Electronegativity, scale

Selected Values from the Pauling Electronegativity Scale... 

An estimate of the polarity of a bond between two atoms may be obtained by reference to the electronegativity scale. The electronegativity values of some common elements are given in Table 5.1. The higher the value the greater the electronegativity. 

The quantum mechanical argument used in deriving the original electronegativity scale involved the amount of ionic character of a normal covalent bond A—B, and it was evident that the amount of ionic character and accordingly the value of the electric dipole moment of the bond would be closely correlated with the difference Ax = xA — xB of the two atoms A and B. In the first edition of The Nature of the Chemical Bond (1939) the following equation was advanced ... 

We can use Pauling s empirical formula, which defines the Pauling electronegativity scale, for dissociation energies of a diatomic gold molecule. 

In this equation, Z is the effective nuclear charge, which takes into account the fact that an outer electron is screened from experiencing the effect of the actual nuclear charge by the electrons that are closer to the nucleus (see Section 2.4). In principle, the Allred-Rochow electronegativity scale is based on the electrostatic interaction between valence shell electrons and the nucleus. 

A simple way to bypass the vagaries of empirical fits is to define an alternative, purely theoretical electronegativity scale based on a proposed functional relationship between /ah and jca, analogous, e.g., to Eq. (3.57) or Eq. (3.58). A particularly simple and attractive one-parameter exponential relationship is... 

We again choose the value of the exponential parameter in Eq. (3.76) to be a = —0.45 so that sigma- and pi-electronegativity scales have the same units. The specification of the scale is then completed by choosing the value of xA(7I) for a single atom. For this purpose it is convenient to assign to the carbon atom a common value of a- and 7t-electronegativity,... 

See Pauling, note 14, pp. 97ff. Coulson, note 21, p. 141, states Now there must be some relation between the percentage ionic character and the electronegativity difference xB — xA. It is of the very essence of the idea of an electronegativity scale that this should be so. ... 

A simple linear shift of the entire electronegativity scale by —0.56 (i.e., shifting F downward from its arbitrarily chosen value 4.0 on the Pauling scale to new value 3.44) would make xnb(N) = 0, the more obvious electronegativity of a nonexistent ligand. 

Electronegativity does not have a unit. The most widely used electronegativity scale today was derived by Linus Pauling. He used bond energy values in the preparation of this scale. 

In this equation //A and r/B are values of electronegativity assigned to atoms A and B. In the original formulation of the electronegativity scale, the electron volt was used as the unit of energy this use accounts for the approximate factor 23 in the equation. (leV = 1.60217653(14) 10 19J,Avogadro number 6.0221415(10) 1023mol 1). 

On the basis of this equation, systematically considering all the available thermochemical data and arbitrarily assigning to the most electronegative element, fluorine, a value of about 4, Pauling was able to prepare a complete electronegativity scale. This is shown in Table 2.1. 

The results can be used to predict unknown structural parameters and to establish a group electronegativity scale that will include... 

Saxena S. K. (1977). A new electronegativity scale for geochemists. In Energetics of Geological Processes, S. K. Saxena and S. Bhattacharji, eds. New York-Heidelberg-Berlin Springer-Verlag. 

Carbon occupies an intermediate position in the electronegativity scale (with a value of 2.5) and forms covalent bonds with neighbouring elements in the Periodic Table. 

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