Equation Screening constant

Later methods, especially that of Gordy (1955), and later Allred and Rochow (1958) make use of screening constants of the electron strucmre for the nuclear charge of each atom. This determines die attraction between the nucleus of the atom and an electron outside the normal electron complement, and is die effective nuclear charge. The empirical equation for the values of electronegativity obtained in this manner by Allred and Rochow is... 

It is customary to express the empirical data concerning term values in the X-ray region by introducing an effective nuclear charge Zeff e in the place of the true nuclear charge Ze in an equation theoretically applicable only to a hydrogen-like atom. Often a screening constant S is used, defined by the equation... 

This equation, including succeeding terms, was obtained originally by Sommerfeld from relativistic considerations with the old quantum theory the first term, except for the screening constant sQ> has now been derived by Heisenberg and Jordan] with the use of the quantum mechanics and the idea of the spinning electron. The value of the screening constant is known for a number of doublets, and it is found empirically not to vary with Z. 

In Table IV are given values of the mole refraction of gaseous ions calculated from equations (24) and (29) with the use of the values found above for SE and ASe. Values for hydrogen-like atoms and ions are also included these are, of course, accurate, since no screening constant is needed. Table IV is... 

The molal diamagnetic susceptibilities of rare gas atoms and a number of monatomic ions obtained by the use of equation (34) are given in Table IV. The values for the hydrogen-like atoms and ions are accurate, since here the screening constant is zero. It was found necessary to take into consideration in all cases except the neon (and helium) structure not only the outermost electron shell but also the next inner shell, whose contribution is for argon 5 per cent., for krypton 12 per cent., and for xenon 20 per cent, of the total. 

The value of the size screening constant SSoo is accordingly given by equation (26), r/t being replaced by 2. 

As before, we may expect the values of Ss calculated for Z large to be valid for actual ions with the helium and neon structures. For the other structures we introduce the empirical corrections based upon those used for the mole refraction screening constant, with the aid of the principle of the constancy of the ratios of corresponding screening defects, already used for the diamagnetism screening constant. In this way the values of SSo and ASs given in Table VIII are obtained. An equation similar to equation (29) is to be used to find individual values of Ss. 

Fig. 5. Contribution of ionic terms to the binding energy as a function of internuclear distance, (a) Contribution /i of ionic terms for H2 molecule (see equation 25) calculated with best screening constant of a Heitler-London wave function (after Coulson and Fischer (134)). (b) Variation of the binding energy...

NMR chemical shifts are perhaps best understood in terms of an additive dependency of the paramagnetic, OoP, and diamagnetic, screening constants Equation (1) [8]. 

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