Elements in the periodic table are arranged in periods (rows) and groups (columns). Each of the seven periods is filled sequentially by atomic number. Groups include elements having the same electron configuration in their outer shell, which results in group elements sharing similar chemical properties. The electrons in the outer shell are termed valence electrons. Valence electrons determine the properties and chemical reactivity of the element and participate in chemical bonding. The Roman numerals found above each group specify the usual number of valence electrons. There are two sets of groups. The group A elements are the representative elements, which have s or p sublevels as their outer orbitals. The group B elements are the nonrepresentative elements, which have partly filled d sublevels (the transition elements) or partly filled f sublevels (the lanthanide series and the actinide series). The Roman numeral and letter designations give the electron configuration for the valence electrons (e.g., the valence electron configuration of a group VA element will be s2p3 with 5 valence electrons).