1) The document discusses the early attempts to classify elements, including Lavosiere's classification of metals and non-metals, and Dobereiner's classification of elements into triads based on similarities in atomic mass and properties. 2) It then describes John Newlands' law of octaves, where he observed that elements arranged by atomic mass had repeating properties every eighth element. 3) The main focus is on Mendeleev's periodic table, where he arranged elements by atomic mass and was able to predict properties of undiscovered elements, establishing the periodic law that properties repeat periodically with increasing atomic mass. 4) The modern periodic table arranged by atomic number rather than mass is then discussed, with M