Question

Why is the arrangement of electrons in energy levels levels for the ground state configuration of potassium 2,8,8,1 not 2,8,9

Answer 1

since in this case 4s is filled first, and not 3d orbital. The sum of quantum numbers is less for 4s than for 3d => it is the 4s sublevel that will be filled first with electrons, although it is formally located farther from the core than 3d.