[jmaxwell98]

Calcium's electron configuration is 2,8,8,2. Why isn't it 2,8,9,1? According to the Aufbau Principle electrons are placed in the energetically lowest shell. The third shell is energetically lower than the fourth and can hold up to 18 electrons.

You can see this on other elements too. It appears that electrons could be placed in a lower electron shell but they aren't. Also, does anyone know why Palladium has 18 valence electrons? For other elements the limit is 8

Thanks!

[Hunter2]

It is the octett law. The last shell can only hold 8 Electrons, all noble gases beside Helium have that.

[kriggy]

Why it should be 2-8-9-1? Its
1s2
2s2 2p6
3s2 3p6
4s2

you cant fill 3rd layer with 1 more electron, there is no space left.
Paladium doesnt have 18 valence electrons, only 10.
What do you mean by "It appears that electrons could be placed in a lower electron shell but they aren't" ? Do you mean irreguralities in filling the d shell? like Ru is 4d7 5s1 instead of 4d6 5d2?