How are elements organized in the same period of the periodic table?

Elements in the same period of the periodic table are organized based on the number of electron shells they possess, which is directly related to their energy levels. Each period in the periodic table corresponds to the filling of a particular shell of electrons. For instance, all elements in the first period have one electron shell, while those in the second period have two electron shells. As you move from left to right across a period, the elements have an increasing number of protons and electrons, yet they maintain the same number of occupied electron shells. This is a key feature of periodic organization; it reflects how electron configurations change within the same principal energy level.

Consequently, the reason elements in the same period have different chemical properties or reactivities stems from their varying numbers of valence electrons, which are also related to the electron configuration, rather than the number of shells. Options suggesting atomic masses, comparable reactivity, or the number of protons pertain to different classifications within the periodic table and do not define the fundamental organization of periods.