In chemistry and atomic physics, the main group is the group of elements whose lightest members are represented by helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, and neon as arranged in the periodic table of the elements. The main group includes the elements (except hydrogen) in groups 1 and 2 (s-block), and groups 13 to 18 (p-block). Group 12 elements are usually considered to be transition metals; however, zinc (Zn), cadmium (Cd), and mercury (Hg) share some properties of both groups, and some scientists believe they should be included in the main group.