Final answer:
The first f energy sublevel appears as the 4f sublevel and begins to be populated with electrons after the 6s sublevel, marking the start of the lanthanide series with up to 14 electrons.
Step-by-step explanation:
The first f energy sublevel appears in the periodic system of elements starting with the 4f sublevel. This sublevel begins to fill with electrons after the 6s sublevel is filled, indicating that f sublevels are generally two principal energy levels behind in terms of their position in the energy hierarchy. The 4f sublevel introduction is significant as it marks the beginning of the lanthanides series, which contains elements with atomic numbers ranging from 58 (cerium) to 71 (lutetium). These elements are known for being hidden behind one another in the periodic table, thus the origin of the term derived from Greek. The 4f sublevel can accommodate up to 14 electrons, and this capacity contributes to the characteristic length of the f-block within the periodic table.