Final answer:
The 1st Principal Energy Level (PEL) contains only the 1s sublevel. The 1s sublevel is filled first according to the Aufbau principle, and it can hold two electrons in its single orbital.
Step-by-step explanation:
The question pertains to the electron configuration in atoms, specifically the sublevels that fit in the 1st Principal Energy Level (PEL). The 1st PEL contains only one sublevel, the 1s sublevel. According to the Aufbau principle, this sublevel is the lowest in energy and is always filled first. It includes a single s orbital that can hold up to two electrons. When we talk about constructing the electron configuration of an atom, or understanding how electrons are distributed among the orbitals, we use the Aufbau principle to fill the sublevels in order of increasing energy.
The 1s sublevel, being the lowest in energy, is filled with the two electrons before any higher sublevels are considered. As we move up to higher elements with more electrons, these electrons fill the next lowest energy sublevels following the sequence dictated by increasing energy levels.