Electronegativity and Periodic variation of electronegativity

What is electronegativity ?

The tendency or power of an atom to attract electrons towards itself, when combined in a covalent compound is termed as electronegativity of atom .

Unit of electronegativity.

Electronegativity is the tendency of an atom to attract covalently bonded electrons toward itself. So electronegativity has no unit.

The most electronegative element in the world is fluorine ( F ) and the less electronegative element in the world is cesium ( Cs ) .

The most electronegative compound in the world is sodium perxenate,  Na₄[XeO₆ ].The term electronegativity has been defined differently by differently investigators .

 The approaches of some of the investigators are discussed  below.

Pauling ‘s Approaches :

Pauling defined electronegativity as the power of an atom in a molecule to attract electrons to itself. He calculated electronegativities of different elements from thermodynamics data.

Pauling consider a reaction of the given type for calculating of electronegativity of elements are shown below,

 1/2( A – A ) +  1/2 ( B – B ) → ( A – B )………………………..( I )

The bond dissociation energy of (A – B)  is higher than the mean of the bond dissociation energies of  ( A – A )  and ( B – B ) bonds and that their

difference  Δ  is related to the difference in the electronegativities of A and B, according to the following empirical equation,

Δ  = E _{A – B} ­­ -  1/2( E  _{A – A}  +  E  _{B – B}  )  = 23 ( χ_A - χ_B )2 ……………………….( II )

Where   E _{A – B} , E _{A – A}  and  E _{B – B} represents the bond dissociation energies of ( A –B ) ,(  A – A ) and ( B – B ) bonds respectively and  χ_A  and χ_B denote the  electronegativities of A and B respectively.

Thus,   Δ = 23 ( χ_A - χ_B )2 ………………………………( III )

Or  0.208 √Δ  =  χ_A - χ_B ……………………………………( IV )

Pauling later took the geometric mean instead of the arithmetric mean of E _{A – A}  and  E _{B – B} and suggested the following empirical relation,

  E _{A – B} ­­ -   ( E  _{A – A}  x  E  _{B – B}  )1/2  = Δ’ = 30( χ_A - χ_B )2 …………………….( V ) 

Or  0.182√Δ’ = χ_A - χ_B  ……………………………( VI )

In Pauling’s equation , the dissociation energies are taken in Kcal per mol.

Assuming arbitrarily  the electronegativity of hydrogen to be 2.1 , Pauling determined electronegativities of other elements with the help of equation    (VI ) .

Allred and Rochow's Approach :

Allred and Rochow defined electronegativity as the electrostatic force of attraction exerted by the nucleus of an atom on the valency electrons.

Allred and Rochow , by applying effective nuclear charge, Zeff , gave an empirical relation for calculating the electronegativity,

Where χ is the electronegativity and  r is the covalent radius of the atom .

Mulliken's Approach:

Mulliken suggested that the mean of the ionization potential and electron affinity of an atom should be a measure of the electronegativity of the atom.

Mulliken proposed the following empirical relation for determining   electronegativities  of various elements are ,

Where χ is the electronegativity and  r is the covalent radius of the atom .

Mulliken's Approach:

Mulliken suggested that the mean of the ionization potential and electron affinity of an atom should be a measure of the electronegativity of the atom.

Mulliken proposed the following empirical relation for determining electronegativities  of various elements are ,

In this equation, ionization potential as well as  electron affinity are in volts.

Periodic variation of electronegativity:

The electronegativity of elements gradually increases from left to right along a period and electronegativity decreases from top to bottom along a group. 

The change of electronegativities of elements according to Pauling's scale are as follows,

 Summary : 



What is electronegativity ? unit of electronegativity.
Periodic variation of electronegativity.
The most and the least electronegative elements in the world. 

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