Final answer:
According to the Aufbau principle, electrons fill atomic orbitals from the lowest to the highest energy levels. The last sublevels to fill would generally be the 7p or 5f sublevels when considering known elements. The 4f sublevel starts filling after the 6s sublevel.
Step-by-step explanation:
According to the Aufbau principle, the order in which sublevels fill is determined by their increasing energy levels. The principle indicates that electrons occupy the lowest energy orbitals available before filling higher energy orbitals. Based on the given information, the sublevels fill in the following order: 1s, 2s, 2p, 3s, 3p, and 4s. The 3d sublevel is logically expected to fill after the 3p, but the 4s is actually slightly lower in energy, so the 4s fills before the 3d. After the 4s sublevel is filled, electrons then fill the 3d sublevel, followed by 4p, then 5s, and so on. The 4f sublevel, which is filled after the 6s sublevel, is one of the last to be filled.
The Aufbau principle is a guide to the sequence of orbital occupation and is sometimes referred to as the 'building up' principle. It starts with the lowest energy sublevel, which is 1s, occupied by the hydrogen atom's single electron in its ground state. As atoms become larger, electrons are added according to the sublevels' increasing energy, respecting the Pauli exclusion principle and quantum numbers.
To summarize, the sublevels are filled in an order that starts from the lowest energy and progresses to higher energies. Although the 4f sublevel does not begin to fill until after the 6s sublevel, the actual sublevels filled last are those at the highest energies, which would typically be the 7p or 5f sublevels, depending on how far along the periodic table one is considering. For elements currently known, 7p would be the last sublevel to fill.