Halogen ions are negatively charged ions formed when halogen elements, such as fluorine, chlorine, bromine, and iodine, gain one electron. These ions are commonly found in nature and play essential roles in various chemical reactions. The most common halogen ions include fluoride (F⁻), chloride (Cl⁻), bromide (Br⁻), and iodide (I⁻). Halogen ions are important in many applications, including water treatment, where chloride ions help disinfect water. They also play a crucial role in biological systems, such as the function of iodide in thyroid hormone production. Understanding halogen ions is essential for studying chemistry and environmental science.