Why do electron configurations differ for elements with Z<20 and Z>90?

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Discussion Overview

The discussion revolves around the differences in electron configurations for elements with atomic numbers (Z) less than 20 compared to those greater than 90. Participants explore the implications of these configurations in the context of atomic structure and the filling of electron subshells, particularly focusing on the transition from lighter to heavier elements.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions why the electron configuration differs for Z<20 and Z>90.
  • Another participant provides a proposed electron configuration sequence for Z<20 and Z>90, noting the filling order of subshells.
  • A later reply clarifies that while the actinides have different configurations, the 4s subshell is always filled before the 3d subshell, and emphasizes the importance of the f subshell filling in actinides.
  • Participants discuss the Aufbau principle in relation to the filling order of orbitals, with one suggesting that further reading on this principle may be beneficial.
  • One participant expresses curiosity about the source of the information regarding electron configurations and asks for clarification on what it states about elements with atomic numbers between 20 and 90.
  • A participant mentions that their understanding comes from a class on crystal fracture, where this topic was briefly addressed without detailed explanation.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the reasons behind the differences in electron configurations, and multiple viewpoints regarding the filling order and the role of the f subshell in actinides are presented.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about electron configurations and the lack of detailed explanations in the referenced class materials. The discussion also reflects varying levels of understanding of the Aufbau principle and its implications for electron filling.

Who May Find This Useful

Readers interested in atomic structure, electron configurations, and the behavior of elements in different regions of the periodic table may find this discussion relevant.

asdf1
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why is the electron configuration different for Z<20 and Z>90?
 
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Can you please develop?
 
Z<20:
1s<2s<2p<3s<3p<4s<3d<4p
but
Z>90
1s<2s<2p<3s<3p<3d<4s<4p ?
 
Well, at 90 the actinides start. They have different electron configuration, but not in the way you say (4s is always ahead of 3d). When moving on to the actinides, the f subshell is being filled.

When doing chemical/physical notation of orbitals it is correct to write

1s2s2p3s3p3d4s4p

aswell as:

1s2s2p3s3p4s3d4p

However, it is important to remember that the (>3)d orbital is filled before the (>4)s orbital.
 
is there a reason why that is so?
 
Do you mean why the actinides are filling their f sunshell or why 4s is filled before 3d?
 
read up on the Aufbau principle
 
asdf1 said:
Z<20:
1s<2s<2p<3s<3p<4s<3d<4p
but
Z>90
1s<2s<2p<3s<3p<3d<4s<4p ?
Out of curiosity :

1. Where did you read this ?

2. What does the source say about 20 < z < 90 ?
 
well, I'm taking a class called crystal fracture and that came up in the lecture notes, but didn't explain why...
 

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