Final answer:
Electrons in successive atoms on the periodic table tend to fill low-energy orbitals first. The filling order is based on observed experimental results and theoretical calculations. The 4s subshell is filled before the 3d subshell because it has lower energy.
Step-by-step explanation:
Electrons in successive atoms on the periodic table tend to fill low-energy orbitals first. The filling order is based on observed experimental results and 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 l increases, the electrons are less penetrating, meaning there is less electron density found close to the nucleus. Electrons that are closer to the nucleus slightly repel electrons that are farther out, offsetting the more dominant electron-nucleus attractions slightly. For larger atoms, the electron arrangement becomes more complicated because after the 3p subshell is filled, filling the 4s subshell first leads to a lesser overall energy than filling the 3d subshell.