Final answer:
The properties of elements repeat in a pattern when arranged by increasing atomic number, which forms the basis of the modern periodic table.
Step-by-step explanation:
The modern periodic law states that the properties of elements repeat in a regular pattern when the elements are arranged in order of increasing atomic number. This law is the foundation of the contemporary periodic table, where elements are ordered by atomic number and grouped based on similar properties. Dmitri Mendeleev, who initially organized elements by increasing atomic mass, contributed significantly to the development of the periodic table, but it was later found that atomic number is a more reliable organizing principle.
Elements on the periodic table are configured into rows, or periods, and columns, or groups. Each group contains elements that exhibit similar chemical behaviors due to having the same number and distribution of electrons in their valence shells. The periodic variations of element properties such as atomic size, ionization energy, and electron affinity are also important patterns observed in the periodic table.