Final answer:
Any element in the second period of the periodic table has 2 occupied principal energy levels. The 2s sublevel fills first, then the 2p sublevel as one moves across the period, ending with neon which completes this energy level.
Step-by-step explanation:
The element that has 2 occupied principal energy levels is any element in the second period of the periodic table. This is because the second period marks the filling up of the second principal energy level. Starting with lithium (Z = 3), which adds one electron to the 2s sublevel, each subsequent element in the period increases the number of electrons within the second level. When we reach beryllium (Z = 4), the 2s sublevel is filled and the 2p sublevel begins to fill with boron (Z = 5). The 2p sublevel has three orbitals, and each orbital can hold two electrons, leading to a total of six elements with their outermost electrons in the 2p sublevel. The process culminates with neon (Z = 10), which completes the second principal energy level with a 2s²2p⁶ electron configuration, indicating a full octet.