnetic fields. Such substances possess completely filled orbitals i.e. all the electrons present in them are paired. Ex - benzene, NaCl etc.
2. Paramagnetic substances - The substances which are attracted by the magnetic fields are called paramagnetic substances. These substances possess permanent magnetic dipoles due to the presence of atoms, ions or molecules with unpaired electrons. However they lose their magnetism in the absence of a magnetic field
3. Ferromagnetic substances - These substances are strongly attracted by magnetic fields and show permanent magnetism even in the absence of magnetic field. Ferromagnetism is due to the spontaneous alignment of magnetic moments (due to the presence of unpaired electrons) in the same direction. Ex- Iron, cobalt etc.
4. Antiferromagnetic substances - The substances which possess zero net magnetic moment inspite of the presence of unpaired electrons are called antiferromagnetic substances. Antiferromagnetism is due to the alignment of moments in a compensatory way. Ex - MnO
5. Ferrimagnetic substances - The substances which possess only small net magnetic moments inspite of the presence of a large no. of unpaired electrons are called ferrimagnetic substances. In this substance, the moments are aligned in parallel and antiparallel directions in unequal numbers resulting in small net magnetic moments. Ex - Ferrites. 
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