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A superatom is any cluster of atoms that seem to exhibit some of the properties of elemental atoms.

Sodium atoms, when cooled from vapor, naturally condense into clusters, preferentially containing a magic number of atoms (2, 8, 20, 40, 58, etc.). The first two of these can be recognized as the numbers of electrons needed to fill the first and second shells, 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.

Aluminium clusters

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[1]. 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 iodide ion or better still a bromide ion. 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.[2][3]

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.[4]

• Al7 = the property is similar to germanium atoms.
• Al13 = the property is similar to halogen atoms, more specifically, chlorine.
• Al13Ix, where x = 1–13.[5]
• Al14 = the property is similar to alkaline earth metals.
• Al14Ix, where x = 1–14.[5]
• Al23
• Al37

Other clusters

• Li(HF)3Li = the (HF)3 interior causes 2 valence electrons from the Li to orbit the entire molecule as if it were an atom's nucleus.[6]
• VSi16F = has ionic bonding.[7]
• A cluster of 13 platinum becomes highly paramagnetic, much more so than platinum itself.[8]
• A cluster of 2000 rubidium atoms.[9]

Superatom complexes

Superatom complexes are a special group of superatoms that incorporate a metal core which is stabilized by organic ligands. In thiolate-protected gold cluster complexes a simple electron counting rule can be used to determine the total number of electrons (ne) which correspond to a magic number via,

$$n_{e}=N\nu _{A}-M-z$$

where N is the number of metal atoms (A) in the core, v is the atomic valence, M is the number of electron withdrawing ligands, and z is the overall charge on the complex.[10] For example the Au102(p-MBA)44 has 58 electrons and corresponds to a closed shell magic number.[11]
Gold superatom complexes

• Au25(SMe)18 [12]
• Au102(p-MBA)44
• Au144(SR)60 [13]

Other superatom complexes

• Ga23(N(Si(CH3)3)2)11[14]
• Al50(C5(CH3)5)12[15]
• Re6Se8Cl2 - In 2018 researchers produced 15-nm-thick flakes of this superatomic material . They anticipate that a monolayer will be a superatomic 2-D semiconductor and offer new 2-D materials with unusual, tunable properties.[16]

Bose–Einstein condensate
Quantum dot

References

Bergeron, D. E. (2 April 2004). "Formation of Al13I−: Evidence for the Superhalogen Character of Al13". Science. American Association for the Advancement of Science (AAAS). 304 (5667): 84–87. doi:10.1126/science.1093902. ISSN 0036-8075. PMID 15066775.
Philip Ball, "A New Kind of Alchemy", New Scientist Issue dated 2005-04-16.
Bergeron, D. E. (14 January 2005). "Al Cluster Superatoms as Halogens in Polyhalides and as Alkaline Earths in Iodide Salts". Science. American Association for the Advancement of Science (AAAS). 307 (5707): 231–235. doi:10.1126/science.1105820. ISSN 0036-8075. PMID 15653497.
Harris, I. A.; Kidwell, R. S.; Northby, J. A. (17 December 1984). "Structure of Charged Argon Clusters Formed in a Free Jet Expansion". Physical Review Letters. American Physical Society (APS). 53 (25): 2390–2393. doi:10.1103/physrevlett.53.2390. ISSN 0031-9007.
Sun, Xiao-Ying; Li, Zhi-Ru; Wu, Di; Sun, Chia-Chung (2007). "Extraordinary superatom containing double shell nucleus: Li(HF)3Li connected mainly by intermolecular interactions". International Journal of Quantum Chemistry. Wiley. 107 (5): 1215–1222. doi:10.1002/qua.21246. ISSN 0020-7608.
Koyasu, Kiichirou; Atobe, Junko; Akutsu, Minoru; Mitsui, Masaaki; Nakajima, Atsushi (2007). "Electronic and Geometric Stabilities of Clusters with Transition Metal Encapsulated by Silicon". The Journal of Physical Chemistry A. American Chemical Society (ACS). 111 (1): 42–49. doi:10.1021/jp066757f. ISSN 1089-5639. PMID 17201386.
Platinum nanoclusters go magnetic Archived 2007-10-15 at the Wayback Machine, nanotechweb.org, 2007
Ultra Cold Trap Yields Superatom, NIST, 1995
Walter, M.; Akola, J.; Lopez-Acevedo, O.; Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Whetten, R. L.; Gronbeck, H.; Hakkinen, H. (1 June 2008). "A unified view of ligand-protected gold clusters as superatom complexes". Proceedings of the National Academy of Sciences. 105 (27): 9157–9162. doi:10.1073/pnas.0801001105. ISSN 0027-8424. PMC 2442568. PMID 18599443.
Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Bushnell, D. A.; Kornberg, R. D. (19 October 2007). "Structure of a Thiol Monolayer-Protected Gold Nanoparticle at 1.1 Å Resolution". Science. American Association for the Advancement of Science (AAAS). 318 (5849): 430–433. doi:10.1126/science.1148624. ISSN 0036-8075. PMID 17947577.
Akola, Jaakko; Walter, Michael; Whetten, Robert L.; Häkkinen, Hannu; Grönbeck, Henrik (2008). "On the Structure of Thiolate-Protected Au25". Journal of the American Chemical Society. American Chemical Society (ACS). 130 (12): 3756–3757. doi:10.1021/ja800594p. ISSN 0002-7863. PMID 18321117.
Lopez-Acevedo, Olga; Akola, Jaakko; Whetten, Robert L.; Grönbeck, Henrik; Häkkinen, Hannu (16 January 2009). "Structure and Bonding in the Ubiquitous Icosahedral Metallic Gold Cluster Au144(SR)60". The Journal of Physical Chemistry C. American Chemical Society (ACS). 113 (13): 5035–5038. doi:10.1021/jp8115098. ISSN 1932-7447.
Hartig, Jens; Stößer, Anna; Hauser, Petra; Schnöckel, Hansgeorg (26 February 2007). "A Metalloid Ga23{N(SiMe3)2}11 Cluster: The Jellium Model Put to Test". Angewandte Chemie International Edition. Wiley. 46 (10): 1658–1662. doi:10.1002/anie.200604311. ISSN 1433-7851. PMID 17230594.
Clayborne, Peneé A.; Lopez-Acevedo, Olga; Whetten, Robert L.; Grönbeck, Henrik; Häkkinen, Hannu (13 May 2011). "The Al50Cp*12 Cluster - A 138-Electron Closed Shell (L = 6) Superatom". European Journal of Inorganic Chemistry. Wiley. 2011 (17): 2649–2652. doi:10.1002/ejic.201100374. ISSN 1434-1948.

Zyga, Lisa. "Researchers create first superatomic 2-D semiconductor". Phys.org. Retrieved 2018-02-18.

Designer Magnetic Superatoms, J.U. Reveles, et al. 2009
Gold Superatom Complexes, M. Walter et al. 2008
Gold Superatom Complexes P.D. Jadzinsky et al. 2007
Multiple Valence Superatoms, J.U. Reveles, S.N. Khanna, P.J. Roach, and A.W. Castleman Jr., 2006
On the Aluminum Cluster Superatoms acting as Halogens and Alkaline-earth Metals, Bergeron, Dennis E et al., 2006
Clusters of Aluminum Atoms Found to Have Properties of Other Elements Reveal a New Form of Chemistry, innovations report, 2005. Have a picture of Al14.
Clusters of Aluminum Atoms Found to Have Properties of Other Elements Reveal a New Form of Chemistry, Penn State, Eberly College of Science, 2005
Research Reveals Halogen Characteristics innovations report, 2004. Have pictures of Al13.

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