The periodic table's most electronegative element is fluorine (3.98 on the Pauling scale and top right in the periodic table). The least electronegative element is cesium (0.79 on the Pauling scale ...

Mendeleev discovered the periodic table (or Periodic System, as he called it ... but Mendeleev saw that iodine was very similar to the rest of the halogens (fluorine, chlorine, bromine), and tellurium ...

The zigzag, staircase, line in the diagram of the periodic table above separates the metals, on the left, from non-metals, on the right. Hydrogen is a non-metal, but it is often put in the middle.