Super-átomos Uma tabela periódica tridimensional?

Super-átomos
Uma tabela periódica tridimensional?
Prof. Dr. Arnaldo Dal Pino Júnior
Dep. de Física do ITA
Lavoisier divided the few elements known in the
1700's into classes.
John Dalton
The mass of an atom was it's most important property
"The chemical elements are composed of... indivisible particles
of matter, called atoms... atoms of the same element are
identical in all respects, particularly weight." - Dalton
Fósforo (P).
Primeiro elemento a ser descoberto.
Ponto de partida para a construção
da tabela periódica".
Cloro, bromo e iôdo;
A tríade da primeira
tentativa.
Periodic table begins with German chemist Johann Dobereiner (1780-1849)
who grouped elements based on similarities.
Dobereiner noticed the atomic weight of strontium fell midway between the
weights of calcium and barium: Ca Sr Ba (40 + 137) ÷ 2 = 88
coincidence?
Dobereiner noticed the same pattern for the alkali metal triad (Li/Na/K) and the
halogen triad (Cl/Br/I).
Li
Na
K
Cl
Br
I
7
23 39
35
80 127
In 1829 Dobereiner proposed the Law of Triads: Middle element
in the triad had atomic weight that was the average of the other
two members.
Soon other scientists found chemical relationships extended
beyond triads. Fluorine was added to Cl/Br/I group; sulfur,
oxygen, selenium and tellurium were grouped into a family;
nitrogen, phosphorus, arsenic, antimony, and bismuth were
classified as another group
First Periodic Table
Alexandre Beguyer de Chancourtois (1820-1886)
professor of geology at the School of Mines in Paris
recognized periodicity in the physical properties of the elements.
1862 list of all the known elements.
The list was constructed as a helical graph wrapped around a
cylinder--elements with similar properties occupied positions on
the same vertical line of cylinder (the list also included some
ions and compounds). Using geological terms and published
without the diagram, de Chancourtois ideas were completely
ignored until the work of Mendeleev.
Parafuso Telúrico de Chancourtois
Law of Octaves 1863
English chemist John Newlands (18371898), having arranged the 62 known
elements in order of increasing atomic
weights, noted that after interval of eight
elements similar Physical /chemical
properties reappeared.
Newlands was the first to formulate the
concept of periodicity in the properties of
the chemical elements.
In 1863 he wrote a paper proposing the Law of Octaves:
Elements exhibit similar behavior to the eighth element
following it in the table.
Lítio, potássio e sódio;
pela primeira vez,
juntos no modelo das
oitavas de Newlands
Mendeleev's Periodic Table
In 1869, Russian chemist Dimitri Mendeleev (1834-1907)
proposed arranging elements by atomic weights and
properties.
Mendeleev's periodic table of 1869 contained 17 columns
with two partial periods of seven elements each (Li-F &
Na-Cl) followed by two nearly complete periods (K-Br &
Rb-I).
In 1871 Mendeleev revised the 17-group table with eight
columns (the eighth group consisted of transition
elements). This table exhibited similarities not only in
small units such as the triads, but showed similarities in
an entire network of vertical, horizontal, and diagonal
relationships.
The table contained gaps but Mendeleev predicted the
discovery of new elements. In 1906, Mendeleev came
within one vote of receiving the Nobel Prize in chemistry.
Em 1869, enquanto escrevia seu livro de química inorgânica,
organizou os elementos na forma da tabela periódica atual.
Mendeleev criou uma carta para cada um dos 63 elementos
conhecidos.
Cada carta continha o símbolo do elemento, a massa atômica e
suas propriedades químicas e físicas.
Colocando as cartas em uma mesa, organizou-as em ordem
crescente de suas massas atômicas, agrupando-as em
elementos de propriedades semelhantes. Formou-se então a
tabela periódica.
Julius Lothar Meyer (1830 - 1895).
Em 1869, Meyer e Mendeleyev, trabalhando
independentemente, lançaram classificações periódicas
semelhantes. Mas o brilhantismo das previsões de
Mendeleyev ofuscou por completo o resultado das pesquisas
de Lothar Meyer.
