Effective nuclear charge determination

The effective nuclear charge for transition metals is determined from the atomic spectral terms ° because few experimental data on diatomic transition metal hydrides are available (see e.g.. Ref. 41). The effective nuclear charges determined for transition elements are given by... 

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... 

Fig. 1-16. Moseley plot for Ka2 lines. The curvature at high Z is due to a change in the effective nuclear charge (Z — 1). The insert shows the atomic number Z to be more fundamental than the atomic weight M. X-rays made possible the first experimental determinations of Z. Crosses = atomic weight dots = atomic number.

A question which has been keenly argued for a number of years is the following if it were possible continuously to vary one or more of the parameters determining the nature of a system such as a molecule or a crystal, say the effective nuclear charges, then would the transition from one extreme bond type to another take place continuously, or would it show discontinuities For example, are there possible all intermediate bond types between the pure ionic bond and the pure electron-pair bond With the development of our knowledge of the nature of the chemical bond it has become evident that this question and others like it cannot be answered categorically. It is necessary to define the terms used and to indicate the point of view adopted and then it may turn out, as with this question, that no statement of universal application can be made. 

Why does the octet rule work What factors determine whether an atom is likely to gain or to lose electrons Clearly, electrons are most likely to be lost if they are held loosely in the first place—that is, if they feel a relatively low effective nuclear charge, Zeff, and therefore have small ionization energies. Valence-shell electrons in the group 1A, 2A, and 3A metals, for example, are shielded from the nucleus by core electrons. They feel a low Zeff, and they are therefore lost relatively easily. Once the next lower noble gas configuration is reached, though, loss of an additional electron is much more difficult because it must come from an inner shell where it feels a high Zeff. 

In another alternative it was proposed that electronegativity be defined as a potential arising from the effective nuclear charge (Ze/f) determined by partial screening of the nucleus, measured at the covalent radius ... 

The radius of an ion is determined mainly by the principal quantum number and the effective nuclear charge. As the atomic number increases in the transition-metal series, the principal quantum number remains the same, but the effective charge of the valence electrons increases hence, the ionic radius should decrease smoothly with an increase in atomic number. 

The variation theorem can be nsed to provide a quantum mechanical determination of the energy E of the helium atom. This is expressed in terms of the effective nuclear charge Z experienced by each of the electrons as... 

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