Group 17 on the periodic table is a column of elements known as the halogens, a name derived from Greek roots meaning “salt-former.” This refers to their strong tendency to react with metals to create various salts, such as sodium chloride. The group includes fluorine, chlorine, bromine, iodine, astatine, and the synthetic element tennessine. Halogens are unique because they feature elements existing in all three classic states of matter at standard temperature.
The Defining Chemical Properties of Halogens
The chemical behavior of Group 17 elements is governed by their electron configuration, as each possesses seven valence electrons in its outermost shell. This structure is just one electron short of the highly stable, eight-electron configuration found in the noble gases. Because of this electron deficiency, halogens exhibit an exceptionally strong tendency to attract and gain an additional electron from other atoms.
Halogens are the most electronegative elements on the periodic table, with fluorine holding the distinction as the most electronegative. When a halogen atom successfully gains an electron, it forms a halide ion with a negative one charge, achieving the stable octet. Halogens are extremely reactive and are rarely found in nature as free, single atoms.
To achieve stability, halogens exist naturally as diatomic molecules, such as \(F_2\) or \(Cl_2\). In compounds with metals, the halogen typically exhibits a negative one oxidation state. However, chlorine, bromine, and iodine can also display positive oxidation states (one, three, five, or seven) when bonded to more electronegative atoms like oxygen. Astatine and tennessine are highly unstable and radioactive, with properties deviating somewhat from the lighter halogens.
Physical States and Observable Trends
At standard room temperature, the lightest halogens, fluorine and chlorine, exist as gases. Bromine is a reddish-brown liquid, making it one of only two elements that is liquid at room temperature. Iodine is a dark violet or grayish-black solid.
This change in state is mirrored by a trend in color, which generally becomes darker down the group, ranging from the pale yellow gas of fluorine to the deep violet vapor of iodine. The transition from gas to liquid to solid is directly related to the increasing strength of intermolecular forces between the diatomic molecules. As the atoms become larger and have more electrons moving down the group, the temporary attractive forces known as van der Waals forces become stronger.
Consequently, the melting and boiling points of the halogens consistently increase from fluorine to iodine. More energy is required to overcome these stronger forces of attraction between the larger molecules, which explains why the heavier elements require higher temperatures to change their physical state. This predictable physical trend contrasts with their chemical reactivity, which decreases as the atoms get larger and the outer electrons are farther from the nucleus.
Essential Role in Daily Life and Industry
Halogen compounds are found in countless applications. Chlorine’s strong oxidizing power makes it a widely used disinfectant for treating municipal drinking water and swimming pools, effectively killing harmful bacteria and microorganisms. It is also employed on a massive scale in industrial processes, including the production of polyvinyl chloride (PVC) plastic, which is used in pipes and construction materials.
Fluorine is best known for its role in dental health, where fluoride ions are added to toothpaste and sometimes public water supplies to strengthen tooth enamel and prevent decay. Fluorine-containing compounds are used to create specialized materials, such as polytetrafluoroethylene (PTFE), widely known as Teflon, which is valued for its non-stick and chemical-resistant properties.
Iodine is a necessary component for the thyroid gland to produce hormones that regulate metabolism. To ensure adequate intake, iodine is often added to table salt, which is referred to as iodized salt. Iodine is also an effective antiseptic, used medically as a topical solution to prevent infection in wounds.
Bromine is utilized in flame retardants to reduce the flammability of materials like plastics and textiles. Compounds containing bromine, such as silver bromide, were historically essential components of photographic film. Bromine is also used in some water purification systems as an alternative disinfectant to chlorine.