down a group (or column or family). Thus, the most metallic elements (such as caesium and francium ) are found at the bottom left of traditional periodic tables and the most nonmetallic elements ( oxygen , fluorine , chlorine ) at the top right. The combination of horizontal and vertical trends in metallic character explains the stair-shaped dividing line between metals and nonmetals found on some periodic tables, and the practice of sometimes categorizing several elements adjacent to that line, or elements adjacent to those elements, as metalloids . [ 41 ] [ 42 ] History First systemization attempts In 1789, Antoine Lavoisier published a list of 33 chemical elements , grouping them into gases , metals , nonmetals , and earths ; [ 43 ] Chemists spent the following century searching for a more precise classification scheme. In 1829, Johann Wolfgang Döbereiner observed that many of the elements could be grouped into triads based on their chemical properties. Lithium , sodium , and potassium , for example, were grouped together in a triad as soft, reactive metals. Döbereiner also observed that, when arranged by atomic weight, the second member of each triad was roughly the average of the first and the third; [ 44 ] this became known as the Law of Triads . [ 45 ] German chemist Leopold Gmelin worked with this system, and by 1843 he had identified ten triads, three groups of four, and one group of five. Jean-Baptiste Dumas published work in 1857 describing