- The document discusses the early attempts to classify elements, including by Lavoisier, Dobereiner, and Newlands. It then focuses on Mendeleev's periodic table from 1869, which arranged elements by atomic mass and allowed for predictions of undiscovered elements. - The modern periodic table, developed based on Moseley's work relating element properties to atomic number rather than mass, is now used. It represents an improved understanding of periodic trends and the atomic-level basis for element properties.