Em 1882, porém, os dois cientistas receberam a Medalha
Davy, a mais alta honraria da Associação Britânica para o
Progresso da Ciência.
Vale lembrar também que, em 1887, outra injustiça foi
reparada. A mesma medalha foi oferecida a Newlands, o
cientista que fora ridicularizado por sua classificação baseada
nas oitavas musicais.
A consagração de Mendeleyev
O cientista russo deixou alguns espaços vagos na sua tabela,
justificando que esses locais eram reservados para o eventual
ordenamento de elementos, na época, ainda desconhecidos,
denominando-os de:
Eka-boro (abaixo do boro);
Eka-aluminio (abaixo do aluminio);
Eka-silício (abaixo do silício).
Demonstrando grande sagacidade científica, Mendeleyev definiu
as propriedades desses elementos ainda desconhecidos.
Previsões
Suas previsões se mostraram corretas:
em 1874, o Eka-alumínio foi descoberto por L. Boisbaudran,
recebendo o nome de Gálio;
cinco anos depois, Lars F. Nilson descobriu o Eka-boro, cuja
denominação passou a ser de Escândio;
finalmente, em 1886, Clemens Alexander Winkler descobriu o
Eka-silício, elemento hoje denominado de Germânio.
Para melhor compreensão, observe:
A tabela abaixo mostra as propriedades do germânio e
as propriedades previstas por Mendeleev para esse
elemento, que na época era desconhecido e o qual
Mendeleev nomeou de eka-silício.
Propriedades
Eka-Silício
Germânio
Massa Atômica
72
72,6
Densidade(g/c
m3)
Cor
5,50
5.47
Cinzento
Cinza claro
4,7
4,7
Densidade
Óxido
A tabela abaixo mostra as propriedades do germânio e
as propriedades previstas por Mendeleev para esse
elemento, que na época era desconhecido e o qual
Mendeleev nomeou de eka-alumínio.
Propriedades
Eka-Alumínio
Gálio
Massa Atômica
68
69,7
Densidade(g/c
m3)
Pto. de Fusão
5,9
5.94
Baixo
30,15 0C
Óxido
(EAl)2O3
Ga2O3
Lord Rayleigh (1842-1919)
William Ramsey (1852-1916)
In 1895 Rayleigh reported the discovery of a new gaseous
element named argon. This element was chemically inert and did
not fit any of the known periodic groups.
Ramsey followed by discovering the remainder of the inert gases
and positioning them in the periodic table.
So by 1900, the periodic table was taking shape with elements
were arranged by atomic weight. For example, 16g oxygen reacts
with 40g calcium, 88g strontium, or 137g barium.
Rayleigh (physics) and Ramsey (chemistry) were awarded Nobel prizes in 1904. The first inert gas compound
was made in 1962 (xenon tetrafluoride) and numerous compounds have followed
Soon after Rutherford's landmark
experiment of discovering the
proton in 1911, Henry Moseley
(1887-1915) subjected known
elements to x-rays. He was able
to derive the relationship between
x-ray frequency and number of
protons.
Moseley's Periodic Law
When Moseley arranged the elements according to increasing
atomic numbers and not atomic masses, some of the
inconsistencies associated with Mendeleev's table were
eliminated. The modern periodic table is based on Moseley's
Periodic Law (atomic numbers).
At age 28, Moseley was killed in action during World War I and as a direct result Britain adopted the policy of
exempting scientists from fighting in wars. Shown below is a periodic table from 1930:
1872 - A tabela periódica de Mendeleyev.
Os espaços marcados com traços representam elementos que Mendeleyev deduziu
existirem mas que ainda não haviam sido descobertos àquela época. Os símbolos no
topo de cada coluna são as fórmulas moleculares escritas no estilo do século XIX.
Modern Periodic Table
The last major change to the periodic table resulted from
Glenn Seaborg's work in the middle of the 20th century.
