The trace alkali elements of Group 1 are: lithium (Li), rubidium (Rb), and cesium (Cs). The common characteristic of the alkali elements is a single electron in the outermost energy level, resulting in highly reactive chemical behavior. The relative bonding force holding these monovalent cations is presented most often in the order Cs+ > Rb+ > Li+. These cations do not usually form complex ionic species, but can be bound in some chelates and organometallic compounds. Although their properties differ (Table 6.1) these lithophilic metals are relatively similar, especially Rb and Cs, in their behavior in crystallochemical and geochemical processes. They are closely associated with the major crustal components and are likely to enter silicate minerals.