What is determined by the period number?

The elements in the periodic table refer to groups and periods. Periodic table groups are columns. The periods of the periodic table are rows. Elements of the same period have the same principal quantum number, which describes both the size and energy of the outermost electron shell of an atom. The period number is the number that is given to a group of elements in the periodic table that have made a circle from the completion of its outer electron shell. This pattern usually starts with a Group I element and ends with a Group 8 element.