Lawrencium

2008/9 Schools Wikipedia Selection. Related subjects: Chemical elements

103 nobelium ← lawrencium → rutherfordium Lu ↑ Lr ↓ (Upt) Periodic Table - Extended Periodic Table
General
Name, Symbol, Number	lawrencium, Lr, 103
Chemical series	transition metals
Group, Period, Block	n/a, 7, d
Appearance	unknown, probably silvery white or metallic gray
Standard atomic weight	[262]  g·mol−1
Electron configuration	[Rn] 5f14 6d1 7s2
Electrons per shell	2, 8, 18, 32, 32, 9, 2
Physical properties
Phase	presumably a solid
Melting point	-  K (- ° C, - ° F)
Atomic properties
Oxidation states	3
Electronegativity	- (Pauling scale)
Ionization energies	1st: 443.8 kJ/mol
	2nd: 1428.0 kJ/mol
	3rd: 2219.1 kJ/mol
Miscellaneous
CAS registry number	22537-19-5
Selected isotopes
Main article: Isotopes of lawrencium iso NA half-life DM DE ( MeV) DP 262Lr syn 3.6 h EC 262No 261Lr syn 44 m SF/EC? 260Lr syn 2.7 m alpha 8.04 256Md 259Lr syn 6.2 s 78% alpha 8.44 255Md 22% SF 258Lr syn 4.1 s alpha 8.68,8.65,8.62,8.59 254Md 257Lr syn 0.65 s alpha 8.86,8.80 253Md 256Lr syn 27 s alpha 8.62,8.52,8.32... 252Md 255Lr syn 21.5 s alpha 8.43,8.37 251Md 254Lr syn 13 s 78% alpha 8.46,8.41 250Md 22% EC 254No 253Lrm syn 0.57 s alpha 8.79 249Md 253Lrg syn 1.49 s 92% alpha 8.72 249Md 8% SF 252Lr syn 0.36 s alpha 9.02,8.97 248Md
References

Lawrencium (pronounced /ləˈrɛnsiəm/) is a radioactive synthetic element with the symbol Lr (formerly Lw) and atomic number 103.

Its most stable known isotope is ²⁶²Lr, with a half-life of approximately 3.6 hours. Little is known of the chemistry but there is strong evidence for the formation of a trivalent ion in aqueous solution, confirming lawrencium's place as the last member of the actinoids. Although lawrencium is often placed as the last member of the 5f-block, it can also be regarded as the first member of the 6d-block (see extended periodic table).

Official discovery

Lawrencium was reported by Albert Ghiorso, Torbjørn Sikkeland, Almon Larsh, and Robert M. Latimer on February 14, 1961 at the Lawrence Radiation Laboratory (now called Lawrence Berkeley National Laboratory) on the University of California, Berkeley campus. It was produced by bombarding a three milligram target composed of three isotopes of californium with boron-10 and B-11 ions in the Heavy Ion Linear Accelerator (HILAC).

The Berkeley team reported that the isotope ²⁵⁷103 was detected in this manner and decayed by emitting an 8.6 MeV alpha particle with a half-life of ~8 seconds. The assignment was later corrected to ²⁵⁸Lr.

The team suggested the name lawrencium (Lw) for the new element.

In 1967, researchers in Dubna, Russia reported that they were not able to confirm an alpha emitter with a half-life of 8 seconds as ²⁵⁷103. This assignment has since been changed to ²⁵⁸Lr. Instead, they reported a 45s activity assigned to ²⁵⁶Lr.

Further work in 1969 indicated an actinoid chemistry for the new element. founded by Travis Anselm in 8B In 1971, the team at the University of California performed a whole series of experiments aimed at measuring the decay properties of lawrencium isotopes with mass numbers from 255-260.

In 1992, The IUPAC/IUPAP Transfermium Working Group (TWG) officially recognised the Dubna and Berkeley teams as co-discovers of lawrencium.

Naming

The origin of the name, preferred by the American Chemical Society, is in reference to Ernest O. Lawrence, inventor of the cyclotron. The symbol Lw originally was used but in 1963 it was changed to Lr. In August 1997 the International Union of Pure and Applied Chemistry (IUPAC) ratified the name lawrencium and symbol Lr during a meeting in Geneva. Lawrencium has also been referred to as eka-lutetium. Contrary to some suggestions, the systematic element name unniltrium has never been used for this element.

