The periodic table of elements is an ordered arrangement of all known chemical elements, organized in rows called periods and columns called groups, based on increasing atomic number (the number of protons in the nucleus). Elements with similar chemical properties are grouped together in the same columns, reflecting recurring trends in their behavior. The table visually represents the periodic law, which shows that when elements are arranged by atomic number, their properties recur in a predictable pattern. It serves as a fundamental tool in chemistry and other sciences for understanding and predicting element properties. The table currently lists 118 known elements, with the first 94 found naturally on Earth and others synthesized in labs. It was first broadly accepted with Dmitri Mendeleev's 1869 version, which he used to predict undiscovered elements, and has been refined over time with advancements in atomic theory and quantum mechanics.