Variation of Electronegativity

Electronegativity is a variant property of an atom and shows a quite significant dependence on its environments. Some of the important factors affecting electronegativity values are given here.

(i)    Oxidation number. Electronegativity of an atom varies form one bonding situation to another depending upon the oxidation number of the atom in the molecule. For example, Pauling’s electronegativity value is 1.62 for T(I) , whereas it is around 2.04 for T (III). Similarly, electronegativity of S in sf₆ is higher than that in SCI₂ because the former molecule is in higher oxidation state of s. The higher the oxidation state, the greater the attraction electrons and higher the electronegativity.

(ii)    Hybridization. Hybridization affects electronegativity values considerably because s-orbital die to its higher penetration effect is more tightly held than other orbital’s and hence has lower energy. Consequently, it will have grater tendency to attract electrons and hence show higher electronegativity values. Fore example, carbon in Ch₄ is sp³ hybridized which has 25% s-character. The electronegativity of C in Ch₄ is 2.53. In ethylene H₂C=CH₂, carbon is sp2 hybridized which has 33% s-character. The electronegativity of C in C₂H₄ is 2.75. In case of acetylene HC =CH  carbon is sp hybridized which has 50% s-character. Thus, electronegativity of C in C₂H₂ is enhanced significantly (as evidenced by the higher acidity of C₂H₂) and its value is reported to be 3.3.

Similarly, electronegativity of N in sp³, sp² and sp- hybridized configuration  Shows an increase with values 3,7,4.1 and 5.1 respectively. That is, in its higher s-characters hybrids (sp) nitrogen becomes more electronegative than F and carbon approaches oxygen.

(iii)    Nature of Constituents. Electronegative of a group varies with the nature of substations. For example the group electronegativity of CH₃ – 2.3 while that of CF₃=3.35 and that of CCI₃ =3.0 and CN =3.3. These values are based on dipole moment, atomic electronegative and kinetic data.

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