Halides are defined as binary compounds containing a halogen. Because halogens are highly electronegative, their atoms normally retain negative charge in these compounds. The magnitude of this negative charge depends on the difference in the two constituent's electronegativities.

The nature of the bonds between molecules determines the physical properties of the halides, and the distinction between ionic and covalent halides provides a basis for understanding these molecules. In the ionic halides, such as sodium chloride, all of the bonding electrons are transferred to the halogen, producing a halide ion. Interionic forces are large, so these halides have high melting and boiling points. In covalent halides, such as carbon tetrachloride, the bonding electrons are localized in the carbon-chlorine bonds of the individual molecules. The intermolecular forces in covalent halides are so weak that these molecules usually exist as gases, liquids, or low melting-point solids. Many elements form fluorides, however, that cannot be labeled covalent or ionic; these halides exhibit the characteristics of polymeric compounds.

The group IA elements (lithium, sodium, potassium, rubidium, cesium) readily lose their single valence electron to form singly charged cations. All of the halides of these elements are ionic compounds, and they all have high melting points. Examples include LiF, NaCl, and NaI.

The group IIA and IIB elements (beryllium, magnesium, calcium, strontium, barium, zinc, cadmium, mercury) also tend to form ionic halides. Examples include MgCl₂, and CaBr₂.

The group III and IV elements (boron, aluminum, gallium, indium, carbon, silicon, germanium, tin), on the other hand, tend to form covalent halides. Examples include BF₃ and SiCl₄.

The group V and VI elements (nitrogen, phosphorous, arsenic, antimony, oxygen, sulfur, selenium, tellurium) form covalent halides exclusively. Examples include NF₃ and SCl₂.

The group VII elements (fluorine, chlorine, bromine, and iodine) can form simple halides like I₂, but also more complex molecules like ClF₃.