In the modern periodic table, the number of period of the element is the same as

The correct option is principal quantum number.

In the modern periodic table, elements are arranged in horizontal rows called periods and vertical columns called groups. There are seven periods in total, numbered 1 to 7.

The period number of an element corresponds to the highest occupied energy level of that element's atoms in its ground state. In quantum mechanics, the main energy level or shell of an electron is designated by the principal quantum number, represented by the symbol n.

For example, if an element is in Period 3, it means the highest energy level that contains electrons in its ground-state configuration is the third shell, which corresponds to a principal quantum number of $n=3$.
Similarly, for Period 4, the highest energy level being filled is characterized by $n=4$.

Therefore, the number of the period of any element is directly equal to the maximum value of the principal quantum number (n) of the electrons in that element's atoms.