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= Lawrencium =
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Introduction
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Lawrencium is a synthetic chemical element; it has symbol Lr (formerly
Lw) and atomic number 103. It is named after Ernest Lawrence, inventor
of the cyclotron, a device that was used to discover many artificial
radioactive elements. A radioactive metal, lawrencium is the eleventh
transuranium element, the third transfermium, and the last member of
the actinide series. Like all elements with atomic number over 100,
lawrencium can only be produced in particle accelerators by bombarding
lighter elements with charged particles. Fourteen isotopes of
lawrencium are currently known; the most stable is 266Lr with
half-life 11 hours, but the shorter-lived 260Lr (half-life 2.7
minutes) is most commonly used in chemistry because it can be produced
on a larger scale.
Chemistry experiments confirm that lawrencium behaves as a heavier
homolog to lutetium in the periodic table, and is a trivalent element.
It thus could also be classified as the first of the 7th-period
transition metals. Its electron configuration is anomalous for its
position in the periodic table, having an s2p configuration instead of
the s2d configuration of its homolog lutetium. However, this does not
appear to affect lawrencium's chemistry.
In the 1950s, 1960s, and 1970s, many claims of the synthesis of
element 103 of varying quality were made from laboratories in the
Soviet Union and the United States. The priority of the discovery and
therefore the name of the element was disputed between Soviet and
American scientists. The International Union of Pure and Applied
Chemistry (IUPAC) initially established 'lawrencium' as the official
name for the element and gave the American team credit for the
discovery; this was reevaluated in 1992, giving both teams shared
credit for the discovery but not changing the element's name.
History
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In 1958, scientists at Lawrence Berkeley National Laboratory claimed
the discovery of element 102, now called nobelium. At the same time,
they also tried to synthesize element 103 by bombarding the same
curium target used with nitrogen-14 ions. Eighteen tracks were noted,
with decay energy around and half-life around 0.25 s; the Berkeley
team noted that while the cause could be the production of an isotope
of element 103, other possibilities could not be ruled out. While the
data agrees reasonably with that later discovered for 257Lr (alpha
decay energy 8.87 MeV, half-life 0.6 s), the evidence obtained in this
experiment fell far short of the strength required to conclusively
demonstrate synthesis of element 103. A follow-up on this experiment
was not done, as the target was destroyed. Later, in 1960, the
Lawrence Berkeley Laboratory attempted to synthesize the element by
bombarding 252Cf with 10B and 11B. The results of this experiment were
not conclusive.
The first important work on element 103 was done at Berkeley by the
nuclear-physics team of Albert Ghiorso, Torbjørn Sikkeland, Almon
Larsh, Robert M. Latimer, and their co-workers on February 14, 1961.
The first atoms of lawrencium were reportedly made by bombarding a
three-milligram target consisting of three isotopes of californium
with boron-10 and boron-11 nuclei from the Heavy Ion Linear
Accelerator (HILAC). The Berkeley team reported that the isotope
257103 was detected in this manner, and that it decayed by emitting an
8.6 MeV alpha particle with a half-life of . This identification was
later corrected to 258103, as later work proved that 257Lr did not
have the properties detected, but 258Lr did. This was considered at
the time to be convincing proof of synthesis of element 103: while the
mass assignment was less certain and proved to be mistaken, it did not
affect the arguments in favor of element 103 having been synthesized.
Scientists at Joint Institute for Nuclear Research in Dubna (then in
the Soviet Union) raised several criticisms: all but one were answered
adequately. The exception was that 252Cf was the most common isotope
in the target, and in the reactions with 10B, 258Lr could only have
been produced by emitting four neutrons, and emitting three neutrons
was expected to be much less likely than emitting four or five. This
would lead to a narrow yield curve, not the broad one reported by the
Berkeley team. A possible explanation was that there was a low number
of events attributed to element 103. This was an important
intermediate step to the unquestioned discovery of element 103,
although the evidence was not completely convincing. The Berkeley team
proposed the name "lawrencium" with symbol "Lw", after Ernest
Lawrence, inventor of the cyclotron. The IUPAC Commission on
Nomenclature of Inorganic Chemistry accepted the name, but changed the
symbol to "Lr". This acceptance of the discovery was later
characterized as being hasty by the Dubna team.
: + → * → + 5
The first work at Dubna on element 103 came in 1965, when they
reported to have made 256103 in 1965 by bombarding 243Am with 18O,
identifying it indirectly from its granddaughter fermium-252. The
half-life they reported was somewhat too high, possibly due to
background events. Later 1967 work on the same reaction identified two
decay energies in the ranges 8.35-8.50 MeV and 8.50-8.60 MeV: these
were assigned to 256103 and 257103. Despite repeat attempts, they were
unable to confirm assignment of an alpha emitter with a half-life of 8
seconds to 257103.
