StellarOrbitals
As mentioned previously, the electrons orbit the nucleus at fixed positions. The electron orbitals are split into separate shells, which are numbered from 1 to 7. Shell 1 is the closest to the nucleus while Shell 7 is the furthest. However, they are broken down even further into subshells, which go s, p, d, f. The s subshell can hold 2 electrons, p can hold 6 electrons, d can hold 10 electrons, and f can hold 14 electrons. Note that only certain subshells can exist, which will be explained in the next paragraph.
The Aufbau Principle states electrons will fill up these shells in order of least energy to most energy. From Lesson 2.1, you know that least energy means closer to the nucleus while most energy means further from the nucleus. Due to some of the subshells crossing over with the other subshells, the order in which they fill up will look like this: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s.