Final answer:
The only element that does not fill the 1s sublevel is hydrogen, which has a single electron in the 1s sublevel as 1s¹, while helium is the element that completes it with the configuration 1s².
Step-by-step explanation:
According to the Aufbau principle, the only element that does not fill the 1s sublevel is the first element in the periodic table, hydrogen. Hydrogen has only one electron, which occupies the 1s sublevel as 1s1. In contrast, the second element, helium, completes the 1s sublevel with a configuration of 1s2. Starting from lithium, the next available sublevel starts to fill, in this case the 2s sublevel. As per the Aufbau process, electrons fill up orbitals in order of increasing energy, and subsequent electrons will occupy higher energy sublevels such as 2s, 2p, 3s, and continue as such following the patterns that are dictated by their energy levels and the orbital filling diagrams.