The most active nonmetals belong to the halogen family, which sits to the left of the noble gases on the right side of the periodic table. The halogens are so reactive that they are never found in nature by themselves.
The elements fluorine, chlorine, bromine, iodine and astatine make up the halogen family. Halogens have seven valence electrons, which means that they have seven electrons in their outermost shells. These valence electrons are the cause of an element's reactivity.
The tendency of atoms is to fill this outermost shell to capacity to attain a stable arrangement of electrons. For example, the noble gases, which contain eight electrons in the outermost energy level, are extremely stable elements because they already have their full complement of electrons. In nature, the noble gases generally do not form compounds with any other element. All other elements need to find situations in which they can either donate or receive electrons to achieve their full complement. Halogens, because they need one more electron to have a stable arrangement, generally try to grab electrons from other elements; therefore, halogens are considered electronegative elements.
Halogens tend to form stable arrangements called diatomic molecules in which two of the same halogen element share an electron. Electron donors such as metals like to combine with halogens to make stable compounds. Sodium chloride, ordinary table salt, is the result of a chemical reaction between the metal sodium and the halogen chlorine.