Matter in the bulk form (that is, massive matter) behaves much differently from single atoms or molecules. For example, bulk iron is a ferromagnetic substance characterized by high remanence (residual magnetism) and saturation magnetization values; differently, a single atom of iron is a not magnetic specie. At interface between single atoms and bulk matter, there is a region where matter is not bulk anymore but not yet in the atomic or molecular form. In this dimensional regime the miniaturized solids change their properties with varying of size. In particular, the variation of physical and chemical properties takes place in a size regime typically ranging from a few nanometers to several nanometers tens (nanoscopic regime). In such ‘transition zone’ these solids with molecular size change their properties on the basis of the own dimensionality that can be expressed in different ways. In the case of atomic clusters, the dimensionality is usually expressed in terms of nuclearity, that is the number of atoms contained in the atomic cluster. As an example, let’s consider the reaction for the formation of an atomic cluster of iron: xFe → Fe_x, in this case x atoms of iron bond together to generate an iron cluster and the x value represents the cluster nuclearity, that is the number of iron atoms contained in the cluster. Nuclearity does not change continuously but in a discrete manner, and precisely it varies according to ‘magic numbers’ since cluster stability depends on nuclearity. The fundamental cluster (i.e., the smallest stable cluster) of gold has a nuclearity equal to 13 (i.e., Au₁₃), then there is Au₅₅, Au₁₄₇, etc. Clusters with a nuclearity corresponding to a magic number have a stable electronic configuration just like the noble gasses. Such dependence of properties on size is a fundamental concept of the nanomaterial science and it is expressed by a three dimensional representation of the periodic table of elements ^[1]. In particular, the development along the z axis of this 3D periodic table indicates the variation of the element properties with the atomic cluster nuclearity. For example, the remanence disappears in the case of iron particles with a size inferior to that of single domain (ca. 13nm). This kind of magnetism is named superparamagnetism and single-domain iron particles have a size-dependent superparamagnetic behavior. Further examples can be the tunable fluorescence of semiconductor quantum-dots, the change of metal cluster melting point with size, etc.