What is the definition of the period number?

The period number in the Periodic Table refers to the horizontal rows that indicate the highest principal energy level of the electrons in an atom. Each period corresponds to the filling of a different electron shell, with elements in the same period having the same number of electron shells. For example, elements in period 1 have one electron shell, while those in period 2 have two. The period number helps categorize elements based on their electron configuration and overall chemical properties.