Dmitri Mendeleev was a Russian chemist best known for creating the first version of the Periodic Table of Elements. In 1869, he organized the known elements based on their atomic mass and properties, which revealed patterns that helped predict the existence of undiscovered elements. His work laid the foundation for modern chemistry. Mendeleev's table was revolutionary because it grouped elements with similar characteristics together. This organization allowed scientists to understand how elements interacted with one another. His predictions about elements like germanium and gallium were later confirmed, showcasing the accuracy of his periodic law and solidifying his legacy in science.