Nuclear chemistry: Lawrencium bridges a knowledge gap

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Nature News and Views feature concerning our article in Nature on the measurement of the first ionization potential of lawrencium (Nature 520, 209 (2015), cover page feature).

Accurately measuring the chemical properties of elements that have an atomic number (Z) greater than 100 is exceedingly difficult because of the limited availability of these elements. But by synthesizing single, short-lived atoms of lawrencium (Z = 103) in nuclear fusion reactions, Satoet al.¹ (page 209 of this issue) have now measured this element's first ionization potential — the energy required to remove a single electron from an uncharged gaseous atom. This allowed them to compare the experimental value with theoretical calculations and to bridge a gap in our knowledge of the chemistry of the actinide elements, of which lawrencium is the final member in the periodic table.

Einstein's theory of relativity is perhaps best known for its pivotal role in high-energy physics and astrophysics, but it also has implications for the properties of atoms. The higher the nuclear charge of an atom, the faster the orbiting electrons move, attaining velocities corresponding to a considerable fraction of the speed of light. The relativistic mass of the electrons therefore increases, causing certain inner electron orbitals to contract and stabilize, and thus increasing the shielding of the nuclear charge. This in turn causes other orbitals to expand and destabilize. In addition, 'spin–orbital' interactions become very large for electrons in heavy elements, changing the energy levels of certain orbitals and causing them to split in response to relativistic effects. Theoretical chemists must thus include relativity in their calculations of atomic properties — indeed, this is the only way to explain why gold has its characteristic yellow colour² or why mercury is a liquid at room temperature³.

Experimental study of the superheavy elements that exhibit the greatest relativistic effects is difficult. The heaviest element currently reported has an atomic number of 118, which makes it a member of the noble gases, below xenon and radon in the periodic table. All the other elements in the seventh row of the table (which includes element 118) have also been synthesized. Elements heavier than fermium (Z = 100) can be produced only as single atoms in nuclear fusion reactions at heavy-ion accelerators, and even the longest-lived isotopes have short half-lives, ranging from several hours to less than one millisecond. The half-life of the lawrencium isotope made by Satoet al. is 27 seconds.

The authors synthesized lawrencium by firing a beam of energetic boron-11 ions at a target of californium-249 atoms, producing one lawrencium atom every few seconds in a nuclear fusion reaction. The lawrencium atoms recoil from the californium target because of momentum transfer during fusion, and so the authors captured them by using a helium gas atmosphere seeded with aerosol particles of cadmium iodide — the atoms rapidly diffused to the surface of the aerosol particles.

The helium–aerosol mixture was then passed through a capillary to an ionization site, efficiently transporting the atoms; the ionization site consisted of a metal surface heated to 2,700 or 2,800 kelvin. The researchers measured the efficiency of the ionization of lawrencium atoms on the surface by extracting the ions and sending them through a mass analyser. Cadmium iodide evaporates completely at this temperature, and so did not interfere with the surface ionization process.

From their measurements, Sato and co-workers deduced that the first ionization potential of lawrencium is 4.96 electronvolts. This compares favourably with the calculated value of 4.963(15) eV, which is also reported by the authors in the current paper. Their calculation accounts not only for relativistic effects, but also for minor contributions from quantum electrodynamic effects. These latter effects cause an extra shift in orbital energy levels because of the interaction of electrons with their own electromagnetic radiation field and because of the magnetostatic interaction between electrons.

Lawrencium thus has the lowest first ionization potential of all the lanthanide and actinide elements (Fig. 1). This confirms the proposed picture of the arrangement of the element's outer electrons: the 5f orbitals are filled with 14 electrons; a relativistically stabilized 7s orbital contains two electrons; and an additional, weakly bound electron resides in the 7p1/2 valence orbital.

Figure 1: Periodic table of the elements.

Bar sizes represent the energy of the first ionization potential for each element — the energy required to remove a single electron from an uncharged gaseous atom. Sato et al.¹ have measured the first ionization potential for lawrencium (Lr; red). The binding energy of the least-bound valence electron of Lr, which can be determined from its ionization potential, is the lowest of those of all the elements, except for group-1 elements heavier than sodium (Na). The first ionization potentials for the elements with symbols depicted in blue have not been experimentally determined. Ln, lanthanides; An, actinides.

Kazuaki Tsukada

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About 10¹² atoms were needed to determine the first ionization potentials of elements in the past, but, with their spectacular experiment, Sato et al. have lowered the amount of material needed by many orders of magnitude. Does this mean that we will now be able to use this technique to accurately measure all first ionization potentials for superheavy elements up to element 118? Unfortunately not. The ionization potentials for the elements rutherfordium (Z = 104) to copernicium (Z = 112) are expected⁴ to rise from 6 eV to a staggering 12 eV, the highest value for any known metal. Surface ionization techniques are sadly inefficient for ionization potentials of this magnitude.

Furthermore, Sato and colleagues benefited from being able to generate a few thousand atoms of lawrencium, but for heavier elements it is currently possible to generate only a few atoms — about 1 per week for element 118 (ref. 5). Major improvements in experimental techniques would therefore be needed to study those elements, especially because they are radioactive and have short half-lives. Ideally, an atom (or ion) should be confined in a trap, and several measurements of its properties taken before it decays. Ion traps for nobelium (Z = 102) and lawrencium have been reported⁶, but not for heavier elements. Much ingenuity will be required to push the limits beyond lawrencium, although there is certainly hope that this will become possible.

In the meantime, spectacular developments in relativistic quantum theory and computational algorithms have allowed highly accurate calculations to be made of the basic atomic properties of the heaviest elements. This is just as well because, probably for years to come, theoretical investigations will be the only way to learn more about the chemical properties of some superheavy elements. Sato and co-workers' experimental validation of the predictive power of theoretical methods is therefore highly reassuring.