What does the periodic law state?

The periodic law states that the properties of elements are periodic functions of their atomic numbers. This principle was established by Dmitri Mendeleev and later supported by Moseley, who demonstrated that the atomic number, rather than atomic mass, is the more accurate basis for the organization of elements on the periodic table.

As elements are arranged in order of increasing atomic number, patterns emerge in their physical and chemical properties. For example, elements in the same group (column) of the periodic table display similar characteristics, such as reactivity and ionization energy. This observation underscores why the atomic number is pivotal in understanding the relationship between an element's position in the periodic table and its properties, leading to predictions about its behavior and interactions with other elements.

The other options fail to capture the essence of the periodic law—atomic mass is not the basis for periodic trends, and claiming properties are constant disregards the variations observed among different groups. Additionally, the periodic law is universally applicable to all elements, not limited to specific categories like transition metals.