[N64 Gamer]

My textbook has a figure like the one above.

But for some reasons it doesn't always work. Like for example the electron configuration for Cadmium. According to my facits:


I don't understand why the 3d^10 comes before the 4s^2

According to the figure, that shouldn't be the case?

I remember a long time ago I heard that the figure above does not count for transition metals? Or was it ions? I don't remember it exactly and my textbook gives me nothing, so could somebody please verify that and give me some explanation? When can I use the figure above and when can't I? And if I can't use the figure above, what other methods do I use?

[Borek]

Filling order is not necessarily reflected in the notation, it is common to group the orbitals according to the main quantum number.

In other words:

1s²2s²2p⁶3s²3p⁶4s²3d⁵

and

1s²2s²2p⁶3s²3p⁶3d⁵4s²

mean exactly the same.

There are some minor exclusions from the Aufbau principle, but I guess that's now what you are looking for.

[Corribus]

This might help:

http://www.chemguide.co.uk/atoms/properties/3d4sproblem.html

[N64 Gamer]

Filling order is not necessarily reflected in the notation, it is common to group the orbitals according to the main quantum number.

In other words:

1s²2s²2p⁶3s²3p⁶4s²3d⁵

and

1s²2s²2p⁶3s²3p⁶3d⁵4s²

mean exactly the same.

There are some minor exclusions from the Aufbau principle, but I guess that's now what you are looking for.

Actually, my book mentions the Aufbau principle but they don't explain it well enough. I think it might be the Aufbau principle that got me confused.
I remember there was a technique where you had to count how many columns an element was from the noble gas group.
I'm not sure if you know what method I'm referring to.

[antimatter101]

Shells don't necessarily have the same energy level, thus giving rise to hypervalence like in sulphuric acid and phosphoric acid.