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One of the curious things about modern space fiction films is that while they try to get the astronomy and physics reasonably correct, the scriptwriters often completely ignore the rules of chemical structure. For example, Captain Kirk in Star Trek finds an alien spaceship whose hull is made of some unknown element with bizarre properties. Any chemist knows that such a discovery is just about impossible because every stable element that can exist in the Universe is already known.

But that may not be completely true. Over the past two or three decades, American, German and Soviet research groups have been trying to create artificial elements that are extremely heavy. The heaviest elements made so far are not very stable and, therefore, are extremely radioactive but some theorists think that it might be possible to make even heavier elements that are more stable.

An element consists of atoms, each of which has a nucleus with a characteristic number of positively charged protons, defined by the atomic number, and a certain number of neutrons which stabilise the nucleus by ‘diluting’ the repulsive electrical charge between the protons. Most elements come in several varieties called isotopes which have differing numbers of neutrons. The total number of protons and neutrons are denoted by the atomic mass number (for example, carbon-12 has six protons and six neutrons). The lightest element hydrogen, contains atoms with a nucleus of one proton, but another isotope called deuterium contains nuclei with one proton and one neutron, while a third isotope, tritium, contains two neutrons and a proton. This isotope is unstable, or radioactive, disintegrating to give off beta particles, or negative electrons. Helium isotopes contain nuclei with two protons and one or two neutrons. The properties of successively heavier elements are described by the periodic table continuing to, and beyond, the element with atomic number 92, uranium.

Uranium is, in fact, the last and heaviest naturally occurring element in the periodic table. All its naturally occurring isotopes, uranium-234, 235 and 238, are radioactive, indicating that their nuclei are less stable than those further down the periodic table. Many elements have some isotopes that are unstable. They transmute into another element by emitting particles – beta particles or alpha particles (which are helium nuclei of two protons and two neutrons). These kinds of transformations are important in synthesising and detecting new elements. They can happen in seconds or take decades. In fact, the half-life of an element, or the time taken for half of its atoms to break up, defines its stability.

In the past 50 years, researchers have been systematically extending the periodic table by making artificial elements with higher atomic numbers than uranium, starting from neptunium (atomic number 93) and plutonium (atomic number 94). There are now 15 other elements, ranging from atomic number 95 to 109. None of these is very stable, and they become increasingly difficult to make with higher atomic numbers. In fact researchers have only made three atoms of element 109 and it survives fleetingly for about 3.4 milliseconds.

Indeed, until 1970 both theory and experimental data seemed to indicate that a practical limit to the periodic table would be reached at about element 108, after which nuclei would immediately disintegrate. The data indicated that the half-lives of the longest-lived isotopes would become so short (less than one-billionth of a second) as to prevent them from being observed or studied. However, between 1966 and 1972, several calculations based on newly developed theories of nuclear structure by the Soviet physicist Vilen Strutinsky predicted that much heavier elements with an atomic number of around 114 could be relatively stable. Some people even suggested that such ‘superheavy’ nuclei could have half-lives of the order of the age of the Universe. As a result people looked for superheavy elements in nature, thinking that they might have been formed, like other heavy elements, in natural processes of nuclear synthesis such as those that happen in supernovae. Now more recent calculations predict that superheavy elements can exist but they would have relatively short half-lives compared with the age of the Earth.

Such predictions depend on having a good understanding of the structure of a nucleus. As the atomic number rises, the repulsion between the protons increases and so proportionally more neutrons are needed to overcome the repulsion. According to a very simplified description of nuclear structure, protons and neutrons are arranged in concentric shells which fill up in a similar way to the way electrons fill shells in an atom. A nucleus with completely filled shells is particularly stable in the same way as the filled electronic shells of the noble gases are. The stability of a nucleus also relates to its shape, with spherical nuclei usually being the most stable.