The Russians proposed the name "rutherfordium" for the new element in
1967: this name was later proposed by Berkeley for element 104.
: + → * → + 5
Further experiments in 1969 at Dubna and in 1970 at Berkeley
demonstrated an actinide chemistry for the new element; so by 1970 it
was known that element 103 is the last actinide. In 1970, the Dubna
group reported the synthesis of 255103 with half-life 20 s and alpha
decay energy 8.38 MeV. However, it was not until 1971, when the
nuclear physics team at University of California at Berkeley
successfully did a whole series of experiments aimed at measuring the
nuclear decay properties of the lawrencium isotopes with mass numbers
255 to 260, that all previous results from Berkeley and Dubna were
confirmed, apart from the Berkeley's group initial erroneous
assignment of their first produced isotope to 257103 instead of the
probably correct 258103. All final doubts were dispelled in 1976 and
1977 when the energies of X-rays emitted from 258103 were measured.
In 1971, the IUPAC granted the discovery of lawrencium to the Lawrence
Berkeley Laboratory, even though they did not have ideal data for the
element's existence. But in 1992, the IUPAC Transfermium Working Group
(TWG) officially recognized the nuclear physics teams at Dubna and
Berkeley as co-discoverers of lawrencium, concluding that while the
1961 Berkeley experiments were an important step to lawrencium's
discovery, they were not yet fully convincing; and while the 1965,
1968, and 1970 Dubna experiments came very close to the needed level
of confidence taken together, only the 1971 Berkeley experiments,
which clarified and confirmed previous observations, finally resulted
in complete confidence in the discovery of element 103. Because the
name "lawrencium" had been in use for a long time by this point, it
was retained by IUPAC, and in August 1997, the International Union of
Pure and Applied Chemistry (IUPAC) ratified the name lawrencium and
the symbol "Lr" during a meeting in Geneva.
Physical
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Lawrencium is the last actinide. Authors considering the subject
generally consider it a group 3 element, along with scandium, yttrium,
and lutetium, as its filled f-shell is expected to make it resemble
the other 7th-period transition metals. In the periodic table, it is
to the right of the actinide nobelium, to the left of the 6d
transition metal rutherfordium, and under the lanthanide lutetium with
which it shares many physical and chemical properties. Lawrencium is
expected to be a solid under normal conditions and have a hexagonal
close-packed crystal structure ('c'/'a' = 1.58), similar to its
lighter congener lutetium, though this is not yet known
experimentally. The enthalpy of sublimation of lawrencium is estimated
at 352 kJ/mol, close to the value of lutetium and strongly suggesting
that metallic lawrencium is trivalent with three electrons
delocalized, a prediction also supported by a systematic extrapolation
of the values of heat of vaporization, bulk modulus, and atomic volume
of neighboring elements to lawrencium. This makes it unlike the
immediately preceding late actinides which are known to be (fermium
and mendelevium) or expected to be (nobelium) divalent. The estimated
enthalpies of vaporization show that lawrencium deviates from the
trend of the late actinides and instead matches the trend of the
succeeding 6d elements rutherfordium and dubnium, consistent with
lawrencium's interpretation as a group 3 element. Some scientists
prefer to end the actinides with nobelium and consider lawrencium to
be the first transition metal of the seventh period.
Lawrencium is expected to be a trivalent, silvery metal, easily
oxidized by air, steam, and acids, and having an atomic volume similar
to that of lutetium and a trivalent metallic radius of 171 pm. It is
expected to be a rather heavy metal with a density of around 14.4
g/cm3. It is also predicted to have a melting point of around 1900 K
(1600 °C), not far from the value for lutetium (1925 K).
Chemical
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In 1949, Glenn T. Seaborg, who devised the actinide concept, predicted
that element 103 (lawrencium) should be the last actinide and that the
ion should be about as stable as in aqueous solution. It was not
until decades later that element 103 was finally conclusively
synthesized and this prediction was experimentally confirmed.
