Q.How many days pass between two successive full moon nights?
What type of motion does a pendulum exhibit?
Give some other examples of periodic events.

Mendeleev's Periodic Law: the properties of elements are a periodic function of their atomic weights.

Dmitri Mendeleev, in 1869, arranged the then-known elements in a table in order of increasing atomic weight, and observed that elements with similar chemical and physical properties recurred at regular (periodic) intervals in this arrangement. He stated this observation as a law: the properties of elements are a periodic function of their atomic weights. This allowed him to group elements into families (columns) and even predict the properties of then-undiscovered elements (like eka-silicon = germanium) by leaving gaps in the table.

…

Modern periodic law states that the physical and chemical properties of elements repeat periodically when the elements are arranged in order of increasing atomic number.

Mendeleev's original periodic law (1869) was based on atomic mass, which led to a few anomalies (e.g., Ar and K, Co and Ni appearing out of mass order). Henry Moseley (1913), through his X-ray spectra studies, showed that the atomic number (number of protons/nuclear charge) — not atomic mass — is the fundamental property that determines an element's pr …