Halogen
Halogens are the five nonmetallic elements of Group 17 — fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At) — each with seven valence electrons, making them one electron short of a full outer shell and therefore highly reactive oxidising agents. They readily gain one electron to form stable 1− anions (halides) and react with metals to form ionic salts, and with hydrogen to form hydrogen halides such as HCl and HF. Halogens have important industrial and biological applications: chlorine disinfects water supplies, iodine is essential for thyroid hormone synthesis, and fluorine is used in making Teflon and fluoride toothpaste.
Key Formula
X2 + 2e⁻ → 2X⁻
LaTeX: \text{X}_2 + \text{2e}^- \rightarrow \text{2X}^-
| Symbol | Meaning | Unit |
|---|---|---|
| X2 | Diatomic halogen molecule | — |
| X⁻ | Halide ion formed | — |
| e⁻ | Electron gained | — |
Worked Example
Problem
Chlorine water is added to a solution containing bromide ions (Br⁻). Explain the displacement reaction and write the ionic equation.
Solution
Step 1 – Identify oxidising agent: Cl2 is a stronger oxidising agent than Br2 (higher electronegativity, higher reduction potential: Cl2/Cl⁻ = +1.36 V vs Br2/Br⁻ = +1.09 V). Step 2 – Displacement occurs: Cl2 oxidises Br⁻ to Br2, while Cl2 is reduced to Cl⁻. Step 3 – Ionic equation: Cl2(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br2(aq) Step 4 – Observation: The solution turns orange/brown due to formation of Br2.
Answer
Cl2(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br2(aq); solution turns orange-brown
Properties of the halogens
| Element | Symbol | State at 25°C | Colour | Electronegativity |
|---|---|---|---|---|
| Fluorine | F | Gas | Pale yellow | 3.98 |
| Chlorine | Cl | Gas | Yellow-green | 3.16 |
| Bromine | Br | Liquid | Red-brown | 2.96 |
| Iodine | I | Solid | Purple-black | 2.66 |
| Astatine | At | Solid | Black (est.) | 2.20 |
Interactive Tools
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Related Terms
Chemical Group
A chemical group (also called a family) is a vertical column in the periodic table containing elements that share the same number of valence electrons, and therefore exhibit similar chemical properties and reactivity patterns. The modern IUPAC system numbers groups 1 through 18 from left to right. Elements within a group show predictable trends: for example, reactivity increases down Group 1 (alkali metals) because the outermost electron is progressively further from the nucleus and more easily lost.
Noble Gas
Noble gases are the six elements of Group 18 — helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn) — characterised by completely filled outer electron shells, which make them extremely stable and almost entirely unreactive under normal conditions. Their full valence shells give them very high ionisation energies and near-zero electronegativity, meaning they do not readily form chemical bonds. Noble gases have important applications in lighting (neon signs, argon-filled incandescent bulbs), inert atmospheres for welding and chemical synthesis, and medical imaging (xenon anaesthesia, krypton in lung ventilation scans).
Electronegativity Trend
Electronegativity is a measure of the tendency of an atom to attract a shared pair of electrons towards itself in a covalent bond, and its periodic trend describes how this property changes systematically across the periodic table. Electronegativity increases across a period (left to right) because increasing nuclear charge pulls bonding electrons more strongly, and decreases down a group because the bonding electrons are further from the nucleus and shielded by additional inner electron shells. On the Pauling scale, fluorine is assigned the highest electronegativity value of 3.98, making it the most electronegative element, while caesium and francium have the lowest values near 0.79.
From Greek "halos" (salt) + "gennao" (to produce) — literally "salt-formers." The term was coined by Swedish chemist Jöns Jacob Berzelius around 1825, reflecting the fact that these elements readily combine with metals to form salt compounds such as sodium chloride (NaCl).