Final answer:
The 4th to 7th PELs start filling with their respective 's' sublevels, followed by 'd', and 'p' sublevels with d and f sublevels being filled one and two PELs behind respectively.
Step-by-step explanation:
Sublevels in the 4th to 7th Principal Energy Levels (PEL)
The sublevels that fit within the 4th to 7th principal energy levels (PEL) follow a specific order based on the increasing energy of sublevels. The 4th PEL begins filling with the 4s sublevel, followed by the 3d and then the 4p sublevel. The 5th PEL starts with the 5s sublevel, continues with the 4d sublevel, and is succeeded by the 5p. The 6th PEL introduces a new sublevel, filling the 6s followed by the 4f, which accommodates lanthanides, and afterward is the 5d and finally the 6p. In the 7th PEL, after the 7s is filled, the 5f sublevel starts filling (which includes the actinides), and it is followed by the 6d and the 7p sublevels. It is important to remember that the d and f sublevels are actually filled one and two principal energy levels behind, respectively. This means the 3d sublevel fills during the 4th PEL, the 4f sublevel fills during the 6th PEL, and accordingly for higher sublevels.
Each sublevel consists of a varying number of orbitals that can hold up to two electrons each. The energy overlaps between these sublevels mean the order of electron filling does not strictly follow the numerical order of the PELs.