Final answer:
The filling order of electrons is based on observed experimental results and confirmed by theoretical calculations.
Step-by-step explanation:
The filling order of electrons is based on observed experimental results, and has been confirmed by theoretical calculations. As the principal quantum number, n, increases, the size of the orbital increases and the electrons spend more time farther from the nucleus. Within each shell, as the value of 1 increases, the electrons are less penetrating, in the order s > p > d > f. Electron filling follows a specific order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, and so on.