Studies on the element, performed in 1969, showed that lawrencium
reacts with chlorine to form a product that was most likely the
trichloride, . Its volatility was found to be similar to the chlorides
of curium, fermium, and nobelium and much less than that of
rutherfordium chloride. In 1970, chemical studies were performed on
1500 atoms of 256Lr, comparing it with divalent (No, Ba, Ra),
trivalent (Fm, Cf, Cm, Am, Ac), and tetravalent (Th, Pu) elements. It
was found that lawrencium coextracted with the trivalent ions, but the
short half-life of 256Lr precluded a confirmation that it eluted ahead
of in the elution sequence. Lawrencium occurs as the trivalent ion
in aqueous solution and hence its compounds should be similar to those
of the other trivalent actinides: for example, lawrencium(III)
fluoride () and hydroxide () should both be insoluble in water. Due to
the actinide contraction, the ionic radius of should be smaller than
that of , and it should elute ahead of when ammonium
α-hydroxyisobutyrate (ammonium α-HIB) is used as an eluant. Later 1987
experiments on the longer-lived isotope 260Lr confirmed lawrencium's
trivalency and that it eluted in roughly the same place as erbium, and
found that lawrencium's ionic radius was , larger than would be
expected from simple extrapolation from periodic trends. Later 1988
experiments with more lawrencium atoms refined this to and calculated
an enthalpy of hydration value of . It was also found that the
actinide contraction at the end of the actinides was larger than the
analogous lanthanide contraction, with the exception of the last
actinide, lawrencium: the cause was speculated to be relativistic
effects.
It has been speculated that the 7s electrons are relativistically
stabilized, so that in reducing conditions, only the 7p1/2 electron
would be ionized, leading to the monovalent ion. However, all
experiments to reduce to or in aqueous solution were unsuccessful,
similarly to lutetium. On the basis of this, the standard electrode
potential of the 'E'°() couple was calculated to be less than −1.56 V,
indicating that the existence of ions in aqueous solution was
unlikely. The upper limit for the 'E'°() couple was predicted to be
−0.44 V: the values for 'E'°() and 'E'°() are predicted to be −2.06 V
and +7.9 V. The stability of the group oxidation state in the 6d
transition series decreases as RfIV > DbV > SgVI, and lawrencium
continues the trend with LrIII being more stable than RfIV.
In the molecule lawrencium dihydride (), which is predicted to be
bent, the 6d orbital of lawrencium is not expected to play a role in
the bonding, unlike that of lanthanum dihydride (). has La-H bond
distances of 2.158 Å, while should have shorter Lr-H bond distances
of 2.042 Å due to the relativistic contraction and stabilization of
the 7s and 7p orbitals involved in the bonding, in contrast to the
core-like 5f subshell and the mostly uninvolved 6d subshell. In
general, molecular and LrH are expected to resemble the corresponding
thallium species (thallium having a 6s26p1 valence configuration in
the gas phase, like lawrencium's 7s27p1) more than the corresponding
lanthanide species. The electron configurations of and are expected
to be 7s2 and 7s1 respectively. However, in species where all three
valence electrons of lawrencium are ionized to give at least formally
the cation, lawrencium is expected to behave like a typical actinide
and the heavier congener of lutetium, especially because the first
three ionization potentials of lawrencium are predicted to be similar
to those of lutetium. Hence, unlike thallium but like lutetium,
lawrencium would prefer to form than LrH, and LrCO is expected to be
similar to the also unknown LuCO, both metals having valence
configuration σ2π1 in their monocarbonyls. The pπ-dπ bond is expected
to be seen in just as it is for and more generally all the . The
complex anion is expected to be stable with a configuration of 6d1
for lawrencium; this 6d orbital would be its highest occupied
molecular orbital. This is analogous to the electronic structure of
the analogous lutetium compound.
Atomic
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Lawrencium has three valence electrons: the 5f electrons are in the
atomic core. In 1970, it was predicted that the ground-state electron
configuration of lawrencium was [Rn]5f146d17s2 (ground state term
symbol 2D3/2), per the Aufbau principle and conforming to the
[Xe]4f145d16s2 configuration of lawrencium's lighter homolog lutetium.
But the next year, calculations were published that questioned this
prediction, instead expecting an anomalous [Rn]5f147s27p1
configuration. Though early calculations gave conflicting results,
more recent studies and calculations confirm the s2p suggestion. 1974
relativistic calculations concluded that the energy difference between
the two configurations was small and that it was uncertain which was
the ground state. Later 1995 calculations concluded that the s2p
configuration should be energetically favored, because the spherical s
and p1/2 orbitals are nearest to the atomic nucleus and thus move
quickly enough that their relativistic mass increases significantly.
