The Halogens belong to Group 17 (7A) of the Periodic Table. They are a family of highly reactive and strongly electronegative nonmetals, whose name (from Greek, “salt-formers”) reflects their strong tendency to react with metals to produce halides, such as sodium chloride (NaCl), the well-known table salt. In their natural state, halogens are found as diatomic molecules (F2, Cl2, Br2, I2).
Which are the Halogens?
- Fluorine (F) – pale yellow gas, the most electronegative element in the Periodic Table;
- Chlorine (Cl) – yellow-green gas, widely used in disinfection;
- Bromine (Br) – volatile liquid, dark reddish-brown at room temperature;
- Iodine (I) – solid, violet-black, sublimates easily;
- Astatine (At) – radioactive, extremely rare, with partially metallic behavior;
- Tennessine (Ts) – a synthetic superheavy element (formerly Ununseptium, Uus), unstable and without practical applications.
Position, configuration, and electronegativity
Halogens are representative p-block elements. Their valence shell ends with ns² np5, giving them 7 outer electrons. They tend to gain 1 electron (forming X−) or share electrons in covalent bonds. Electronegativity (Pauling scale) increases upward in the group (At < I < Br < Cl < F), with Fluorine being the most electronegative of all elements.
Physical states and trends down the group
- Physical state at 25 °C: F2 and Cl2 are gases, Br2 is a liquid, I2 and At are solids;
- Color: darkens from pale yellow (F2) to violet (I2);
- Oxidizing reactivity: decreases down the group (F > Cl > Br > I);
- Melting/boiling points: increase downward (greater mass and stronger van der Waals forces);
- Aqueous solutions: Cl2 undergoes disproportionation (forming HCl and HClO); Br2 and I2 are less soluble, requiring nonpolar solvents.
Characteristic reactivity
- With metals: form ionic halides MX (e.g., NaCl, KBr, CaCl2);
- With nonmetals: produce covalent compounds (e.g., PCl3, SF6);
- With hydrogen: form hydrogen halides HX; in water, HCl, HBr, and HI are strong acids, while HF is a weak acid but extremely toxic and corrosive;
- Displacement/redox: a more reactive halogen displaces a less reactive one from its salts (Cl2 oxidizes I− to I2, for example);
- Interhalogens: halogens bond with each other to form compounds such as ClF3, BrF5, IF7, all strong oxidizers/fluorinating agents;
- With noble gases: Fluorine reacts with Xenon and Krypton to form fluorides (XeF2, XeF4, XeF6, KrF2) under controlled conditions.
Compounds and applications
- Chlorine (Cl2): water disinfection, pool sanitation (hypochlorites), bleaching, PVC production (vinyl chloride);
- Fluorine/fluorides: production of fluoropolymers (PTFE), uranium hexafluoride (UF6), dental treatments (controlled fluoridation);
- Bromine: brominated flame retardants (regulated use), silver bromide in photography, organic synthesis;
- Iodine: iodized salt (nutrition), antiseptics (povidone-iodine), analytical chemistry (iodometry);
- Oxyhalides/oxoacids (e.g., chlorates, perchlorates, iodates) used in specific industrial and chemical processes.
Safety, toxicity, and environmental impact
- All elemental halogens are hazardous: F2 is extremely reactive, Cl2 is toxic and corrosive, Br2 is dangerous and volatile, I2 is less toxic but still irritant, At is radioactive;
- HF (hydrofluoric acid): highly dangerous (penetrates tissues, complexes with Ca2+/Mg2+), requiring extreme precautions;
- Volatile chlorinated/brominated compounds affect the ozone layer (e.g., CFCs/halons, now banned under the Montreal Protocol);
- Handling requires proper ventilation, PPE, compatible materials, and safe neutralization procedures.
Quick notes
- Electronegativity: increases upward; Fluorine is the highest of all elements.
- Oxidizing power: decreases down the group (F > Cl > Br > I).
- Physical states: gases (F, Cl) → liquid (Br) → solids (I, At); colors darken downward.
- Hydrogen halides (HX): HCl/HBr/HI are strong acids, HF is weak but extremely corrosive.
- Interhalogens and Xenon fluorides are examples of advanced halogen chemistry.
FAQ – Frequently Asked Questions about Halogens
Which are the halogen elements?
Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), Astatine (At), and Tennessine (Ts) (synthetic).
Why are they called “salt-formers”?
Because they react readily with metals to form ionic halides (MX), such as NaCl (table salt).
What is their typical electronic configuration?
Valence shell ns² np5, with 7 outer electrons; they tend to gain 1 electron (X−) or share electrons covalently.
Are all halogens toxic?
In their elemental forms, yes. However, Iodine is nutritionally essential in trace amounts, though toxic in excess. HF and F2 are among the most dangerous substances handled in laboratories.
Do halogens react with noble gases?
Fluorine reacts with Xenon and Krypton, forming fluorides (XeF2, XeF4, XeF6, KrF2) under controlled conditions.
What is the order of oxidizing power?
F > Cl > Br > I. For instance, Cl2 can displace I− from its salts, but the reverse is not possible.
What are their main applications?
Water disinfection (Cl), fluoropolymers (PTFE), iodized salt (I), organic synthesis, flame retardants (Br, regulated), and advanced fluorinated materials.