The elements in group 7 are known as the halogens. These are fluorine, chlorine, bromine, iodine and astatine. These elements are non-metals that are poisonous. All halogens have similar reactions as they have seven electrons in their outermost shell. Halogens are diatomic, meaning they form molecules made of pairs of atoms sharing electrons (forming a single covalent bond between the two halogen atoms).

The melting and boiling points of the halogens increase as you go down the group. This is due to increasing intermolecular forces as the atoms become larger, so more energy is required to overcome these forces. Fluorine and chlorine are gases, bromine is a liquid and iodine is crumbly solid. The colours of the halogens also change as you descend the group - they become darker.

Reactivity of the halogen

Reactivity of group 7 elements decreases as you go down the group. As you go down the group 7, the number of shells of electrons increases, the same as with all other groups.

Fluorine is the smallest halogen, which means its outermost shell is the closest to the positive nucleus of all the halogen. Therefore, the ability to attract an electron is strongest in fluorine making it the most reactive.

A halogen displacement reaction occurs when a more reactive halogen displaces a less reactive halogen from an aqueous solution of its halide. The solution becomes orange as bromine is formed or the solution becomes brown as iodine is formed

Cl₂ + 2KBr → 2KCl + Br₂ (Orange)
Cl₂ + 2KI → 2KCl + I₂ (Brown)

The halogens react with some metals to form ionic compounds which are metal halide salts. The halide ion carries a -1 charge so the ionic compound formed will have different numbers of halogen atoms, depending on the valency of the metal

2Na + Cl₂ → 2 NaCl
Ca + Br₂ → CaBr₂

The halogens react with non-metals to form simple molecular covalent structures. For example, the halogens react with hydrogen to form hydrogen halides (e.g., hydrogen chloride)