Lawrencium – facts and properties
- Symbol
- Lr
- Atomic number
- 103
- Atomic mass
- 266 u
- Period
- 7
- Category
- actinide
- Electron configuration
- [Rn] 5f¹⁴ 7s² 7p¹
- Electronegativity
- 1.3
- Valence electrons
- 3
- State at standard conditions
- solid
- Melting point
- 1627 °C
- Discovered
- 1961
Lawrencium is an artificial, radioactive element with atomic number 103, and it is the last of the actinoids. It is known only from experiments on single atoms made in accelerators. Does it belong in group 3 of the periodic table, together with lutetium? And why does its outermost electron seem to sit in a 7p orbital, when lutetium, just above it, has a 5d electron there? Since 2015 the second question has had a measurement behind it. The first still has no final answer.
Position in the periodic table
Occurrence and extraction
No lawrencium is found in nature. Every known isotope is radioactive. The longest-lived one, lawrencium-266, has a half-life of several hours, and many others last only seconds or minutes.
The element was discovered in 1961 at Berkeley, California. A team led by Albert Ghiorso fired boron ions at a californium target and detected a new alpha emitter. The first assignment of that signal later had to be corrected, and a group at Dubna produced results of its own in the 1960s. The symbol was first written Lw before it was changed to Lr.
The name honours Ernest Lawrence, who invented the cyclotron. This circular accelerator for charged particles made it possible to build new elements by bombardment.
Today lawrencium atoms are made by shooting heavy ions at actinoid targets, or they turn up as daughter nuclei when still heavier elements decay. The yield is a handful of atoms.
Properties and reactions
Lawrencium is in period 7 and is the fifteenth actinoid, in the actinoid row to the right of nobelium. Lutetium sits above it in the lanthanoid row. Almost every property is predicted rather than measured. That applies to the electronegativity of 1.3, which is only an estimate, and to the melting point of 1627 °C, which is likewise only an estimate. The atomic mass is given as 266 u, the mass number of lawrencium-266.
The electron configuration [Rn] 5f¹⁴ 7s² 7p¹ is a prediction as well, and its 7p electron is the striking part. In lutetium the outermost electron occupies a 5d orbital. For lawrencium, calculations put it in a 7p orbital instead. The reason is relativity: with such a large nuclear charge, electrons close to the nucleus move at a sizeable fraction of the speed of light. The 7p orbital reaches in close to the nucleus, which lowers its energy below that of the 6d level. In 2015 a team from the Japan Atomic Energy Agency measured the ionization energy of lawrencium for the first time. It comes out just under five electron volts, lower than for any other lanthanoid or actinoid, and it agrees with the relativistic calculations.
In solution, ion-exchange experiments show lawrencium as a triply charged ion, as with lutetium. That is where the group 3 debate starts. Some tables put lanthanum and actinium under scandium and yttrium, others put lutetium and lawrencium there. Arguments from electronic structure favour the second layout, and in 2021 a provisional report by an IUPAC task group recommended it as well. IUPAC itself has made no binding decision. Depending on the table, lawrencium therefore ends the actinoid row or sits in group 3.
Research and synthesis
Outside research, lawrencium is not needed. Because it is so hard to obtain, it serves as a test of how well theory describes the end of the actinoid series.
The 2015 measurement shows how demanding that is. It used lawrencium-256, whose half-life is only about half a minute. The atoms were caught in a gas stream, ionized on a hot metal surface and counted one by one. A run takes days.
With so little material, the element itself poses no hazard. Radiation protection matters mainly at the accelerators.
Quick question
What is the atomic number of lawrencium (Lr)?
FAQ
Does lawrencium belong in group 3?
People still argue about it. A provisional report by an IUPAC task group from 2021 favours lutetium and lawrencium as the lower members of group 3, while other tables list lanthanum and actinium there. Lawrencium can be studied only atom by atom, so the choice rests mostly on theory.
Why does lawrencium have a 7p electron?
Calculations say relativistic effects pull the 7p level down in energy, so the outermost electron goes into a 7p orbital rather than a 6d one. The ionization energy measured in 2015 fits that prediction.
Has anyone seen lawrencium as a metal?
No. Visible amounts have never been made. The melting point quoted is an estimate, and most other properties of a bulk sample have not been measured.