Trends in Group 1 and Group 17

Groups 1 and 17 offer the clearest illustrations of how properties change down a group, and their reactions are staples of the chemistry laboratory. The alkali metals of Group 1 (lithium, sodium, potassium, rubidium, and cesium) each have a single valence electron, which they lose readily to form +1 ions. The halogens of Group 17 (fluorine, chlorine, bromine, and iodine) each have seven valence electrons and tend to gain one electron to form -1 ions. Though the two groups sit on opposite sides of the table and behave in opposite ways, both show reactivity trends that follow directly from atomic size and the ease of gaining or losing electrons.

For the alkali metals, reactivity increases down the group. Each metal must lose its single outer electron to react, and that electron becomes progr