chemistry

ELECTRONEGATIVITY

The tendency of an atom to attract the shared electron pair towards itself in a molecule is called its electro negativity.

Electron affinity is the property of an isolated atom and electronegativity is the property of a bonded atom.

Electron affinity is absolute phenomenon electro negativity is relative phenomenon. Electron affinity has units and electronegativity has no units.

Electron affinity of an element is fixed and electronegativity is slightly variable. Electronegativity depends on

i)     Size of atom: EN. decreases with increase in size of atom

ii)   Nuclear charge: E.N. increases with increase in nuclear charge

iii)Screening effect: E.N. decreases with increase in screening effect.

iv) Number of valence electrons: E.N increases with increase in number of valence e^-s.

v)   S-character of hybrid orbital: E.N. increases with increase in S-character of hybrid orbital.

TREND:

Electronegativity decreases with increase in size and increase in screening effect in a group from top to bottom.

Among all groups, I-A group has least E.N. and VII-A group has highest E.N. Values.

Electronegativity increases with decrease in size and increase in nuclear charge in a period from left to right.

In a given period I-A element has least E.N. and VII-A element has highest E.N. value. 'O' group elements have zero E.N values.

E.N of the 1^stfew elements:

F > O > N ≈ CI > Br > C ≈ I > H

In Pauling's scale electroegativities are calculated from the bond energies.

SCALES OF ELECTRONEGATIVITY :

Pauling scales :

It is widely accepted scale.

X_A – X_B = 0.208.   if ∆ is in kcal/mole or X_A – X_B = 0.1017   if ∆ is in kJ/mole.

X_Aand X_B are the electronegativities of two elements A and B.

-difference between calculated and experimental bond energies of A – B bond.

=  ionic - resonance energy

=  polarity of A - B bond.

In the determination of electronegativities of other elements. Hydrogen is reference element. Fluorine is the most electronegative and Cs is the least electronegative.

Electronegative value of Hydrogen is 2.1.

MULLIKAN SCALE:

Mullikan suggested that the electronegativity of an element is the average of its ionization energy and electron affinity.

ionization energy + electron affinity

E.N=                         ²                         (eV/atom)

These values are about 2.8 times greater than the values on Pauling's scale.

The commonly accepted electronegataivity values are obtained from the formula.

					ionization potential + electron affinity
				E.N =	544	(kJ/mole)
Mulliken scale is not much useful due to the following drawbacks.
i)	It is applicable only for monovalent elements.
	ii) As per Mulliken scale, Inert gases should posses higher E.N values.
	Conversion:
		1 unit of Mulliken
	1 Unit of pauling =		2.8

APPLICATIONS OF ELECTRONEGATIVITY:

i)         If the electronegativity difference of the bonded atoms is greater than or equal to 1.7, the bond is ionic. If the electronegativity difference is less than 1.7 the bond is covalent or polar covalent.

ii)       Electronegativity values are useful in writing the formulae of compounds.

iii)     In calculation of oxidation states.

iv)     In predicting oxidising power of elements.