Period 1 element lithium and fluorine (appearing twice), or left floating above the other elements and not assigned to any group in the periodic table. Helium is almost always placed above neon (which is in the p-block) in the periodic table as a noble gas, although it is occasionally placed above beryllium due to their similar electron configuration. Hydrogen (H) is the chemical element with atomic number 1. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H. With an atomic mass of 1.00794 amu, hydrogen is the lightest element. Hydrogen is
Period 3 element characteristic odor to rotting eggs and other biological processes. "Chlorine" (symbol Cl) is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine. It has the highest electron affinity and the third highest electronegativity of all the elements; for this reason, chlorine is a strong oxidizing agent. The most common compound of chlorine, sodium chloride, has been known since ancient times; however, around 1630, chlorine gas was obtained by the Belgian chemist and physician Jan Baptist van Helmont. The synthesis and characterization of elemental chlorine occurred in 1774