The alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals are called the Group 1 elements. Previously, they were called the Group IA elements, with "A" representing the main group elements and Roman numeral "I" representing the first group within the main-group series. The alkali metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs)[note 1] and francium (Fr).[4] Hydrogen (H), although nominally also a member of Group 1,[5] very rarely exhibits behavior comparable to the alkali metals. This group lies in the s-block of the periodic table, which means that all its elements have their outermost electron in an s-orbital. The s-block also includes alkaline earth metals, plus hydrogen and helium. The alkali metals provide one of the best examples of group trends in properties in the periodic table, with well characterized homologous behavior down the group.