FLUORINE
Fluorine atoms have nine electrons, one fewer than neon, and electron configuration 1s2 2s2 2p5 : two electrons in a filled inner shell and seven in an outer shell requiring one more to be filled. Fluorine's first ionization energy is third-highest among all elements, behind helium and neon.Fluorine atoms have a small covalent radius of around 60 picometers, similar to those of its period neighbors oxygen and neon. The bond energy of difluorine is much lower than that of either Cl2 or Br2 and similar to the easily cleaved peroxide bond.At room temperature, fluorine is a gas of diatomic molecules.It has a characteristic halogen-like pungent and biting odor detectable at 20 ppb. Fluorine condenses into a bright yellow liquid at −188 °C (−306 °F), a transition temperature similar to those of oxygen and nitrogen.
