- The halogens make up Group 17 of the periodic table and include the elements fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At), with tennessine (Ts) as a synthetic member.
- The name “halogen” comes from the Greek words halo (salt) and genes (forming), meaning “salt-former,” because these elements readily combine with metals to form salts such as sodium chloride (table salt).
- Halogens are a family of nonmetals, although astatine exhibits some metallic characteristics, and they are known for their high reactivity due to having seven valence electrons (ns² np⁵ configuration). This leaves them just one electron short of a stable noble gas configuration, giving them a strong tendency to gain an electron and form –1 anions (halide ions).
- In terms of physical properties, halogens display a distinct trend down the group. Fluorine and chlorine exist as pale yellow and greenish-yellow gases, respectively; bromine is a reddish-brown volatile liquid; iodine is a shiny violet-black solid that sublimates into purple vapor; and astatine is a rare, radioactive element with metallic-like behavior. Their melting and boiling points increase as we move down the group due to stronger intermolecular van der Waals forces between larger molecules. They are diatomic in their elemental form (e.g., F₂, Cl₂), meaning each molecule consists of two atoms.
- Chemically, halogens are among the most reactive nonmetals. Their reactivity decreases down the group: fluorine is the most reactive halogen and one of the most reactive elements in the periodic table, while iodine and astatine are less reactive. They readily form ionic compounds with metals (e.g., NaCl, CaCl₂) and covalent compounds with nonmetals (e.g., hydrogen halides such as HCl, which dissolve in water to form strong acids like hydrochloric acid). Halogens also act as powerful oxidizing agents, with fluorine being the strongest due to its high electronegativity and small atomic size. A key property of halogens is their ability to undergo displacement reactions: a more reactive halogen can displace a less reactive halogen from its salt solution, such as chlorine displacing bromine from sodium bromide.
- Halogens have significant biological and industrial importance. Fluorine is used in toothpaste (as fluoride) to strengthen tooth enamel and in Teflon production. Chlorine plays a vital role in disinfection, water treatment, and the production of plastics like PVC. Bromine is used in flame retardants and photographic chemicals. Iodine is essential for thyroid function in humans, preventing goiter, and is also used as an antiseptic. Astatine, being radioactive and extremely rare, has no widespread applications but is being studied for potential use in targeted cancer treatments. Tennessine, as a synthetic element with a very short half-life, has only been produced in trace amounts, and its properties remain largely theoretical.
- Overall, the halogens are characterized by their high reactivity, tendency to form salts, and strong oxidizing power. Their unique combination of properties makes them essential both in natural biological systems and in a wide range of industrial and medical applications. From disinfecting drinking water to aiding human health and powering chemical industries, the halogens demonstrate the remarkable diversity and importance of Group 17 elements.
