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Superatoms are clusters of atoms which seem to exhibit some of the properties of elemental atoms.
Sodium atoms when cooled from vapor, naturally condense into clusters, more so into clusters of 2, 8, 20, 40, 58 or 92 atoms (the magic numbers), than into the other numbers. The first two of these can be recognized as the numbers of electrons needed to fill s and p orbitals, respectively. The superatom suggestion is that free electrons in the cluster occupy a new set of orbitals that are defined by the entire group of atoms, i.e. cluster, rather than each individual atom separately (non-spherical or doped clusters show deviations in the number of electrons that form a closed shell as the potential is defined by the shape of the positive nuclei.) Superatoms tend to behave chemically in a way that will allow them to have a closed shell of electrons, in this new counting scheme. Therefore, a superatom with one more electron than a full shell should give up that electron very easily, similar to an alkali metal, and a cluster with one electron short of full shell should have a large electron affinity, such as a halogen.
Certain aluminium clusters have superatom properties. These aluminium clusters are generated as anions (Aln- with n = 1,2,3...) in helium gas and reacted with a gas containing iodine. When analyzed by mass spectrometry one main reaction product turns out to be Al13I-. These clusters of 13 aluminium atoms with an extra electron added do not appear to react with oxygen when it is introduced in the same gas stream. Assuming each atom liberates its 3 valence electrons, this means that there are 40 electrons present, which is one of the magic numbers noted above for sodium, and implies that these numbers are a reflection of the noble gases. Calculations show that the additional electron is located in the aluminium cluster at the location directly opposite from the iodine atom. The cluster must therefore have a higher electron affinity for the electron than iodine and therefore the aluminium cluster is called a superhalogen. The cluster component in Al13I- ion is similar to an iodine ion or better still a bromine atom. The related Al13I2- cluster is expected to behave chemically like the triiodide ion.
Similarly it has been noted that Al14 clusters with 42 electrons (2 more than the magic numbers) appear to exhibit the properties of an alkaline earth metal which typically adopt +2 valence states. This is only known to occur when there are at least 3 iodine atoms attached to an Al14- cluster, Al14I3-. The anionic cluster has a total of 43 itinerant electrons, but the three Iodine atoms each remove one of the itinerant electrons to leave 40 electrons in the jellium shell.
It is particularly easy and reliable to study atomic clusters of inert gas atoms by computer simulation because interaction between two atoms can be approximated very well by the Lennard-Jones potential. Other methods are readily available and it has been established that the magic numbers are 13, 19, 23, 26, 29, 32, 34, 43, 46, 49, 55, etc. [I. A. Harris et al. Phys. Rev. Lett. Vol. 53, 2391-94 (1984).]
Three-dimensional periodic table
With more discovered superatoms that act similar to a particular atom, a new branch is starting to form on the periodic table.
- Formation of Al13I-: Evidence for the Superhalogen Character of Al13 D. E. Bergeron, A.W. Castleman Jr., T. Morisato, S. N. Khanna Science, Vol 304, Issue 5667, 84-87 , 2 April 2004 Abstract MS spectra
- Philip Ball, "A New Kind of Alchemy", New Scientist Issue dated 2005-04-16.
- Al Cluster Superatoms as Halogens in Polyhalides and as Alkaline Earths in Iodide Salts D. E. Bergeron, P. J. Roach, A.W. Castleman Jr., N.O. Jones, S. N. Khanna Science, Vol 307, Issue 5707, 231-235 , 14 January 2005 Abstract MS spectrum
- Expanding the periodic table? (Biomedcentral)
- On the Aluminum Cluster Superatoms acting as Halogens and Alkaline-earth Metals, Bergeron, Dennis E et.al., 2006
- Research Reveals Halogen Characteristics innovations report, 2004. Has pictures of Al13.
- Clusters of Aluminum Atoms Found to Have Properties of Other Elements Reveal a New Form of Chemistry, innovations report, 2005. Has a picture of Al14.
- Beyond The Periodic Table, Computational Chemistry Portal, 2006de:Superatom