The Roman numerals on a periodic table of elements define the chemical group of the elements in that column and identify the number of valence electrons of each element. Group IA elements have one valence electron; group IIA elements have two valence electrons and so forth.
Since each element within a group has the same number of valence electrons in the outer shell of its nucleus, it shares similar chemical properties with other elements in the group. Valence electrons are the electrons involved in forming chemical bonds. The ability of an element to bind with another element relies on this characteristic and determines its electrical charge. The groups are labeled IA, IIA, IIIA, IVA, VA, VIA, VIIA and VIIA. For example, group IA includes the element Lithium, which is assigned the atomic number 3. The Lithium atom has three electrons: two near the nucleus and one in the outer shell.