How are elements arranged in the modern periodic table?

Elements in the modern periodic table are arranged by increasing atomic number, which corresponds to the number of protons in an atom's nucleus. This organization creates a systematic arrangement that reflects the periodic law, which states that the properties of elements are a periodic function of their atomic numbers. As one moves across a period or down a group in the periodic table, not only do the atomic numbers increase, but this arrangement also reveals trends in elemental properties, such as electronegativity, ionization energy, and atomic radius.

The use of atomic number over atomic mass stems from the realization that atomic mass can vary due to isotopes, which may lead to inconsistencies in the arrangement. Therefore, organizing elements by atomic number provides a more reliable and consistent framework for understanding chemical behavior and relationships among elements. This method distinctly highlights the periodicity observed in elemental properties.