The halogens have seven valence electrons and often gain one electron to form halide ions. Fluorine, chlorine, bromine and iodine also form diatomic elemental molecules, while their compounds range from simple salts to strongly covalent interhalogens and oxo-species.
Fluorine, chlorine, bromine and iodine belong to group 17. They are reactive non-metals with seven electrons in their outer shell.
F · Cl · Br · I · At · Ts
Halogens are usually found in compounds rather than as free elements. Chloride is abundant in seawater and table salt, while fluorides, bromides and iodides occur in minerals, waters and living systems in very different amounts.
Their physical appearance changes down the group. Fluorine and chlorine are gases under ordinary conditions, bromine is a liquid and iodine is a solid. This visible progression reflects stronger attractions between larger molecules.
Halogen chemistry is important in disinfection, medicines, polymers and many industrial processes. The same reactivity that makes these elements useful also means the pure substances require careful handling.
02Elemental oxidizing strength changes down the group
03Physical states change with polarizability
Electron gain explains halides
An X− ion completes the valence shell for a neutral halogen atom. This makes halide formation common in ionic compounds with electropositive metals.
Elemental oxidizing strength changes down the group
F2 is an exceptionally strong oxidizing agent. Broadly, the tendency of X2 to accept electrons decreases down the common halogens, although aqueous redox comparisons also involve bond and solvation energies.
Physical states change with polarizability
At room conditions F2 and Cl2 are gases, Br2 is a liquid and I2 is a solid. Increasing electron-cloud polarizability strengthens dispersion forces down the group.
Worked example
Why can chlorine displace bromide ions from solution?
Cl2 is the stronger oxidizing agent in the usual aqueous comparison, so it can accept electrons from Br− to form Cl− and Br2.
Common traps to avoid
Halogens are not present in table salt as neutral Cl atoms; chloride ions are present.
Astatine and tennessine cannot simply be assigned every bulk trend from macroscopic samples.