The three-dimensional map (below left) shows very clearly how the stability of atomic nuclei varies as the numbers of protons and neutrons increase. There is a ‘mountainous peninsula’ of stability running diagonally across the map. In the lower part of the map, stable nuclei have roughly equal numbers of protons and neutrons. Further up, they have more neutrons. The mountain peaks are the so-called magic numbers – numbers of protons or neutrons producing the extra stable closed shells. Surrounding the peninsula is a sea of instability. At the far end, at atomic number 114 is the ‘island of stability’ where nuclei with 114 protons and 184 neutrons are predicted to have particularly stable spherical shapes because the proton and neutron shells are completely filled. These elements would be more stable than many elements with a lower atomic number.

Over the past three decades, we at the Lawrence Berkeley Laboratory in California, as well as European research groups at the Laboratory for Heavy Ion Research in Darmstadt in Germany and the Laboratory of Nuclear Reactions in Dubna near Moscow have been trying to make a leap to this island, hoping to make a new range of elements.

How do you synthesise a new element? The ‘transuranium’ elements 93, 95, 99 and 100 were produced by bombarding a starting element with neutrons, which the nuclei then absorb (see Table). This is how Edwin McMillan and Philip Abelson made neptunium-239 from uranium-238, and one of us (Glenn Seaborg) and colleagues created americium from plutonium-239. Because a neutron is electrically neutral, it can easily slip into the nucleus of, for example, a uranium or plutonium atom. Here the neutron is absorbed and bound to other protons and neutrons by the strong, nuclear force. This process does not create a new element, because capturing a neutron does not affect the number of protons in a nucleus. Instead a new isotope of the starting element is created.

This additional neutron may make the nucleus unstable because the forces binding the protons and neutrons together are strongest when the ratio of protons to neutrons is close to a certain favoured value. As a result the additional neutron is converted into a proton. This creates a new element with an atomic number one unit higher. To conserve electric charge, a negative beta particle, or electron, is created and ejected from the nucleus. As we described earlier, so-called beta decay is a common mode of radioactivity.

Another method of creating new elements depends on bombarding a target of the starting element with the nuclei of other elements. These projectiles might be protons (nuclei of the element hydrogen), deuterons (the deuterium nuclei) or nuclei of helium, carbon, nitrogen, oxygen and other elements. The protons present in both the target and in the bombarding nuclei have a positive electrical charge, so there is a strong electrical repulsion when they approach each other. But the nuclei must actually touch each other if they are to react. Because the radii of nuclei are very small, the positive charges must be brought very close together. This means that the bombarding particles must have a high kinetic energy. We achieve this by accelerating them in machines running at a high voltage, such as cyclotrons and linear accelerators.

Once the bombarding nucleus penetrates the target nucleus, the short range nuclear forces bind the two nuclei into a compound nucleus of a new element. This compound nucleus will be formed in an ‘excited’ state (with a higher energy than that of the lowest energy state of the nucleus) whose excess energy must be dissipated before the nucleus can stabilise.

In the heaviest elements, such as the transuranium elements, this excitation energy is usually lost by emitting gamma rays and ‘boiling off’ neutrons from the excited nucleus. The nucleus of the new element is radioactive, and it will strive to become more stable by changing its internal structure. This might involve losing beta particles or alpha particles, as we previously described, or spontaneous fission when the nuclei breaks up into two smaller nuclei.

Elements 93 to 106 (except elements 95, 99 and 100) were synthesised using a cyclotron or linear accelerator in the Lawrence Berkeley Laboratory, while elements 107 to 109 were made at Darmstadt. Elements 101 to 109 were made one atom at a time. The last four elements have not been named. The researchers at Dubna are claiming to have made elements 106 to 108 first, and the discoverers have the privilege of suggesting the names. The Soviet scientists also contest the discovery of elements 104 and 105.

This is also the approach we have used to try to make superheavy nuclei. But first we need to be able to predict the properties of superheavy nuclei as accurately as possible, to decide which ones might be the easiest to make and characterise. This is extremely difficult. It involves extrapolating what we know about the structure of the heaviest known elements to the region of the periodic table with a much higher atomic number of 110 or more where we have no knowledge. Even the smallest errors are greatly magnified in this extrapolation process. As a result, our estimates have changed quite a lot over the years.

