Chemistry,atomic structure

The third shell can held 2n^2 = 2(3)^2 = 18 electrons.

I think you are a Form 4 student. It can held maximum of 18 electrons. In transition elements like Sc, Ti, V, Cr, Mn ....... Zn, there are more than 8 electrons in the third shell. But it is not a good idear to talk about it in form 4. It requires to use another notation like 1s, 2s, 2p ........... When you become a Form 6 student, you are going to discuss this matter using those notations.

2006-11-26 21:40:18 補充：
Sorry ! idear is typing error. it is idea

i think it can let Ca to become stable more easily(only lose 1 shell can be stable)
but my teacher said that this is for some kind of 'safety reason'