Final answer:
After the 3p sublevel, the next sublevel filled is the 4s sublevel, due to it being lower in energy than the 3d sublevel. Sublevel energy and ordering are essential in determining electron configurations.
Step-by-step explanation:
Following the 3p sublevel, according to the Aufbau principle, the next sublevel to be filled in with electrons is the 4s sublevel. Despite what might seem logical, the 4s sublevel is actually lower in energy than the 3d sublevel, and consequently, electrons fill the 4s orbital before filling the 3d. It's only after the 4s is completely occupied that electrons start to occupy the 3d sublevel.
Sublevel filling order is crucial for understanding electron configurations in chemistry, reflecting how electrons populate atomic orbitals within an atom. Remembering that each p sublevel consists of three orbitals, one should note that when moving beyond the 3p sublevel, the aforementioned energy overlapping of sublevels in different principal energy levels plays a crucial role in determining the order of electron filling.