Chemistry: Number of electrons in the shells...?

Posted on 2016-03-11 18:42:00 in Chemistry by ZORRO

I'm currently teaching grade 9 ESL students about chemistry, and when it comes to atoms and their electron shells, I ran into a problem. I know that the first shell has two electrons max, the second has eight max and the third has 18 max, with 32 in the fourth. It WAS my understanding that a shell must be complete before a new shell can begin to fill up. 
HOWEVER, some elements have shells which are like this: 2, 8, 8, 1 (Potassium). 
Why isn't the third shell filled before the fourth is started?

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