The Central Field Approximation for Many-Electron Atoms

In summary, the effective nuclear charge for an electron in an atom is a function of its distance from the nucleus.
  • #1
rtareen
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My book (Young and Freedman 14th) doesn't go into detail about the central field approximation other than saying the potential energy only has a radial component.
Attached is my book's section on many-electron atoms. It says that in the central field approximation, an electron's potential energy is a function of its distance from the nucleus. Later on it says there is an effective atomic number. Does this mean that in this approximation, all charges (protons and electrons) are taken to be in the nucleus? That's very simple to understand. But if that's not the case, how does it actually work? Are the other electrons given certain distances from the electron of interest? How would it work for electrons that closer to the nucleus or farther out?
 

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rtareen said:
Does this mean that in this approximation, all charges (protons and electrons) are taken to be in the nucleus?

No, for two reasons.

First, as noted in the discussion around equation 41.45, if all of the charges except the single electron being considered were treated as being in the nucleus (or at least closer to it than that single electron), then ##Z_{eff}## would be exactly ##1##. In fact it is larger than ##1##, so only a portion of the other electrons' charges are being treated as screening the nuclear charge.

Second, if all of the charge except for the single electron being considered was treated as being in the nucleus, then the potential would just be proportional to ##1 / r##. But, as noted, the potential function ##U(r)## is not that simple. That effectively means that the charge is spread out, not all at the center.

rtareen said:
Are the other electrons given certain distances from the electron of interest?

Not as far as I know; my understanding is that the potential ##U(r)## is derived empirically, not from any specific theoretical assumption about distances of the other charges from the center.

rtareen said:
How would it work for electrons that closer to the nucleus or farther out?

As far as I know, the central field approximation works best for electrons in the outermost shell, i.e., the ones farthest from the nucleus.
 
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  • #3
PeterDonis said:
No, for two reasons.

First, as noted in the discussion around equation 41.45, if all of the charges except the single electron being considered were treated as being in the nucleus (or at least closer to it than that single electron), then ##Z_{eff}## would be exactly ##1##. In fact it is larger than ##1##, so only a portion of the other electrons' charges are being treated as screening the nuclear charge.

Second, if all of the charge except for the single electron being considered was treated as being in the nucleus, then the potential would just be proportional to ##1 / r##. But, as noted, the potential function ##U(r)## is not that simple. That effectively means that the charge is spread out, not all at the center.

Not as far as I know; my understanding is that the potential ##U(r)## is derived empirically, not from any specific theoretical assumption about distances of the other charges from the center.

As far as I know, the central field approximation works best for electrons in the outermost shell, i.e., the ones farthest from the nucleus.

Every time you explain something it is always so clear-cut and easy to understand. I always feel satisfied with your answers. Unfortunately the next chapter is about molecules and condensed matter, which is a subforum I can see you have never posted in. Would you be willing to look out for me over there?
 
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  • #4
rtareen said:
Every time you explain something it is always so clear-cut and easy to understand. I always feel satisfied with your answers.

Thanks! Glad I could help.

rtareen said:
Would you be willing to look out for me over there?

If the question is a general question about how QM models more complex systems like molecules, it could also be asked in this forum.
 
  • #5
My inorganic chemistry textbook (Housecroft & Sharpe) says that for the helium atom, an effective nuclear charge of 1.69 gives the closest approximation to the ground state energy in the central field approximation. If the helium atom is put in an excited state where one electron occupies the 1s orbital and another is somewhere much higher, like 50s orbital, the electron in the high excited state sees an effective nuclear charge of approximately 1.00 because of its greater distance from the nucleus. The same applies to a muonic helium atom where one electron and one muon orbit the nucleus.
 
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FAQ: The Central Field Approximation for Many-Electron Atoms

What is the Central Field Approximation for Many-Electron Atoms?

The Central Field Approximation is a theoretical model used to describe the behavior of many-electron atoms. It assumes that the electrons in an atom are influenced only by the electric field created by the nucleus, and that they do not interact with each other.

How does the Central Field Approximation simplify the study of many-electron atoms?

By assuming that the electrons do not interact with each other, the Central Field Approximation simplifies the complex interactions between multiple electrons in an atom. This allows for easier calculation of the energy levels and other properties of the atom.

What are the limitations of the Central Field Approximation?

The Central Field Approximation is a simplified model and does not accurately represent the full complexity of many-electron atoms. It does not take into account the repulsion between electrons or the effects of electron spin, which can affect the energy levels and properties of the atom.

How is the Central Field Approximation used in practical applications?

The Central Field Approximation is used in a variety of practical applications, such as in quantum chemistry calculations and in the development of new materials. It provides a useful starting point for understanding the behavior of many-electron atoms and can be built upon with more advanced models and techniques.

Are there any alternative models to the Central Field Approximation for studying many-electron atoms?

Yes, there are alternative models such as the Hartree-Fock method and the Density Functional Theory that take into account the interactions between electrons and provide a more accurate description of many-electron atoms. However, the Central Field Approximation remains a useful tool in understanding the behavior of these complex systems.

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