The elements in Group 7 are known as the halogens. They are non-metals and each has seven electrons in its outer shell, so they have similar reactions. Halogens exist as molecules made of pairs of atoms, such as Cl2, Br2 and I2. Chlorine is a gas, bromine is a liquid and iodine is a solid at room temperature.
Going down the group, the relative molecular mass, melting point and boiling point of the halogens increase. The reactivity decreases going down the group: a halogen atom reacts by gaining one electron, and this is harder when the outer shell is further from the nucleus. So chlorine is more reactive than bromine, and bromine is more reactive than iodine.
Halogens react with metals to form ionic compounds containing halide ions, such as sodium chloride. With non-metals they form covalent compounds made of molecules, such as hydrogen chloride.
A more reactive halogen displaces a less reactive halogen from an aqueous solution of its salt. Chlorine added to potassium bromide solution displaces bromine, and the solution turns orange. Bromine does not displace chlorine from potassium chloride solution, so there is no reaction.