September 2012

# New period in the periodic table?

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Geoff Rayner-Canham, Grenfell Campus, Memorial University Cornerbrook, NF A2H 6P9

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When will the first element of Period 8 be synthesized? Which element will it be? Which country will first accomplish the task? Now that each element in Period 7 has been synthesized, these are the questions on the minds of every nuclear chemist and physicist.

## The synthesis of new elements

The new elements are synthesized by firing beams of one element at a target of another element. The target element must have as high an atomic (proton) number as possible. Not only must the projectile have a high enough atomic number, but just as important, it must have as many neutrons as possible. The reason is that the proton:neutron ratio in stable (and long-lived radioactive) isotopes increases from about 1:1 for the early elements to 1:1.6 for uranium-238. One can use a simple model that increasing quantities of neutron “glue” are needed to hold together the positively-charged protons.

## The preference for even numbers of nucleons

Stable element isotopes reflect two phenomena both related to the nuclear shell model of the atom. That is, we can consider protons and neutrons filling shells just like we do electrons. In “normal” chemistry, electron-pairing is important, but for nuclei, proton-pairing and neutron-pairing within the nucleus is even more important. In fact, of the 273 stable nuclei, only four have odd numbers of both neutrons and protons. In addition, elements with even numbers of protons tend to have many stable isotopes, while the odd-number elements have few stable isotopes.

## Magic numbers

Another important point is that the rules governing the shell-filling are different for nucleons from those for electrons. For nucleons, filled shells are attained with 2, 8, 20, 28, 50, 82, and 126 particles (these are referred to as “magic numbers”). Thus in seeking a projectile, we need an isotope with an exceptionally high proportion of neutrons.

To synthesize the later Period 7 elements, calcium-48 has been the projectile of choice. Calcium is an interesting element from the perspective of nuclear chemistry. The common isotope, calcium-40 (96.9% natural abundance), has an abnormally low proton:neutron ratio for its location in the periodic table, that of 1:1. Of course, we can explain the stability of that isotope in terms of the nucleus being “doubly-magic,” with 20 protons and 20 neutrons. But calcium also possesses the next doubly-magic isotope, calcium-48 (0.2% natural abundance) with 28 neutrons, giving a comparatively high ratio, that of 1:1.4 — making that isotope favoured for such syntheses. Also, the filled nucleon shells confer added stability to the nuclei as projectiles.

## The synthesis of elements 117 and 118

The first synthesis of element 118 was accomplished in 2002 — one atom, in fact — followed by two more atoms in 2005. The research was undertaken at the Flerov Laboratory of Nuclear Reactions in Dubna, Russia. The synthetic reaction was:

249/98

It was not until 2010 that atoms of element 117 were synthesized. The greater difficulty of synthesizing element 117 compared to 118 is largely explicable in terms of the nucleus containing an odd number of protons. Instead of californium-249, berkelium-249 was used as the target, while calcium-48 was again the projectile. Some neutrons are always lost in the process, these carrying away much of the excess energy from the impact:

## On to element 119 and 120?

So how can we get any farther? We have run out of long-lived targets. Californium-249 has a half-life of 351 years. It took four months of bombardment to finally have a suitable impact to form Uuo-294 at which time most of the californium still existed. Such a procedure would not be as easy with, for example, einsteinium with the longest-lived isotope of 1.3 years. So the only alternative is to find a more massive projectile.

Titanium-50 (abundance 5.2%) has been proposed. It has the same magic number of neutrons as calcium-48 (thus it is “singly magic”) with two more protons. Therefore impacting californium-249, it should enable the synthesis of an isotope of element 120. Unfortunately, it would be on the low side of the feasible proton:neutron ratio, so the half-life of such a nucleus would be very low — probably in the microsecond range. Similarly, an isotope of element 119 might be expected from the impact of titanium-50 on berkelium-249. But to raise the probability of success, the focus is on the even-numbered element 120.

## Who will win?

There have been three major centres involved in the discovery of past elements: that in Russia, the Lawrence Livermore, California, and the GSI in Darmstadt, Germany. Now a total of 14 laboratories around the world are running the titanium-50/californium-249 experiment, all looking for the first evidence of element 120. Such a discovery would not only add a new period to the periodic table, but there is also the nationalistic factor. The first researchers in that country to establish the existence of this element — even just one atom — would be able to name that element. That name would put their country on the chemical map for all time. So it is not a case of if element 120 is discovered, but when. Will it be in 2012? Or 2013? Expect news soon!

## Sources

- Jean Hein is thanked for bringing to my attention an article in The Economist which generated thoughts for this submission.
- For a previous related article, see: G.W. Rayner-Canham, “Hassium-270: Another triumph for the shell model of the nucleus,” Chem 13 News, 16-17 (September 2007).
- For up-to-date information on elements and isotopes, see Mark Winter’s WebElements (accessed 06 June 2012).
- For a more detailed discussion of nuclear science, see the document by C. Düllman, “Superheavy element research at GSI” (PDF) (accessed 06 June 2012).

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