Halogen
The halogens are the nonmetallic elements found in Group 17 of the periodic table of the elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). The name "halogen" (Greek halos, salt and -genes, forming) is derived from their tendency to form a salt when reacted with a metal.
Chemistry
The valence shells of halogen atoms need only one more electron to form a completely filled shell of either eight or eighteen electrons. As such, the halogens have a strong tendency to form ions with a charge of -1 in order to complete their valence shells. They also do not exist as isolated atoms in their elemental forms; instead, they each share one electron with one other atom to form diatomic molecules.
A third consequence of the halogens' nearly filled valence shell is high reactivity towards cationic or low-ionization-potential species. Halogen anions in the body can bind to potassium, calcium and magnesium cations, rendering them useless. This is particularly true for fluoride ions, which can easily diffuse through the skin and enter the bloodstream.
Hydrohalic acids, diatomic compounds of one hydrogen atom and one halogen atom, are particularly strong owing to high halogen electron affinity. Care should be taken when handling these acids, particularly at high concentrations.
The halogens are found in some hypervalent compounds (eg. SF6) and were instrumental in the formation of the first compounds of the noble gases (eg. XEF4). They can also react with each other to form the so-called interhalogens.
Properties
The halogens demonstrate several periodic trends very well. Their electronegativity and electron affinities decrease going down the group, and their sizes, melting and boiling points increase.
| Halogen | Atomic Mass (u) | Melting Point (K) | Boiling Point (K) | Electronegativity (Pauling) |
| Fluorine | 18.998 | 53.53 | 85.03 | 3.98 |
| Chlorine | 35.453 | 171.6 | 239.11 | 3.16 |
| Bromine | 79.904 | 265.8 | 332.0 | 2.96 |
| Iodine | 126.904 | 386.85 | 457.4 | 2.66 |
| Astatine | (210) | 575 | 610 ? | 2.2 |
Fluorine and chlorine are diatomic gases at room temperature, bromine is a liquid, and iodine and astatine are solids. Group 17 is the only group containing elements in all three phases at room temperature.