Electronegativity and atomic size effects

Average of electron affinity and ionization energy Electrostatic attraction proportional to Z /r  

Average energy of valence shell electrons, configuration energies 

Calculation of electronegativities from bond energies requires averaging over a number of compounds to cancel out experimental uncertainties and other minor effects. Methods that use ionization energies and other atomic properties can be calculated more directly. The electronegativities reported here and in Appendix B-4 are suitable for most uses, but the actual values for atoms in molecules may differ from this average, depending on their electronic environment. 

Many of those interested in electronegativity agree that it depends on the structure of the molecule as well as the atom. Jaffe used this idea to develop a theory of the electronegativity of orbitals rather than atoms. Such theories are useful in detailed calculations of properties that change with subtle changes in structure, but we will not discuss this aspect further. The differences between values from the different scales are relatively small, except for those of the transition metals. All will give the same.results in qualitative arguments, the way most chemists use them. 

Remember that all electronegativities are measures of an atom s ability to attract electrons from a neighboring atom to which it is bonded. A critique of all electronegativity scales, and particularly Pauling s, describes conditions that all scales should meet and many of their deficiencies.  

By the VSEPR approach, trends in many bond angles can be explained by electronegativity. Consider the bond angles in the following molecules  

Similar considerations can be made in situations where the outer atoms remain the same, but the central atom is changed, for example. 

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Mor] estimated the solid solubility of fee alloys in tire ternary regime based on the electronic structure calculations by discrete variational DV-X method. They introduced a parameter M, which is the average energy level of d orbitals of the alloying elements, and it carries both electronegativity and atomic size factor effects. At 1200°C, the predicted solubihty underestimates the experimental data. 

Sketch a periodic table, indicating the trend of electronegativities fi-om the lowest to the highest. Briefly relate these trends to effective nuclear charge and atomic size. Your answer should include a concise definition of electronegativity. 

Substitution of one atom for another is a common phenomenon. These mixtures are also called solid solutions. For example, nickel and copper atoms have similar sizes and electronegativities and the same fee crystal structures. Mixtures of the two are stable in any proportion, with random arrangement of the atoms in the alloys. Other combinations that can work well have a very small atom in a lattice of larger atoms. In this case, the small atom occupies one of the interstices in the larger lattice, with small effects on the rest of the lattice but potentially large effects on behavior of the mixture. If the impurity atoms are larger than the holes, lattice strains result and a new solid phase may be formed. 

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