Answer is: 4s orbital has lower energy than 3d orbital.
Aufbau principle is a scheme used to reproduce the electron configurations of the ground states of atoms by successively filling subshells (orbitals) with electrons in a specific order.
Filling subshells in the following order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f.
For example, atom of calcium.
Atomic number of calcium is 20, it means that it has 20 protons and 20 electrons, so atom of calcium is neutral.
Electron configuration of calcium atom: ₂₀Ca 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² or
[Ar] 4s²; last two electrons go to 4s orbital, not in 3d.
