Electronegativity values

In a homonuclear diatomic molecule X₂, the electron density in the region between the nuclei is symmetrical; each X nucleus has the same effective nuclear charge. On the other hand, the disposition of electron density in the region between the two nuclei of a heteronuclear diatomic molecule X^_Ymay be asymmetrical. If the effective nuclear charge of Y is greater than that of X, the pair of electrons in the X^_Y covalent bond will be drawn towards Y and away from X.

Pauling electronegativity values, x^P

In the early 1930s, Linus Pauling established the concept of electronegativity which he defined as ‘the power of an atom in a molecule to attract electrons to itself ’ (the electron withdrawing power of an atom). The symbol for electronegativity is x but we distinguish between different electronegativity scales by use of a superscript, e.g. x ^P for Pauling. Pauling first developed the idea in response to the observation that experimentally determined bond dissociation enthalpy values for heteronuclear bonds were often at variance with those obtained by simple additivity rules. Equation 1.1 shows the relationship between the bond dissociation enthalpy, D, of the homonuclear diatomic X₂ and the enthalpy change of atomization, Δ_aH^o, of X.

                          (1.1)

 Effectively, this partitions bond enthalpy into a contribution made by each atom and, in this case, the contributions are equal.

 In equation 1.2, we apply the same type of additivity to the bond in the heteronuclear diatomic XY. Estimates obtained for D(X–Y) using this method sometimes agree quite well with experimental data (e.g. ClBr and ClI), but often differ significantly (e.g. HF and HCl) as worked example below

   (1.2)

Worked example Bond enthalpy additivity

Given that D(H–H) and D(F–F) in H₂ and F₂ are 436 and 158 kJ mol^-1, estimate the bond dissociation enthalpy of HF using a simple additivity rule. Compare the answer with the experimental value of 570 kJ mol^-1.

Assume that we may transfer the contribution made to D(H–H) by an H atom to D(H–F), and similarly for F.