The highest occupied level is the outermost level of the atom that contains electrons. Atoms have several levels much like a house, and each level of the atom fills with electrons level by level starting at the lowest. The first level of the atom fills up first. Then, the second level fills up and so forth. Not every atom has all of its levels occupied by electrons. For some atoms, the highest occupied level is the first level, while for others it is the third level.
Atoms fill up their electron orbitals according to the Aufbau Principle, which states that electrons fill up lower energy levels first. For instance, 1s, the first energy level, holds two electrons, and it is filled before the second energy level, 2s, receives any electrons. The energy level 2s holds eight electrons and must be filled before the next level is able to accept electrons. This process continues for each level. For example, the third level, 2p, holds 18 electrons, but it won’t accept any electrons until 2s is full. Generally, most atoms follow this rule, which dictates that each level must reach full capacity before the next level can begin to be filled. However, Hund’s rule explains that there are some exceptions to this rule. When an electron has an option of entering two orbitals with the same energy level, it prefers to occupy a third orbital that is empty. As a result, sometimes an atom can have a fourth energy level with electrons even though the third level is not yet full.