How is iodine different from fluorine, chlorine, and bromine?

Iodine is a member of the halogen elements (Group 17), but it differs from fluorine, chlorine, and bromine in several key physical and chemical properties due to its larger atomic size and position in the periodic table.12

Physical State and Appearance

At room temperature and atmospheric pressure, fluorine and chlorine are gases, and bromine is a liquid, whereas iodine exists as a dark violet crystalline solid. Iodine has a moderate vapor pressure and readily undergoes sublimation to form a violet vapor.134

Reactivity and Chemical Behavior

• Oxidizing Strength: Iodine is a weaker oxidizing agent than fluorine, chlorine, and bromine. While the fluoride ion exhibits no reducing properties, the iodide ion acts as a mild reducing agent.41
• Electronegativity: Iodine has the lowest electronegativity among these halogens, meaning it has the weakest tendency to attract shared electrons in a covalent bond.1
• Ionization Energy: Iodine has a significantly lower first ionization energy than the lighter halogens, which facilitates the formation of compounds in positive oxidation states (+1, +3, +5, and +7).42
• Coordination: Due to its larger atomic size, iodine can achieve higher coordination numbers than chlorine or bromine when bonding with other elements like oxygen or fluorine.14