Dmitri Mendeleev was a Russian chemist best known for creating the Periodic Table of Elements. Born in 1834, he organized elements based on their atomic mass and properties, which helped scientists understand how different elements relate to one another. His work laid the foundation for modern chemistry and allowed for the prediction of undiscovered elements. Mendeleev's table was revolutionary because it showed that elements with similar properties appeared at regular intervals. This pattern revealed the periodic nature of elements, leading to the concept of periodicity. His contributions have had a lasting impact on science, influencing future chemists like Henry Moseley and Glenn T. Seaborg.