How is the periodic table primarily organized?

The periodic table is primarily organized by the periodic law, which states that the properties of elements are a periodic function of their atomic numbers. This means that when elements are arranged in order of increasing atomic number, elements with similar chemical properties appear at regular intervals, or "periods." The periodic law highlights the systematic trends in properties such as electronegativity, atomic radius, and ionization energy.

While atomic weight was historically used to organize the table, it has since been replaced by atomic number because the atomic number directly correlates with the number of protons in the nucleus of an atom, which is fundamental to the identity of the element. Chemical properties also play a significant role in how elements are grouped, but the primary framework for organization is based on the periodic law related to atomic number. The number of electrons can influence an element's behavior but is not the basis for the structure of the periodic table itself.