We recently demonstrated that, in gas phase clusters containing aluminum and iodine atoms, an Al 13 cluster behaves like a halogen atom, whereas an Al 14 cluster exhibits properties analogous to an alkaline earth atom. These observations, together with our findings that Al 13 ؊ is inert like a rare gas atom, have reinforced the idea that chosen clusters can exhibit chemical behaviors reminiscent of atoms in the periodic T he formation of materials with properties different from those of the constituent atoms is a known phenomenon in nature. For example, the formation of NaCl molecules/solid with characteristics different from its constituent elements, Na and Cl, is a classic example. One of the objectives of the research on superatoms (1-3) is to explore if one can carry out, what nature does, in a more facile and controlled manner. Developing an understanding of the factors governing the chemical behavior of clusters (4-10) and demonstrating that this knowledge can be used to design stable building blocks for new materials is critical for translating this concept into practice. For metal clusters, a simple electronic shell model called jellium (11) is routinely used to describe the global features of the electronic structure. In this model, the nuclei together with the innermost electrons form a positive-charged background, whereupon the valence electrons coming from individual atoms are then subjected to this potential. For pure metal clusters, within a spherical jellium background, this approach results in a shell structure where the electrons are arranged in electronic shells 1s 2 , 1p 6 , 1d 10 , 2s 2 , 1f 14 , 2p 6 . . . compared with 1s 2 , 2s 2 , 2p 6 , 3s 2 , 3p 6 , 4s 2 , 3d 10 . . . in individual atoms. Similar shell structure is also obtained for square well and harmonic forms of background potential (12), indicating that the shells derived within a jellium picture represent generic features of electronic states in a confined free electron gas. Clusters containing 2, 8, 18, 20, 34, 40 . . . electrons correspond to filled electronic shells and exhibit enhanced stability as seen via abundances in mass spectra of simple metal clusters, higher ionization potential, lower electron affinity, and chemical inertness seen in reactivity experiments. In this respect, an Al 13 cluster with 39 valence electrons and an electronic structure of 1s 2 , 1p 6 , 1d 10 , 2s 2 , 1f 14 , 2p 5 lacks a single electron as do halogen atoms, which, upon addition of a single electron, acquire a filled shell status (13). Indeed, previous studies (14,15) have shown that Al 13 has an electron affinity comparable to halogen atoms, indicating a chemical behavior reminiscent of halogen atoms. In a similar vein, we had recently shown that in cluster compounds with iodine, an Al 14 cluster exhibits behavior analogous to alkaline earth atoms (3). We had shown that Al 14 I 3 Ϫ is a stable species and that its stability can be reconciled by considering Al 14 in a ϩ 2 valence state (3). The electronic shell structure, outlined above, does ...