Lawrencium

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 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 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 trivalent. It could thus 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 synthesis of element 103 of varying quality were made by 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.

Introduction

History

thumb|left|[[Albert Ghiorso updating the periodic table in April 1961, writing the symbol "Lw" in as element 103. Codiscoverers Latimer, Sikkeland, and Larsh (left to right) look on.]]

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 ¹⁴N 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 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 ²⁵⁷Lr was detected in this manner, and that it decayed by emitting an 8.6 MeV alpha particle with a half-life of . This acceptance of the discovery was later characterized as being hasty by the Dubna team. The Russians proposed the name "rutherfordium" for the new element in 1967;

: + → * → + 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 ²⁵⁵Lr with half-life 20 s and alpha decay energy 8.38 MeV. 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 ²⁵⁷Lr instead of the probably correct ²⁵⁸Lr. This makes it unlike the immediately preceding late actinides which are either 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, 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.

Chemical

thumb|right|upright=1.4|Elution sequence of the late trivalent lanthanides and actinides, with ammonium α-HIB as eluant: the broken curve for lawrencium is a prediction.

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.

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

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 ²D_3/2), 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. 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. 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 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. However, shorter-lived isotopes are usually used in chemical experiments because ²⁶⁶Lr 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. 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. 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.