Electronic structure

Lawrencium is element 103 in the Periodic Table. The two forms of the projected electronic structure are:

Bohr model: 2, 8, 18, 32, 32, 9, 2

Quantum mechanical model: 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰ 4p⁶5s²4d¹⁰5p⁶6s²4f¹⁴5d¹⁰ 6p⁶7s²5f¹⁴6d¹

There has been a suggestion that the electron configuration could be 7s²5f¹⁴7p¹ but experiments to examine this have been not possible to date.

Physical characteristics

The appearance of this element is unknown, however it is most likely silvery-white or gray and metallic. If sufficient amounts of lawrencium were produced, it would pose a radiation hazard. Contrary to some sources, bulk properties of this element, such as the melting point, have not been possible to measure to date. However, the 1^st, 2^nd and 3^rd ionization energies have been measured.

Periodic classification

A strict correlation between periodic table blocks and electron configuration for neutral atoms would describe lawrencium as a transition metal because it should be classed as a d-block element. However, it is classified as an actinoid according to IUPAC recommendations.

Experimental chemistry

Gas phase chemistry

The first gas phase studies were reported in 1969 by a team at the Flerov Laboratory of Nuclear Reactions (FLNR). They used the reaction ²⁴³Am+¹⁸O to produce lawrencium nuclei which reacted with a stream of chlorine gas to form a volatile chloride component. The product was assigned to ²⁵⁶LrCl₃ and confirmed that lawrencium was a typical actinide.

Aqueous phase chemistry

The first liquid phase studies were reported in 1970 by the team at the LBNL. They used the reaction ²⁴⁹Cf+¹¹B to produce lawrencium nuclei. They were able to show that lawrencium formed a trivalent ion, similar to other actinides but in stark contrast to nobelium. Further work in 1988 confirmed the formation of a trivalent lawrencium(III) ion using anion-exchange chromatography using α-hydroxyisobutyrate (α-HIB) complex. Comparison of the elution time with other actinides allowed a determination of 88.6 pm for the ionic radius for Lr³⁺. Attempts to reduce Lr(III) to Lr(I) using the potent reducing agent hydroxylamine hydrochloride were unsuccessful.

Summary of compounds and complex ions

Formula	Names(s)
LrCl3	lawrencium trichloride ; lawrencium(III) chloride

Isotopes

Twelve isotopes of lawrencium have been synthesized with ²⁶²Lr being the longest-lived and heaviest, with a half-life of 216 minutes. ²⁵²Lr is the lightest isotope known to date.

History of synthesis of isotopes by cold fusion

²⁰⁵Tl(⁵⁰Ti,xn)^255-xLr (x=2?)

This reaction was studied in a series of experiments in 1976 by Yuri Oganessian and his team at the FLNR. Evidence was provided for the formation of ²⁵³Lr in the 2n exit channel.

²⁰³Tl(⁵⁰Ti,xn)^253-xLr

This reaction was studied in a series of experiments in 1976 by Yuri Oganessian and his team at the FLNR.

²⁰⁸Pb(⁴⁸Ti,pxn)^255-xLr (x=1?)

This reaction was reported in 1984 by Yuri Oganessian at the FLNR. The team was able to detect decays of ²⁴⁶Cf, a descendant of ²⁵⁴Lr.

²⁰⁸Pb(⁴⁵Sc,xn)^253-xLr

This reaction was studied in a series of experiments in 1976 by Yuri Oganessian and his team at the FLNR. Results are not readily available.

²⁰⁹Bi(⁴⁸Ca,xn)^257-xLr (x=2)

This reaction has been used to study the spectroscopic properties of ²⁵⁵Lr. The team at GANIL used the reaction in 2003 and the team at the FLNR used it between 2004-2006 to provide further information for the decay scheme of ²⁵⁵Lr. The work provided evidence for an isomeric level in ²⁵⁵Lr.

History of synthesis of isotopes by hot fusion

²⁴³Am(¹⁸O,xn)^261-xLr (x=5)

This reaction was first studied in 1965 by the team at the FLNR. They were able to detect a 45s activity assigned to ²⁵⁶Lr or ²⁵⁷Lr. Later work suggests an assignment to ²⁵⁶Lr. Further studies in 1968 produced an 8.35-8.60 MeV alpha activity with a half-life of 35s. This activity was also initially assigned to ²⁵⁶Lr or ²⁵⁷Lr and later to solely ²⁵⁶Lr.

