Alkali metals and halogens are likely to form ions because of a surplus or a lack of electrons in their valence (outermost) electron shells. In order for an alkali metal to become a stable ion, it must lose the single electron in its valence shell to a neighboring atom. Because a halogen needs one extra electron in order to become a stable ion, elements in these two groups often combine.
The alkali metals are classified as group one elements on the periodic table because of their surplus of a single electron in the valence electron shell. Halogens, however, are classified as group seven elements, because they have seven electrons in the valence shell. When an alkali metal, such as Na (sodium), combines with a halogen, such as Cl (chlorine), to create NaCl (table salt), the sodium loses its outer electron to chlorine. Because electrons are negatively charged particles, the sodium becomes an ion with a +1 charge, and the chlorine transforms into an ion with a -1 charge. Because alkali metals and halogens are so extremely reactive with each other, they are virtually never found in nature as pure elements by themselves. Alkali metals often react with elements like hydrogen or sulfur to form hydrides and sulfides. Halogens usually combine with metals to create salts, such as bromides and chlorides.