Halogens
Definition and meaning of Halogens in chemistry.
Halogens are the six elements in group 17 of the periodic table: fluorine, chlorine, bromine, iodine, astatine, and tennessine. Each halogen atom has seven valence electrons, just one short of a stable octet. This makes halogens highly reactive, especially with metals, forming salts called halides.
In more detail
Because halogens need only one more electron to fill their outer shell, they have strong electron affinity and, for the lighter members, very high electronegativity. This lets them easily pull an electron from a metal atom to form a negative halide ion, such as chloride (Cl-).
Reactivity and electronegativity decrease down the group, because atomic radius increases and outer electrons are held less tightly. Fluorine sits at the top of the group and is the most reactive and most electronegative element on the periodic table. In their pure elemental form, halogens exist as diatomic molecules: F2, Cl2, Br2, and I2.
At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid. Astatine and tennessine are intensely radioactive and exist only in tiny, short-lived amounts, so their properties are studied mostly through prediction and small-scale experiments. Halogens also form a wide range of useful compounds beyond simple salts.
Chlorine is used to disinfect drinking water and swimming pools by killing bacteria and other pathogens. Iodine plays an essential role in human health, since the thyroid gland needs it to produce hormones that regulate metabolism. Fluoride, the ion form of fluorine, is added to toothpaste and some water supplies to help prevent tooth decay.
Key facts
| Field | General Chemistry |
|---|---|
| Periodic table position | Group 17 (VIIA) |
| Members | F, Cl, Br, I, At, Ts |
| Valence electrons | 7 |
| Elemental form | Diatomic molecules (X2) |
| Most reactive member | Fluorine |
Chlorine gas reacts vigorously with sodium metal to form table salt. The equation is 2Na + Cl2 → 2NaCl. Each sodium atom transfers one electron to a chlorine atom, forming Na+ and Cl- ions that pack into a crystal lattice.
Frequently asked questions
Why are halogens so reactive?
With seven valence electrons, they need just one more to reach a stable noble-gas configuration. This gives them strong electron affinity and, for the lighter members, very high electronegativity.
Are all halogens diatomic gases?
No. Elemental halogens are diatomic (X2), but their physical state varies. Fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid at room temperature.
What is a halide?
A halide is a negative ion formed when a halogen atom gains one electron, such as fluoride (F-), chloride (Cl-), bromide (Br-), or iodide (I-). Halide ions combine with metal cations to form ionic salts.