There are six elements in Group VIIA, the next-to-last column of the periodic table. As expected, these elements have certain properties in common. They all form diatomic molecules (H2, F2, Cl2, Br2, I2, and At2), for example, and they all form negatively charged ions (H-, F-, Cl-, Br-, I-, and At-).
When the chemistry of these elements is discussed, hydrogen is separated from the others and astatine is ignored because it is radioactive. (The most stable isotopes of astatine have half-lives of less than a minute. As a result, the largest samples of astatine compounds studied to date have been less than 50 ng.) Discussions of the chemistry of the elements in Group VIIA therefore focus on four elements: fluorine, chlorine, bromine, and iodine. These elements are called thehalogens(from the Greekhals, "salt," andgennan, "to form or generate") because they are literally the salt formers.