What is the electron configuration for Gd?

[jumbogala]

Homework Statement

What is the electron configuration for Gd?

Homework Equations

None

The Attempt at a Solution

Since Gd has 64 electrons, I would assume that the electron config. is:

[Xe]6s²4f⁸

Apparently this is wrong. The hint the question gives is that the last two orbitals are only half full... so I guess it would be something like:

[Xe]6s²4f⁷5d¹

But why? According to my diagram, the 4f level fills up before the 5d level does, so why wouldn't it take all 14 spaces for the electrons?

And if the 4f level doesn't really fill up before the 5d level, then why do lower levels fill up in weird orders like (1s²,2s²,2p⁶,3s²). Eg. Why does it jump from s to p then back to s?

 

[Borek]

[Xe]6s²4f⁷5d¹

But why? According to my diagram, the 4f level fills up before the 5d level does, so why wouldn't it take all 14 spaces for the electrons?

4f and 5d are very close in terms of energy levels. In such case strange things may happen. Half filled orbital is slightly more stable than orbital with one additional electron. You put 7 electrons on 4f and when you add next electron it may land on 4f - but then you loose half orbital premioum - or on 5d. It happens that the latter version is energetically better.

And if the 4f level doesn't really fill up before the 5d level, then why do lower levels fill up in weird orders like (1s²,2s²,2p⁶,3s²). Eg. Why does it jump from s to p then back to s?

It is all about orbital energy level.

 

[jumbogala]

So is Gd an exception to the rule, then? How do I know then something is energetically better?

 

[Borek]

So is Gd an exception to the rule, then? How do I know then something is energetically better?

It is exception, and honestly, the only way to know these expception is to remember them. There are a few of them, less then five if I recall correctly. Any reasonably good general chemistry book should list them in the electron configuration section.