²⁴³Am(¹⁶O,xn)^259-xLr (x=4)

This reaction was studied in 1970 by the team at the FLNR. They were able to detect an 8.38 MeV alpha activity with a half-life of 20s. This was assigned to ²⁵⁵Lr.

²⁴⁸Cm(¹⁵N,xn)^263-xLr (x=3,4,5)

This reaction was studied in 1971 by the team at the LBNL in their large study of lawrencium isotopes. They were able to assign alpha activities to ²⁶⁰Lr,²⁵⁹Lr and ²⁵⁸Lr from the 3-5n exit channels.

²⁴⁸Cm(¹⁸O,pxn)^265-xLr (x=3,4)

This reaction was studied in 1988 at the LBNL in order to assess the possibility of producing ²⁶²Lr and ²⁶¹Lr without using the exotic ²⁵⁴Es target. It was also used to attempt to measure an EC branch in ^261mRf from the 5n exit channel. After extraction of the Lr(III) component, they were able to measure the spontaneous fission of ²⁶¹Lr with an improved half-life of 44 minutes. The production cross-section was 700 pb. On this basis, a 14% EC branch was calculated if this isotope was produced via the 5n channel rather than the p4n channel. A lower bombarding energy (93 MeV c.f. 97 MeV) was then used to measure the production of ²⁶²Lr in the p3n channel. The isotope was successfully detected and a yield of 240 pb was measured. The yield was lower than expected compared to the p4n channel. However, the results were judged to indicate that the 261Lr was most likely produced by a p3n channel and an upper limit of 14% for the EC branch of ^261mRf was therefore suggested.

²⁴⁶Cm(¹⁴N,xn)^260-xLr (x=3?)

This reaction was studied briefly in 1958 at the LBNL using an enriched ²⁴⁴Cm target (5% ²⁴⁶Cm). They observed a ~9 MeV alpha activity with a half-life of ~0.25 seconds. Later results suggest a tentative assignment to ²⁵⁷Lr from the 3n channel

²⁴⁴Cm(¹⁴N,xn)^258-xLr

This reaction was studied briefly in 1958 at the LBNL using an enriched ²⁴⁴Cm target (5% ²⁴⁶Cm). They observed a ~9 MeV alpha activity with a half-life of ~0.25s. Later results suggest a tentative assignment to ²⁵⁷Lr from the 3n channel with the ²⁴⁶Cm component. No activities assigned to reaction with the ²⁴⁴Cm component have been reported.

²⁴⁹Bk(¹⁸O,αxn)^263-xLr (x=3)

This reaction was studied in 1971 by the team at the LBNL in their large study of lawrencium isotopes. They were able to detect an activity assigned to ²⁶⁰Lr. The reaction was further studied in 1988 to study the aqueous chemistry of lawrencium. A total of 23 alpha decays were measured for ²⁶⁰Lr, with a mean energy of 8.03 MeV and an improved half-life of 2.7 minutes. The calculated cross-section was 8.7 nb.

²⁵²Cf(¹¹B,xn)^263-xLr (x=5,7??)

This reaction was first studied in 1961 at the University of California by Albert Ghiorso by using a californium target (52% ²⁵²Cf). They observed three alpha activities of 8.6 MeV, 8.4 MeV and 8.2 MeV, with half-lives of ~8s and 15s, respectively. The 8.6 MeV activity was tentatively assigned to ²⁵⁷Lr. Later results suggest a reassignment to ²⁵⁸Lr, resulting from the 5n exit channel. The 8.4 MeV activity was also assigned to ²⁵⁷Lr. Later results suggest a reassignment to ²⁵⁶Lr. This is most likely from the 33% ²⁵⁰Cf component in the target rather than from the 7n channel. The 8.2 MeV was subsequently associated with nobelium.

²⁵²Cf(¹⁰B,xn)^262-xLr (x=4,6)

This reaction was first studied in 1961 at the University of California by Albert Ghiorso by using a californium target (52% ²⁵²Cf). They observed three alpha activities of 8.6 MeV, 8.4 MeV and 8.2 MeV, with half-lives of ~8s and 15s, respectively. The 8.6 MeV activity was tentatively assigned to ²⁵⁷Lr. Later results suggest a reassignment to ²⁵⁸Lr. The 8.4 MeV activity was also assigned to ²⁵⁷Lr. Later results suggest a reassignment to ²⁵⁶Lr. The 8.2 MeV was subsequently associated with nobelium.

