ABSTRACT: This chapter analyzes the structure of Lithium (Z=3) as the first example of "Core-Mantle" nuclear architecture. Overcoming the inert stability of Helium, nature uses the Alpha Module (4He) as a central seed upon which to graft reactive external modules. MTE theory explains the natural prevalence of Lithium-7 (P=10.3) over Lithium-6 (P=9.00) through the concept of Surface Packing: the planar geometry of the Tritium Module adheres perfectly to the faces of the Alpha tetrahedron, unlike the Deuterium Module. The chapter further demonstrates the impossibility of Lithium-5 due to the "Neutron Bridge Law" and connects the expanded structure of the nucleus (halo nucleus) to Lithium's chemical properties, such as its low density and its nature as the first metal of the periodic table. KEYWORDS: Lithium, Core-Mantle Structure, Alpha Module, Tritium Module, Surface Packing, Halo Nucleus, Neutron Bridge Law, Lithium-7, Lithium-6, MTE, Metallic Bond.