Occurrence, Preparation, and Properties of Halogens

Core Concepts

Occurrence: salts, not free elements

Because halogens are so reactive, they occur only as compounds, almost always as halides (oxidation state -1): chloride in seawater (about 19 g of Cl^- per kg of seawater) and in rock salt (halite, NaCl); fluoride in fluorite (CaF₂) and cryolite (Na₃AlF₆); bromide in seawater and concentrated brines (the Dead Sea is notably rich in bromide); and iodide in seawater, brines, and seaweed. These resources determine the industrial chemistry: seawater and salt deposits are the starting point for chlorine and bromine, while iodine comes mainly from brines and from Chilean caliche deposits.

Preparation: electrolysis and oxidation of halides

Because F^- is essentially impossible to oxidize chemically (fluorine is the strongest oxidizing agent), fluorine is made only by electrolysis of a molten fluoride salt (a KF/HF melt), with the gas collected at the anode:

2F^- → F₂ + 2e^-

Chlorine is made industrially by the electrolysis of brine (the chlor-alkali process):

2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + H₂(g) + Cl₂(g)

Bromine and iodine are recovered by oxidizing their halide ions with a stronger halogen — typically chlorine displaces bromide or iodide from solution:

Cl₂ + 2Br^- → 2Cl^- + Br₂   Cl₂ + 2I^- → 2Cl^- + I₂

This displacement pattern — a halogen oxidizes the halide ions of the halogens below it — is the single most tested idea in halogen chemistry.

Properties and the trend down the group

The elements are diatomic: F₂ (pale yellow gas), Cl₂ (yellow-green gas), Br₂ (red-brown liquid, bp 59 °C), I₂ (violet-black solid, mp 114 °C, sublimes easily). Down the group, atoms get larger, the outermost electrons are farther from the nucleus, and electron affinity, electronegativity, and oxidizing power decrease: F₂ > Cl₂ > Br₂ > I₂. Fluorine is so reactive it attacks almost everything, including glass (forming SiF₄) and even water (liberating O₂):

2F₂ + 2H₂O → 4HF + O₂

Chlorine reacts with water more gently, forming a disinfecting equilibrium with hypochlorous acid:

Cl₂ + H₂O ⇌ HCl + HOCl

The Cl₂/HOCl system is exactly what makes chlorinated pools and municipal disinfection work. Iodine, at the bottom, is the mildest — it still oxidizes many metals, but it needs help (heat or a catalyst) for most reactions, which is why I₂ is the safest elemental halogen to handle with basic precautions.

Hydrogen halides and their acids

The halogens form hydrogen halides, HX: HF, HCl, HBr, HI — all colorless, sharp-smelling gases that dissolve in water to give acids. Their behavior in water is a famous inversion of the reactivity trend: acid strength increases down the group (HF ≪ HCl < HBr < HI), even though H–X bond energy decreases down the group. HF is a weak acid because the H–F bond is so strong and because F⁻ hydrogen-bonds to water; HCl, HBr, and HI are strong acids. HF has its own unique chemistry: it attacks glass (used to etch glass and dissolve silicates) and, dangerously, can penetrate skin and damage bone — a specific hazard that makes HF handling a serious safety matter. Hydrochloric acid (muriatic acid) is a common industrial acid, and the halide salts (NaCl, KCl, NaF, KI) are stable, mostly water-soluble ionic compounds.

Oxoacids and interhalogen chemistry

Chlorine, bromine, and iodine form oxoacids in which the halogen has positive oxidation states: hypochlorous acid (HOCl, Cl at +1), chlorous acid (HClO₂, +3), chloric acid (HClO₃, +5), and perchloric acid (HClO₄, +7). Hypochlorites (e.g., NaOCl, bleach) are common disinfectants; perchlorates are powerful oxidizers used in rocket propellants. The halogens also combine with each other to form interhalogen compounds such as ClF₃ and ICl, and with oxygen in halogen oxides. This range of oxidation states is only possible because, unlike fluorine, the heavier halogens have empty d orbitals and can expand their octets.

How It Works / Step-by-Step Process

From seawater to swimming-pool disinfectant: (1) Brine (concentrated NaCl solution, from seawater evaporation or rock salt) is electrolyzed; (2) at the anode, chloride ions lose electrons to form Cl₂ gas, which is collected and dried; (3) some Cl₂ is dissolved in water, where it forms the HCl/HOCl equilibrium — the hypochlorous acid that kills microorganisms; (4) the rest is liquefied under pressure for transport and dosing into water systems. Every step involves corrosive, toxic gas handling, so production is fully enclosed with scrubbing and monitoring systems — chlorine is never produced or used in open, unventilated settings.

Common Confusions