CHEMISTRY ASSIGNMENT

ROLL NO. 4929-4939

 Electronegativity: 

 The concept of electronegativity was first introduced by Linus Pauling in 1930 as a means of describing bond energies. 
1 When we consider the formation of a covalent bond the attraction of the two nuclei for the electrons is not the same and the electron pair is closer to one of the nuclei.

  2 This tendency of a atom in a molecule to attract the electron density towards itself (or the reluctance to release the electron density) is called electronegativity. 

 3 The electronegativity of a atom depends on the size, effective nuclear charge, oxidation state and the hybridization of the atom in the molecule. It therefore depends on the structure of the molecule and the atom. 

 4 If the size of the atom is small and it has almost closed shell electronic configuration the tendency to attract electrons increase and the atom is highly electronegative.  

 Variation of electronegativity

  1.Electronegativity increases from left to right in a period. 

 2 Electronegativity decreases down the group.   

Many scales of electronegativity have been proposed. One of the most commonly used   scale is the Pauling’s scale. 

 Pauling’s Electronegativity:

 For any covalent bond, the bond energy of the heteronuclear bond E(A-B) is greater than the bond energy of the sum of homonuclear bonds E(A-A) and E(B-B). This excess bond energy can be attributed to ionic contribution in the bonds. He treated this ionic contribution by the equation.
       E(A-B) = [E(A-A)×E(B-B)]1/2 + 96.48(ΧA - ΧB)2
E(A-B) is expressed in kJ mol-1  and ΧA - ΧB is  the difference in "electronegativity" between the two elements. The largest electronegativity difference exists between Cs and F. The value of F was set arbitrarily at 4.0and electronegativity values of all other elements found relative to it

 Allred and Rochow electronegativity:

 Allred and Rochow gave the electronegativity values by considering the electrostatic force exerted by effective nuclear charge, Zeff, on the valence electron. They gave the equation: XAR = (3590 x Zeff/r2cov) + 0.744 

 Mullikan electronegativity: Mullikan proposed that two energies associated with the atom i.e. the electron affinity EAv and the ionization potential IEv  should be a measure of electronegativity. The Mulliken electronegativity, ΧM is related to the electron affinity EAv and the ionization potential IEv by the equation:
ΧM = (IEv + EAv)/2
          The subscript v denotes a specific valence state.
          The Mulliken electronegativity ΧM can be expressed on the Pauling scale by the relationship given below if the values of IE and EA are in MJ mol-1:
ΧM = 3.48[((IEv + EAv)/2) - 0.602] 

BY

VIKAS BHATI