In 1988, a team of scientists led by Eichler calculated that
lawrencium's enthalpy of adsorption on metal sources would differ
enough depending on its electron configuration that it would be
feasible to carry out experiments to exploit this fact to measure
lawrencium's electron configuration. The s2p configuration was
expected to be more volatile than the s2d configuration, and be more
similar to that of the p-block element lead. No evidence for
lawrencium being volatile was obtained and the lower limit for the
enthalpy of adsorption of lawrencium on quartz or platinum was
significantly higher than the estimated value for the s2p
configuration.
In 2015, the first ionization energy of lawrencium was measured, using
the isotope 256Lr. The measured value, , agreed very well with the
relativistic theoretical prediction of 4.963(15) eV, and also provided
a first step into measuring the first ionization energies of the
transactinides. This value is the lowest among all the lanthanides and
actinides, and supports the s2p configuration as the 7p1/2 electron is
expected to be only weakly bound. As ionisation energies generally
increase left to right in the f-block, this low value suggests that
lutetium and lawrencium belong in the d-block (whose trend they
follow) and not the f-block. That would make them the heavier
congeners of scandium and yttrium, rather than lanthanum and actinium.
Although some alkali metal-like behaviour has been predicted,
adsorption experiments suggest that lawrencium is trivalent like
scandium and yttrium, not monovalent like the alkali metals. A lower
limit on lawrencium's second ionization energy (>13.3 eV) was
experimentally found in 2021.
Even though s2p is now known to be the ground-state configuration of
the lawrencium atom, ds2 should be a low-lying excited-state
configuration, with an excitation energy variously calculated as 0.156
eV, 0.165 eV, or 0.626 eV. As such lawrencium may still be considered
to be a d-block element, albeit with an anomalous electron
configuration (like chromium or copper), as its chemical behaviour
matches expectations for a heavier analogue of lutetium.
Isotopes
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Fourteen isotopes of lawrencium are known, with mass number 251-262,
264, and 266; all are radioactive. Seven nuclear isomers are known.
The longest-lived isotope, 266Lr, has a half-life of about ten hours
and is one of the longest-lived superheavy isotopes known to date.
However, shorter-lived isotopes are usually used in chemical
experiments because 266Lr currently can only be produced as a final
decay product of even heavier and harder-to-make elements: it was
discovered in 2014 in the decay chain of 294Ts. 256Lr (half-life 27
seconds) was used in the first chemical studies on lawrencium:
currently, the longer-lived 260Lr (half-life 2.7 minutes) is usually
used for this purpose. After 266Lr, the longest-lived isotopes are
264Lr (), 262Lr (3.6 h), and 261Lr (44 min). All other known
lawrencium isotopes have half-lives under 5 minutes, and the
shortest-lived of them (251Lr) has a half-life of 24.4 milliseconds.
The half-lives of lawrencium isotopes mostly increase smoothly from
251Lr to 266Lr, with a dip from 257Lr to 259Lr.
Preparation and purification
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Most isotopes of lawrencium can be produced by bombarding actinide
(americium to einsteinium) targets with light ions (from boron to
neon). The two most important isotopes, 256Lr and 260Lr, can be
respectively produced by bombarding californium-249 with 70 MeV
boron-11 ions (producing lawrencium-256 and four neutrons) and by
bombarding berkelium-249 with oxygen-18 (producing lawrencium-260, an
alpha particle, and three neutrons). The two heaviest and
longest-lived known isotopes, 264Lr and 266Lr, can only be produced at
much lower yields as decay products of dubnium, whose progenitors are
isotopes of moscovium and tennessine.
Both 256Lr and 260Lr have half-lives too short to allow a complete
chemical purification process. Early experiments with 256Lr therefore
used rapid solvent extraction, with the chelating agent
thenoyltrifluoroacetone (TTA) dissolved in methyl isobutyl ketone
(MIBK) as the organic phase, and with the aqueous phase being buffered
acetate solutions. Ions of different charge (+2, +3, or +4) will then
extract into the organic phase under different pH ranges, but this
method will not separate the trivalent actinides and thus 256Lr must
be identified by its emitted 8.24 MeV alpha particles. More recent
methods have allowed rapid selective elution with α-HIB to take place
in enough time to separate out the longer-lived isotope 260Lr, which
can be removed from the catcher foil with 0.05 M hydrochloric acid.
External links
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*
* [
http://periodic.lanl.gov/103.shtml Los Alamos National Laboratory's
Chemistry Division: Periodic Table - Lawrencium]
* [
http://www.periodicvideos.com/videos/103.htm Lawrencium] at 'The
Periodic Table of Videos' (University of Nottingham)
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Original Article:
http://en.wikipedia.org/wiki/Lawrencium
