Final answer:
The third Principal Energy Level includes the 3s, 3p, and 3d sublevels. The 3s fills first, followed by the 3p, which consists of three orbitals. The 4s is lower in energy than the 3d and fills before the 3d sublevel.
Step-by-step explanation:
The question seeks to know what sublevels are present in the third Principal Energy Level (PEL). In the third PEL, the following sublevels can fit: 3s, 3p, and 3d. First, electrons fill the 3s sublevel. Once the 3s is full, they proceed to fill the 3p sublevel, which consists of three orbitals and can accommodate up to six electrons.
Although it might seem that the 3d should follow, the 4s sublevel is actually lower in energy and gets filled before the 3d. The Hund's rule is applied when filling orbitals within the same sublevel, ensuring that electrons with the same spin occupy individual orbitals before pairing up.