Valence electrons :-
The electrons which revolve in the outermost orbit of an atom are called valance electron .
most of the atoms of different elements do not have their outermost orbit completely filled . FOR EXAMPLE:- SHOW that silicon and germanium atoms have four valance electrons . the outermost orbits can have a maximum of 8 electrons . Valance electrons actually determined the physical and chemical properties of a metal . valance electron have more orbital and least binding energy . consequently , very little energy is required to detach then from their parent atom and make them conductors of electricity and heat . these valence electrons take part in chemical bounding and also determine the valency of the element .
FREE ELECTRON :-
VALANCE electron are loosely bound to the nucleus and can easily detached from the parent atom . IN a metal the atoms are closely packed and hence the energy levels of the neighbouring atoms overlap each other . this result in the releases of valance electron from their atoms . As these electrons are no longer attached to any particular atom , they are called FREE ELECTRONS which move randomly in the metal like molecule of gas . The drift of these free electrons in one direction constitutes electrical current .
electron which are detached from the parent atoms and move randomly in the lattice of a metal are called free electrons .
energy bands of silicon and germanium atom at Bohr's radius | semiconductor |:-
THE energy band structure of a solid determines whether it is a conductor , an insulator or a semiconductor .
A free electron or an electron of an isolated atom may have only certain discreate energies and can move in discrete energy level . energy level of isolated atoms are not affected by distant atoms . AS we bring together large number of identical atoms in order to form a solid , significant changes take place in the energy levels . Each atoms is in the electrostatics field of neighbouring atoms .
THE range of energies processed by electrons in a solid is known as energy band .
since each electron occupies a different position inside the solid , no two electrons can see exactly the same pattern of surrounding changes , AS a result . the orbitals of the electrons are different , since there are million of electrons , belonging to the first orbit of atoms in the solids , the slightly different energy level form a band called first energy band . similarly , second energy band . all the third orbits electrons form the third band ,and so on .
Out of all the energy bands , three energy bands are most important to understand the behaviour of solids . the three energy bands are :- (1) Valance band (2) Conduction band and (3) Forbidden band or gap
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