Why does one electron in a potassium atom go into the fourth energy level instead of squeezing into the third energy level along with eight alredy?

Answer 1

The aufbau principle (text books may also call it the Madelung ordering or the Klechkowski rule) predicts the order in which orbitals are filled. The 4s orbital is filled before the 3d orbitals. Wikipedia has an article see link

Answer 2

Each electron in an atom occupies the lowest energy level possible before moving to higher ones. Since the third energy level is already filled with eight electrons, the next electron must move to the fourth energy level to minimize repulsion between electrons and maintain stability of the atom. This follows the principle that electrons arrange themselves in a way that maximizes stability and minimizes energy.