1. Early chemists attempted to classify elements based on similarities in their properties, including Dobereiner who grouped elements into triads and Newlands who arranged them in octaves based on increasing atomic mass. However, these classifications had limitations as they could not accommodate all known elements. 2. Mendeleev organized the periodic table with elements arranged in order of increasing atomic mass and proposed that elements with similar properties would fall into the same groups. His periodic table allowed for predictions of unknown elements and became widely accepted. 3. Modern periodic tables are based on atomic number rather than mass and include additional periodic trends like atomic size, metallic character, and valency that help explain elements' properties. The current 18-