Halogens form small covalent molecules with the formula, X₂, held together by van der Waals forces. Larger atoms can form more van der Waals forces, strengthening their intermolecular interactions. Therefore, the melting and boiling points of halogens increases as you travel down the group
Electronegativity, χ, is the ability of an element to attract a pair of shared electrons in a covalent bond. The electronegativity of halogens increases as you go up the group.
The neutral molecular halogens readily accept electrons, so they act as oxidising agents, taking electrons from other molecules. The negatively charged halide ions have spare electrons available, so they act as reducing agents, giving electrons to other molecules.
The halogens have many uses. Fluorine is highly reactive, but is used to make teflon, a famously unreactive material. Chlorine has many uses in cleaning products as it can kill bacteria. Bromine is toxic, so has applications in insecticides. Iodine is a useful atom in the synthesis of pharmaceuticals.
A common chemistry experiment is the identification of silver halides by their characteristic solubility properties. The addition of silver nitrate to a halide solution produces the corresponding silver halide. The solubility and colour of the silver halide precipitate, under various conditions, allows the unknown halide to be identified.