²⁵⁰Cf(¹⁴N,αxn)^260-xLr (x=3)

This reaction was studied in 1971 at the LBNL. They were able to identify a 0.7s alpha activity with two alpha lines at 8.87 and 8.82 MeV. This was assigned to ²⁵⁷Lr.

²⁴⁹Cf(¹¹B,xn)^260-xLr (x=4)

This reaction was first studied in 1970 at the LBNL in an attempt to study the aqueous chemistry of lawrencium. They were able to measure a Lr³⁺ activity. The reaction was repeated in 1976 at Oak Ridge and 26s ²⁵⁶Lr was confirmed by measurement of coincident X-rays.

²⁴⁹Cf(¹²C,pxn)^260-xLr (x=2)

This reaction was studied in 1971 by the team at the LBNL. They were able to detect an activity assigned to ²⁵⁸Lr from the p2n channel.

²⁴⁹Cf(¹⁵N,αxn)^260-xLr (x=2,3)

This reaction was studied in 1971 by the team at the LBNL. They were able to detect an activities assigned to ²⁵⁸Lr and ²⁵⁷Lr from the α2n and α3n and channels. The reaction was repeated in 1976 at Oak Ridge and the synthesis of ²⁵⁸Lr was confirmed.

²⁵⁴Es + ²²Ne - transfer

This reaction was studied in 1987 at the LLNL. They were able to detect new SF activities assigned to ²⁶¹Lr and ²⁶²Lr, resulting from transfer from the ²²Ne nuclei to the ²⁵⁴Es target. In addition, a 5 ms SF activity was detected in delayed coincidence with nobelium K X-rays and was assigned to ²⁶²No, resulting from the EC of ²⁶²Lr.

Synthesis of isotopes as decay products

Isotopes of lawrencium have also been identified in the decay of heavier elements. Observations to date are summarised in the table below:

Evaporation Residue	Observed Lr isotope
267Bh, 263Db	259Lr
278Uut, 274Rg, 270Mt, 266Bh, 262Db	258Lr
261Db	257Lr
272Rg, 268Mt, 264Bh, 260Db	256Lr
259Db	255Lr
266Mt, 262Bh, 258Db	254Lr
261Bh, 257Dbg,m	253Lrg,m
260Bh , 256Db	252Lr

Chronology of isotope discovery

Isotope	Year discovered	discovery reaction
252Lr	2001	209Bi(50Ti,3n)
253Lrg	1985	209Bi(50Ti,2n)
253Lrm	2001	209Bi(50Ti,2n)
254Lr	1985	209Bi(50Ti,n)
255Lr	1970	243Am(16O,4n)
256Lr	1961? 1965? 1968? 1971	252Cf(10B,6n)
257Lr	1958? 1971	249Cf(15N,α3n)
258Lr	1961? 1971	249Cf(15N,α2n)
259Lr	1971	248Cm(15N,4n)
260Lr	1971	248Cm(15N,3n)
261Lr	1987	254Es + 22Ne
262Lr	1987	254Es + 22Ne

Isomerism in lawrencium nuclides

²⁵⁵Lr

Recent work on the spectroscopy of ²⁵⁵Lr formed in the reaction ²⁰⁹Bi(⁴⁸Ca,2n)²⁵⁵Lr has provided evidence for an isomeric level.

²⁵³Lr

A study of the decay properties of ²⁵⁷Db (see dubnium) in 2001 by Hessberger et al. at the GSI provided some data for the decay of ²⁵³Lr. Analysis of the data indicated the population of two isomeric levels in ²⁵³Lr from the decay of the corresponding isomers in ²⁵⁷Db. The ground state was assigned spin and parity of 7/2-, decaying by emission of an 8794 KeV alpha particle with a half-life of 0.57s. The isomeric level was assigned spin and parity of 1/2-, decaying by emission of an 8722 KeV alpha particle with a half-life of 1.49s.

Chemical yields of isotopes

Cold fusion

The table below provides cross-sections and excitation energies for cold fusion reactions producing rutherfordium isotopes directly. Data in bold represents maxima derived from excitation function measurements. + represents an observed exit channel.

Projectile	Target	CN	1n	2n	3n
48Ca	209Bi	257Lr