What causes odd electron configurations in elements like Cu, Cr, Ag, and Au?

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Discussion Overview

The discussion centers around the electron configurations of transition metals, particularly elements like chromium (Cr), copper (Cu), silver (Ag), and gold (Au). Participants explore why certain elements exhibit configurations that deviate from expected patterns based on Hund's rule and the filling order of orbitals, focusing on the underlying quantum mechanical principles and energy considerations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants question why chromium has an electron configuration of 4s1 3d5 instead of the expected 4s2 3d4, suggesting that the former is energetically favored.
  • One participant argues that the 3d orbital is shielded more from the nucleus and thus has a higher energy than the 4s orbital, while another counters that 3d does not have a higher energy and is filled before 4s.
  • A participant seeks a quantum mechanical explanation for the filling order of orbitals, indicating a preference for a more nuanced understanding rather than simplified answers.
  • There is a discussion about the differences in orbital filling between manganese (Mn) and chromium, with confusion arising over why they should have similar filling patterns despite differing electron counts.
  • One participant suggests that it is energetically more favorable to have a half-filled 3d orbital than to fully fill the 4s orbital, acknowledging the complexity of multi-electron systems.
  • Another participant introduces the concept of effective nuclear charge and its impact on electron configurations, referencing Slater's rules and the idea of "Z expansion" to explain the ordering of orbitals as atomic number increases.

Areas of Agreement / Disagreement

Participants express differing views on the energy levels of the 3d and 4s orbitals, with no consensus reached on the exact reasons for the observed electron configurations. The discussion remains unresolved regarding the complexities of multi-electron systems and the implications of effective nuclear charge.

Contextual Notes

The discussion highlights limitations in understanding the electron configurations of transition metals, particularly the dependence on quantum mechanical principles and the challenges of applying simplified models to complex systems.

Chemist@
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What causes that some elements have an electron configuration that is not in agreement with Hund's rule (Cu, Cr, Ag, Au...)? E.g. why does chromine have a valent electron configuration of 4s1 3d5 instead of 4s2 3d4. The former configuration is said to be more energetically favored, but why?
 
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Chemist@ said:
What causes that some elements have an electron configuration that is not in agreement with Hund's rule (Cu, Cr, Ag, Au...)? E.g. why does chromine have a valent electron configuration of 4s1 3d5 instead of 4s2 3d4. The former configuration is said to be more energetically favored, but why?

The simplest answer here is that if you look at the two orbitals (4s and 3d), the 3d, on average, is further away from the nucleus than then 4s. Thus, the 3d is shielded more from the nucleus, and thus, will have a higher energy than the 4s orbital.

Zz.
 
3d has higher energy, but why is it filled before 4s becomes completely filled?

I don't need the simplest answer, I need the answer that is nearest to the truth. I'd like a quantum mechanical explanation.
 
Chemist@ said:
3d has higher energy, but why is it filled before 4s becomes completely filled?

I don't need the simplest answer, I need the answer that is nearest to the truth. I'd like a quantum mechanical explanation.

But that's the point, 3d does NOT have a higher energy! If it does, it won't get filled up ahead of the 4s! It is closer to the nucleus than the 4s on average!

That is the QM explanation. If you don't care for it, solve the Schrödinger equation.

Zz.
 
In vanadium, 4s is filled before 3d(!), what causes the difference between Mn and Cr orbital filling?
 
Chemist@ said:
In vanadium, 4s is filled before 3d(!)

Yeah? Isn't that what we have been discussing all along? I feel like I keep having to repeat the same thing.

, what causes the difference between Mn and Cr orbital filling?

I'm confused. What is it exactly that you are asking? Mn and Cr have different number of electrons. Why should they have the same "orbital filling"?

Zz.
 
Okay, I will reformulate my quesiton:
Vanadium has a completely filled 4s orbital and 3 electrons in the 3d orbital, by analogy, chromine should be 4s2 3d4, but instead it is 4s1 3d5. Why isn't the 4s orbital in chromine filled completely?
 
Chemist@ said:
Okay, I will reformulate my quesiton:
Vanadium has a completely filled 4s orbital and 3 electrons in the 3d orbital, by analogy, chromine should be 4s2 3d4, but instead it is 4s1 3d5. Why isn't the 4s orbital in chromine filled completely?

Obviously, it is a lower energy to half-filled the 3d orbital than to fully fill the 4s orbital.

At some point, we simply can no longer simply try to give a hand-waving explanation on why this occurs, because the physics of multi-electron system gets rather complex. Anything beyond Helium will require sophisticated methods, such as DFT, to get the solution to such atoms.

Zz.
 
"At some point, we simply can no longer simply try to give a hand-waving explanation on why this occurs, because the physics of multi-electron system gets rather complex. Anything beyond Helium will require sophisticated methods, such as DFT, to get the solution to such atoms."

That's what I was asking for. Thank you for the answer.
 
  • #10
Maybe a simple explanation is that advancing along a period, the effective nuclear charge seen by the valence electrons increases as the valence electrons are not very effective in shielding each other from the nuclear charge. Compare the Slater rules Slater's rules - Wikipedia, the free encyclopedia.
Now with increasing (effective) nuclear charge, e.g. in going from V to Cr, the effects of many electron effects in an atomic problem become less pronounced and the ordering of the orbitals approaches that found in a hydrogen atom, i.e. 3d falls below 4s.
This is formalized using "Z expansion".
 

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