What does each period in the periodic table correspond to?

Each period in the periodic table corresponds to the number of total principal energy levels in an atom. As you move down the periodic table, each new period indicates that the elements have electrons in an additional principal energy level, or shell. For example, elements in the first period have their electrons in the first energy level, while elements in the second period have electrons in the first and second energy levels, and so on. This organization reflects the structure of electrons around the nucleus and helps to explain trends in properties such as atomic size and ionization energy across periods.

The other options do not correctly define what a period represents: the number of protons in the nucleus corresponds to the atomic number, the number of valence electrons varies in a more complex way across periods, and atomic mass depends on the isotopes of an element rather than its period in the table. Thus, the number of principal energy levels is the defining characteristic of each period in the periodic table.