Starting with plutonium in 1940, Seaborg discovered
transuranium elements 94 to 102 and reconfigured the
periodic table by placing the lanthanide/actinide series at
the bottom of the table. In 1951 Seaborg was awarded the
Nobel Prize in chemistry and element 106 was later
named seaborgium (Sg) in his honor.
Dr. Timmothy Stowe's physicists periodic table.
The periodic spiral of Professor Theodor Benfey
A triangular long form periodic table by Emil Zmaczynski.
Superatoms
Superatoms are clusters of atoms which seem to exhibit some of
the properties of elemental atoms.
Sodium atoms when left to condense in clusters from vapour
naturally form into clusters of 8, 20, 40, 58 or 92 atoms (the magic
numbers).
The suggestion is that free electrons in the cluster form atomic like
structure
The atomic properties of this structure should mimic the atomic
properties of atoms with filled s and p orbitals, i.e. the noble
gases.
Superatoms
Jellium is the theory of interacting electrons in which a
uniform background of positive charge exists. In this
theory at zero temperature the system properties are
dependent only on the density of electrons.
This allows for the simplistic calculation of the electronelectron coupling energy being a ratio between the freeelectron kinetic energy and the Coulomb potential
energy.
The jellium theory is used in nuclear physics. It has been
used to try to explain the properties of superatoms.
Clusters of Aluminum Atoms
Found to Have Properties of Other Elements
A research team has discovered clusters of aluminum atoms
that have chemical properties similar to single atoms of metallic
and nonmetallic elements when they react with iodine.
The discovery opens the door to using 'superatom chemistry'
based on a new periodic table of cluster elements to create
unique compounds with distinctive properties never seen before.
Shiv N. Khanna, professor of physics at Virginia Commonwealth University and A. Welford Castleman Jr., the
Evan Pugh Professor of Chemistry and Physics and the Eberly Family Distinguished Chair in Science at Penn
State University
14 January 2005 issue of the journal Science
Certain aluminium clusters have superatom
properties.
These aluminium clusters are generated as anions (Aln- with n =
1,2,3...) in a helium gas and reacted with a gas containing
molecular iodine.
When analyzed by mass spectroscopy one main reaction product
turns out to be Al13IThese clusters of 13 Al atoms with an extra electron added does
also appear not 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 alkali 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.
Science 2 April 2004: Vol. 304. no. 5667, pp. 84 - 87
Formation of Al13I-: Evidence for the Superhalogen Character of
Al13
Denis E. Bergeron,1 A. Welford Castleman, Jr.,1* Tsuguo Morisato,2 Shiv N.
Khanna2
Al13– is a cluster known for the pronounced stability that arises from
coincident closures of its geometric and electronic shells. We present
experimental evidence for a very stable cluster corresponding to
Al13I–.
Ab initio calculations show that the cluster features a structurally
unperturbed Al13– core and a region of high charge density on the
aluminum vertex opposite from the iodine atom. This ionically bound
magic cluster can be understood by considering that Al13 has an
electronic structure reminiscent of a halogen atom. Comparisons to
polyhalides provide a sound explanation for our chemical
observations.
Al Cluster Superatoms as Halogens in Polyhalides
and as Alkaline Earths in Iodide Salts
D. E. Bergeron, P. J. Roach,1 A. W. Castleman, Jr., N. O. Jones,
S. N. Khanna
Two classes of gas-phase aluminum-iodine clusters have been identified
whose stability and reactivity can be understood in terms of the spherical shell
jellium model. Experimental reactivity studies show that the Al13I –x clusters
exhibit pronounced stability for even numbers of I atoms. Theoretical
investigations reveal that the enhanced stability is associated with
complementary pairs of I atoms occupying the on-top sites on the opposing Al
atoms of the Al13– core. We also report the existence of another series, Al14I –
x, that exhibits stability for odd numbers of I atoms. This series can be
described as consisting of an Al14I –3 core upon which the I atoms occupy ontop locations around the Al atoms. The potential synthetic utility of superatom
chemistry built upon these motifs is addressed.
Pensar é o esporte mais
radical que existe:
pratique-o.
Prof. Dr. Arnaldo Dal Pino Júnior
Dep. de Física do ITA