Superatom
Encyclopedia
Superatoms are clusters
of atom
s 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 or 82). The first two of these can be recognized as the numbers of electron
s 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
.
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 number
s are 13, 19, 23, 26, 29, 32, 34, 43, 46, 49, 55, etc.
via,
ne = NvA - 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. For example the Au102(p-MBA)44 has 58 electrons and corresponds to a closed shell magic number.
Cluster chemistry
In chemistry, a cluster is an ensemble of bound atoms intermediate in size between a molecule and a bulk solid. Clusters exist of diverse stoichiometries and nuclearities. For example, carbon and boron atoms form fullerene and borane clusters, respectively. Transition metals and main group...
of atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s that seem to exhibit some of the properties of elemental atoms.
Sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
atoms, when cooled from vapor
Vapor
A vapor or vapour is a substance in the gas phase at a temperature lower than its critical point....
, naturally condense into clusters, preferentially containing a magic number
Magic number (chemistry)
In case a gas condenses into clusters of atoms, the number of atoms in these clusters varies between a few and hundreds. However, there are peaks at specific sizes, usually one at lower and one at larger numbers. Often, specific numbers dominate...
of atoms (2, 8, 20, 40, 58 or 82). The first two of these can be recognized as the numbers of electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s 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
Doping (semiconductor)
In semiconductor production, doping intentionally introduces impurities into an extremely pure semiconductor for the purpose of modulating its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic...
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
Alkali metal
The alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...
, and a cluster with one electron short of full shell should have a large electron affinity, such as a halogen
Halogen
The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
.
Aluminium clusters
Certain aluminiumAluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
clusters
Cluster chemistry
In chemistry, a cluster is an ensemble of bound atoms intermediate in size between a molecule and a bulk solid. Clusters exist of diverse stoichiometries and nuclearities. For example, carbon and boron atoms form fullerene and borane clusters, respectively. Transition metals and main group...
have superatom properties. These aluminium clusters are generated as anions (Aln– with n = 1,2,3...) in helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
gas and reacted with a gas containing iodine. When analyzed by mass spectrometry
Mass spectrometry
Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles.It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and...
one main reaction product turns out to be Al13I–. These clusters of 13 aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
atoms with an extra electron added do not appear to react with oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
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
Magic number (chemistry)
In case a gas condenses into clusters of atoms, the number of atoms in these clusters varies between a few and hundreds. However, there are peaks at specific sizes, usually one at lower and one at larger numbers. Often, specific numbers dominate...
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
Electron affinity
The Electron affinity of an atom or molecule is defined as the amount of energy released when an electron is added to a neutral atom or molecule to form a negative ion....
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
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
ion or better still a bromine
Bromine
Bromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...
atom. The related Al13I2– cluster is expected to behave chemically like the triiodide
Triiodide
In chemistry, triiodide can have several meanings. Triiodide primarily refers to the triiodide ion, I3−, a polyatomic anion composed of three iodine atoms. For some chemical compounds, triiodide indicates a salt of the named cation with the triiodide anion. Examples include sodium 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
Alkaline earth metal
The alkaline earth metals are a group in the periodic table. In the modern IUPAC nomenclature, the alkaline earth metals are called the group 2 elements. Previously, they were called the Group IIA elements . The alkaline earth metals contain beryllium , magnesium , calcium , strontium , barium and...
which typically adopt +2 valence
Valence (chemistry)
In chemistry, valence, also known as valency or valence number, is a measure of the number of bonds formed by an atom of a given element. "Valence" can be defined as the number of valence bonds...
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
Itinerant
An itinerant is a person who travels from place to place with no fixed home. The term comes from the late 16th century: from late Latin itinerant , from the verb itinerari, from Latin iter, itiner ....
electrons, but the three Iodine atoms each remove one of the itinerant
Itinerant
An itinerant is a person who travels from place to place with no fixed home. The term comes from the late 16th century: from late Latin itinerant , from the verb itinerari, from Latin iter, itiner ....
electrons to leave 40 electrons in the jellium
Jellium
Jellium, also known as the uniform electron gas or homogeneous electron gas , is a quantum mechanical model of interacting electrons in a solid where the positive charges are assumed to be uniformly distributed in space whence the electron densityis a uniform quantity as well in space...
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
Lennard-Jones potential
The Lennard-Jones potential is a mathematically simple model that approximates the interaction between a pair of neutral atoms or molecules. A form of the potential was first proposed in 1924 by John Lennard-Jones...
. Other methods are readily available and it has been established that the magic number
Magic number (chemistry)
In case a gas condenses into clusters of atoms, the number of atoms in these clusters varies between a few and hundreds. However, there are peaks at specific sizes, usually one at lower and one at larger numbers. Often, specific numbers dominate...
s are 13, 19, 23, 26, 29, 32, 34, 43, 46, 49, 55, etc.
Aluminum clusters
- Al7 = the property is similar to germaniumGermaniumGermanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....
atoms. - Al13 = the property is similar to halogenHalogenThe halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
atoms, more specifically, chlorineChlorineChlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
.- Al13Ix–, where x = 1-13.
- Al14 = the property is similar to alkaline metalsAlkaline earth metalThe alkaline earth metals are a group in the periodic table. In the modern IUPAC nomenclature, the alkaline earth metals are called the group 2 elements. Previously, they were called the Group IIA elements . The alkaline earth metals contain beryllium , magnesium , calcium , strontium , barium and...
.- Al14Ix–, where x = 1-14.
- 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.
- VSi16F = has ionic bonding.
- A cluster of 13 platinumPlatinumPlatinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
becomes paramagnetic. - A cluster of 2000 rubidiumRubidiumRubidium is a chemical element with the symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group. Its atomic mass is 85.4678. Elemental rubidium is highly reactive, with properties similar to those of other elements in group 1, such as very rapid...
atoms.
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 numberMagic number (chemistry)
In case a gas condenses into clusters of atoms, the number of atoms in these clusters varies between a few and hundreds. However, there are peaks at specific sizes, usually one at lower and one at larger numbers. Often, specific numbers dominate...
via,
ne = NvA - 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. For example the Au102(p-MBA)44 has 58 electrons and corresponds to a closed shell magic number.
Gold Superatom Complexes
- Au25(SMe)18–
- Au102(p-MBA)44
- Au144(SR)60
Other Superatom Complexes
- Ga23(N(Si(CH3)3)2)11
- Al50(C5(CH3)5)12
External links
- 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
- Beyond The Periodic Table, Computational Chemistry Portal, 2006
- Expanding the periodic table? The Scientist, 2005
- 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.