Group 17 elements are collectively known as the Halogens. This name is derived from the Greek words hals (“salt”) and -gen (“to produce”), translating to “salt-former.” The designation Group 17 is the modern system established by the International Union of Pure and Applied Chemistry (IUPAC). Positioned on the far right of the periodic table, immediately next to the noble gases, these elements share a strong drive to react and form compounds, often ionic salts.
The Elements and Their Physical Diversity
This group contains six elements: Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), Astatine (At), and the synthetic element Tennessine (Ts). The four lightest members exhibit a unique physical progression at standard temperature and pressure. They are the only group on the entire periodic table to contain elements naturally found in all three primary states of matter.
Fluorine and chlorine exist as gases, appearing as a pale yellow and a greenish-yellow color, respectively, under normal conditions. Moving down the column, bromine is a dense, reddish-brown liquid, making it one of only two elements liquid at room temperature. Iodine is a lustrous purple-black solid that easily sublimes, transitioning directly into a violet gas. The heaviest naturally occurring halogen, astatine, is a radioactive solid whose properties are largely inferred due to its extreme scarcity.
Defining Chemical Characteristics
The defining feature of the halogens is their electron configuration, which includes seven valence electrons in the outermost shell. Since they are just one electron short of the stable, full-shell arrangement of a noble gas, they have a powerful tendency to attract and gain a single electron from other atoms.
This strong electron-attracting ability results in high electronegativity; fluorine is the most electronegative of all elements. Because they readily accept electrons, halogens function as potent oxidizing agents. Their high reactivity means they are never found in their pure, elemental form in nature, always occurring as compounds.
Reactivity decreases as one moves down the group from fluorine to iodine. This trend is due to increasing atomic size, which places the outermost electron shell farther from the nucleus. The greater distance and increased shielding reduce the nucleus’s attractive pull on an incoming electron, making the larger elements less eager to react.
Essential Real-World Applications
The powerful, electron-seeking nature of halogens makes them indispensable in a wide range of industrial and consumer applications. Chlorine is widely recognized, primarily used as a disinfectant in water treatment and swimming pools to eliminate bacteria. Its compounds are also utilized in the production of household bleach and in manufacturing plastics like polyvinyl chloride (PVC).
Fluorine compounds, specifically fluoride, are incorporated into toothpastes and public water supplies to promote dental health. Fluoride ions strengthen tooth enamel by converting hydroxyapatite into the harder, acid-resistant fluorapatite mineral. Fluorine is also a component in the production of polytetrafluoroethylene (PTFE), known commercially as Teflon, prized for its chemical inertness and non-stick properties.
Bromine is often used in compounds that act as flame retardants, added to textiles, plastics, and electronic casings to suppress combustion. Iodine is applied extensively in medicine as a topical antiseptic and disinfectant. Iodine is also an essential element for human nutrition, required by the thyroid gland for the production of hormones that regulate metabolism.