The halogen elements all form compounds with hydrogen, the hydrogen halides.
The energy of the hydrogen-halogen bond increases strongly from iodide to fluoride.
The free element is widely used as a water-purification agent, and it is employed in a number of chemical processes.
Sodium chloride, of course, is one of the most familiar chemical compounds.
The energy released in the formation of an ion from a free atom and an electron (brought up from an infinite distance) is called the electron affinity.
Fluorine (Fmolecular lattices, and the sublimation energies rise with increasing size of the molecules.Fluorides are known chiefly for their addition to public water supplies to prevent tooth decay, but organic fluorides are also used as refrigerants and lubricants.Iodine is most familiar as an antiseptic, and bromine is used chiefly to prepare bromine compounds that are used in flame retardants and as general pesticides.Hydrogen fluoride in the crystalline state consists of infinite zigzag chains, as shown in the diagram, in which H represents the hydrogen atoms and (as before) F the fluorine atoms; the solid lines represent covalent bonds between the hydrogen and fluorine atoms within the molecules, and the dotted lines represent secondary bonds, called hydrogen bonds.The hydrogen bonds between hydrogen fluoride molecules are considerably weaker (7 kilocalories per mole) than those within the molecules (135 kilocalories per mole), yet they are retained to a great extent in the liquid state.