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

[asdf1]

why is the electron configuration different for Z<20 and Z>90?

 

[Mattara]

Can you please develop?

 

[asdf1]

Z<20:
1s<2s<2p<3s<3p<4s<3d<4p
but
Z>90
1s<2s<2p<3s<3p<3d<4s<4p ?

 

[Mattara]

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.

 

[asdf1]

is there a reason why that is so?

 

[Mattara]

Do you mean why the actinides are filling their f sunshell or why 4s is filled before 3d?

 

[GCT]

read up on the Aufbau principle

 

[Gokul43201]

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 ?

 

[asdf1]

well, I'm taking a class called crystal fracture and that came up in the lecture notes, but didn't explain why...