why is the electron configuration different for Z<20 and Z>90?
Can you please develop?
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
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
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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