The Pearson Model Electronegativity Equalization

The idea of electronegativity has always interested chemists and there have been many scales published, which can be categorized into one- and two-parameter models. The two-parameter models assume equalization of electronegativities, i.e., charge is transferred between the atoms until they have the same electronegativity. 3.2 3,2 7 single-parameter models use other devices (including hybridization, 

There are several other scales of electronegativity. The Allred-Rochov scale is 

Sanderson calls his electronegativity stability ratio SR. The electronegativity in this scale is x(SR) = n/(4.19 r ), in which n is the number of valence electrons and r the covalent atomic radius. It also expresses the influence that atomic volume has on electronegativity. Sanderson was the first to point out the electronegativity equalization principle. Although the -values are different in the various scales as is clear from Table 2.4, all the electronegativity scales correlate strongly with one another. 

The two-parameter models of Miedema (Section 2.6) and Pearson (this section) are both based on the idea of electronegativity and its equalization on bond formation but each interprets the positive contribution to AH in a different way in the Miedema model it means equalizing the electron density at the surface of the atom, while in the Pearson model it is called chemical hardness and also expresses resistance against charge transfer. 

In the Pearson model the electronegativity x is the first derivative of the energy E with respect to the amount of electronic charge on the atom the chemical hardness rj is the second derivative, both at constant internuclear distance and nuclear charge  

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