Electronic  theory of valency :











The theory of valency is based on the electronic structure of elements and hence it is know as electronic theory of valency . 

                               According to this inert gas do not exhibit valency because they generally contain a set eight [electrons except helium =2 electron] in the outermost orbit of their atoms . 


for example :- Na 11 -2 8 1
                      Mg 12  - 2  8 2 
                     cl   17 - 2 8 7 





Ionisation Energy or ionisatation Potential :


The amount of energy required to remove the most loosely bound electron from is an isolated gaseous atom is called ionisation energy . 



UNIT :- Ionisation energy is expressed either in terms of electron volts per atom or kilo jule per mole of atoms. 

Electronic Configuration :- 

The atoms having half filled or completely filled electronic configurations are more stable and it is difficult to remove electron from them and consequently they have higher ionisation energies. 


  • The noble gases have stable configuration they have highest ionisation energies within their respective periods.

  • The elements like N, P have the configuration in which  the orbits of the same subshell are exactly half filled and are also stabe. Hence they need large energy to remove the electron conequently , ionisation energy are relatively heigh.

  • The elements like Be and Mg have competely filled orbitals their ionisation energy are large .

Electron Affinity :-

electron affinity may be defined as the amount of energy released when an electron is added to an isolated gaseous atom. 


UNIT ;- The value of electron affinity are expressed either in eletron volt per atom or kilo joules per mole of atoms. 




Factor affecting electron affinity 

Same of the important factors  which affact electron affinity are ,



  • Nuclear charge :- Greater the magnitude of nuclear charge greater will be the attraction for the incoming  electron and as a result larger will be the value of electron affinity .

  • Atomic Size :-  Large the size of an atom ,  more will be the distance between the nucleus and the additional electronal electron and smalller will be the value of electron affinity.

  • Electronic Configuration :- stable the electronic configuration of an atom lesser will be its tendency to accept the electron and lower will be the value of its electron affinity. 

  • Electronegativity :-  The tendency of an atom in a molecule to attract bonded to itself is called the electronegativity of an atom .


Factors affecting the electronegativity 


  • Size of atom :- The smaller the size of an atom grater is its tendencies to attract bounded pair of electron 

  •  Charge on the atom :- A cation has higher electronegativity than its parent atom as the cation is always smaller in size than atom which it is formed,

Periodic trends :


In a period , electronegativity increase on moving from left to right in a period .

elements of 2nd period electronegativity  this increase in electronagativity value is due to in creases in nuclear charge. Since the effective nuclear charge of  S and P block element increases rapidly and so electronegativities of S and P block elements increase sharply accross a period .



In a group  in group down in a group of S and P blosk elements the electronegativity value decreases. 







element of 1st period      H      Li       Na    K      Pb        Cs 

electronegativitly          2.1       1.0     0.9    0.8    0.8       0.7




This is apparent  from the fact that as  we   move  down   in a  group  , size  of atom increases consequently the electronegativity  value  decreases. This can be  explainded in ionisation and  electon    affinity  values  which decreases as we move down  in a group.


Chemical  Combination Case :-



  • Octot  rule :- The rule state that atoms of various elements enter into chemical combination so as to attain the configuration of eight in the outermost shell. 

    Tendency to acquire minimum energy . Every system in the universe has a matural tendance attain a        of minimum energy . The stale of minimum energy corresponds to satle of maximum stability .



Types of chemical bonds 





  • Ionic  bond or electrovalent bond :- An ionic bound is formed by the complete transfer of one or more electrons from the outer most energy shell of another one dissmilar atom so that both acquire stable inert gas configuration .


General Properties of  Ionic  Compouds 



  • Physical state :-  Ionic  compound usually exist in  the  form of crystalline solids. 
  • High melting and boiling points Ionic compounds generally have high melting and boiling points.
  • Electrical conductivity ;- Ionic compound do not conduct electicity in the solid  stale because the ions occupy fixed positions and not able to have free movement.
  • Solubility :-- Inoic compound are soluble in water and other solvent which have high dielectric constants .


Covlent Bond :-  Covalent bond is formed mutual sharing of electrons in pairs between atoms. both of which make equal contribution of electrons. When each atom contributes one two or three electrons such that two four six electrons are shared in all . the union so up i s called single or double  or triple covalent bond respectively .




Types of overlapping and natural of covalent bond 



  • Sigma Bond :-  This type of covalent bond is formed by the end to end overlapping of bond orbits along the inter nuclear axis .This overlap is known as head on overlap. 



Formation of sigma bond 



  • Pi bond :- This type of covalent bond formed by the side wise overlapping of the half filled  atomic orbitals of bonding atoms such as overlap is know as side wise or lateral overlap.


Different between sigma and pi bond 



    sigma bond                                                                     

  • sigma bond id formed by overlap of  orbitals along their internuclear axis.
  • sigma bond is formed by overlapping between S-S , S-P , or  P-P orbitals .
  • overlapping is quite large so or bond is a strong bond.
  • sigma bond consists of only one electron cloud symmentrical about the intervuclear axis.
  • free rotation about a sigma bond is possible .


pi bond 



  • pi bond is formed by side wise overlapping of orbitals .
  • pi bond is formed  by  overlap  of P-P orbitals.
  • The extent of side wise overlapping of two orbitals is small. hence pi bond is a weak bond .
  • pi bond consists of two electron clouds , one above the plane of atomic nuclei  and the below it.
  • free rotation about a pi bond is not possible.



Charcteristics of Covalent  Bond 

  • Bond length :-  It is defined on the average distance between the nuclei of two  bond in a mole  cule. Bond length can be  determiner by x-ray diffraction and by spectroscopy gases. Bond length devreases with multiplicity bond formed between the 2 atom.

  • Bond Angle :- it may be defined the internal angel between the orbitals containing electron pairs in the  valence of the central atom in a molecule . 

  • Bond strength or Bond energy :- Bond energy of a particular type of bond  is energy required to break one mole of bonds of that type  ina substance in gaseous stale.