Metals, Metalloids, and Nonmetals

Halogens and Noble Gases

The elements found in group 17 are known as the halogens and have 7 electrons in their outer shell. The elements found in group 18 are known as the noble gases and have a complete number of electrons in their outer electron orbital, called a valence shell, making them stable and mostly chemically inert.

A halogen is an element in group 17 of the periodic table. Halogens generally have very high electronegativity values. As an ion, fluorine always has a –1 oxidation state and is extremely reactive. Electronegativity is higher for elements near the top of the periodic table, with fluorine as the most electronegative element found on the periodic table. The periodic table shows a trend of decreasing electronegativity as elements are listed to the left in each row and down each column.

Commercially, the most important halogen is chlorine. Chlorine is a component of polyvinyl chloride (PVC) and other similar plastics. It is also used to form sodium hypochlorite (NaClO), which is used in bleaching in multiple industries. Fluorine is used to prepare a series of compounds called fluorocarbons. Fluorocarbons are used as refrigerants and in plastics. Teflon, a synthetic resin used to coat nonstick cooking utensils, is an example of a fluorocarbon. Iodine is used as an additive to table salt. Iodized salts include potassium iodide (KI). Bromine is used in flame retardants in small amounts. Astatine, the largest halogen, is very rare and highly radioactive. Astatine does not have a commercial use.

Fluorine is mined from minerals, with fluorite (CaF2) being the most common source of fluorine. Chlorine, bromine, and iodine are found in seawater and also in salt deposits.

An interhalogen is a compound that consists of more than one halogen element. Examples include chlorine monofluoride (ClF) and iodine pentafluoride (IF5). Interhalogens typically have one chlorine, bromine, or iodine atom surrounded by fluorine atoms. Chlorine and bromine atoms are small and cannot hold many fluorine atoms around them. An iodine atom is larger and can hold up to seven fluorine atoms. Interhalogen compounds are strong oxidizing agents.

A noble gas is an element found in group 18 of the periodic table. These nonmetals are gases at room temperature. Because noble gases are very stable, they are also very unreactive. With the exception of helium, noble gases have eight electrons in their outer shell. Since this outer shell is full, they do not attract more electrons and they do not have electronegativity. It is possible to react a noble gas with a highly electronegative element, such as oxygen or fluorine under extreme pressures and temperatures in laboratories. Compounds containing xenon, oxygen, and fluorine are common, including XeF2, XeF6, XeO3, and XeOF4.

Commercially, helium is the most important noble gas. Liquid helium is used as a coolant in extremely low-temperature experiments. Helium boils at 4.2 K (Kelvin). Neon, argon, krypton, and xenon are used in lighting and lasers. Argon is used in electric light bulbs to provide an inert atmosphere in the bulb. Argon is also used in welding. Since argon is a noble gas, it can be used as a protective atmosphere for stable substances that would react with water or oxygen.

Helium is found in some natural gas reserves and is obtained from natural gas wells. Neon, argon, krypton, and xenon are found in trace amounts in the air. These gases can be obtained by distillation. Radon is a radioactive gas and has a short half-life.