Question 5
Potassium has 19 electrons. Why is its configuration 2, 8, 8, 1 rather than 2, 8, 9?
The right answer is the one marked above.
Because when a level is the last one holding electrons in an atom, the maximum it can hold is 8. With 8 already in the third level, the next electron must start the fourth.
The third level really can hold 18 — but only once it is no longer the outermost level, which happens beyond the twenty elements you study here. And potassium is in period 4 because its electrons occupy four levels; that is a consequence of the rule, not the reason for it.