Originally, in 1972, Ray Nix and collaborators at Los Alamos National Laboratory suggested that the peak of the superheavy island of stability centred at element 114 containing 184 neutrons, with a gradual slope to isotopes with fewer neutrons. In 1976 the Danish physicist Jorgen Randrup and colleagues suggested that the slope was, in fact, much steeper, crashing into the sea of instability. This prediction made us think that, because of the various nuclear reactions that are feasible, it would be much more difficult to assemble composite nuclei containing as many as 184 neutrons.

Much more recently, in 1989, a Polish physicist Zygmunt Patyk and his team at the Soltan Institute for Nuclear Studies in Warsaw made a number of useful theoretical predictions. They thought that nuclei containing from 182 to 184 neutrons were the most stable, with a longer region of nuclei with measurable half-lives extending to nuclei with 160 neutrons and atomic number 110. Nuclei with atomic number 112 and 184 neutrons would be spherical and so would be relatively stable, while those with fewer than 166 neutrons would be deformed and, therefore, less stable. These latest calculations suggest that there is no vast sea of instability separating the known nuclei from the superheavy nuclei. Instead, there is a peninsula of relatively stable nuclei extending from atomic number 98 out to atomic number 118.

Synthesising the superheavy elements represents a formidable challenge because the chance of nuclei fusing, followed by the new nuclei surviving is so small. It is much less than one in a billion nuclear reactions. The rest of the reactions result in fission which destroys the nucleus. This corresponds to producing no more than three atoms per day of bombarding particles onto a target. Peter Armbruster, a leading figure in this field who was, until recently, at Darmstadt, devised a rule-of-thumb method for estimating the chance that two nuclei fuse, given the atomic numbers of the projectile and target nuclei. Once fusion has taken place, the survival of the new nucleus depends on whether the composite nucleus can first lose the excitation energy via neutrons. As neutrons ‘boil off’, in the way we described before, they take away the excitation energy at a rate of about 10 megaelectronvolts per neutron. If the excitation energy is low enough, then most of it is lost via neutrons and the new nucleus has more chance of surviving. Otherwise, the nucleus just undergoes fission. From tables of measured or predicted atomic masses we can deduce the excitation energy of the composite nuclei. The survival of the fused system can be estimated by rule of thumb that 100 nuclei will ‘die’ due to fission for every nucleus that survives by emitting a neutron.

Take, for example, the synthesis of element 108. The reaction used involved firing iron-58 nuclei (atomic number 26) at a target of lead-208 (atomic number 82). This produces a composite nucleus of element 108 with an excitation energy of about 20 to 23 megaelectronvolts. Using Armbruster’s method, we estimate that about one in a million reactions leads to fusion. With a probability of survival of 1 in 10 000, this gives a production rate of 1 in 10 000 million reactions, which corresponds well with the measured yield of element 108. If it were not the fact that the composite nucleus has a low excitation energy due largely to strong binding associated with the nuclear shells in the lead target nucleus, it would not have been possible to make this element.

There have been more than 25 reported attempts to synthesise superheavy elements in the laboratory. The most intensively studied reaction is between calcium-48 (atomic number 20) and curium-248 (atomic number 96). This reaction produces the superheavy nuclei with the most neutrons so it should be the most stable. We estimate that the resulting composite nucleus of element 116 should have an excitation energy of about 30 megaelectronvolts. This gives a survival probability of one in a million. If the predicted rate of fusion of the target and projectile nuclei is three in a million reactions, then the predicted formation of a surviving nucleus is three in a million million reactions. But the experimental limit at which the element can be detected is between 10 and 100 in a million million reactions. So it is not surprising that attempts to make this element have so far failed.

There is another way of making superheavy nuclei where, instead of making composite nuclei including all the protons and neutrons of the projectile and target nuclei, only a fraction of the projectile nucleus is transferred to the target nucleus. In fact, spurred on by studies of the reaction between uranium-238 nuclei in which 20 or more protons were transferred from the projectile to the target nucleus creating only moderate excitation energies, some researchers thought that superheavy elements could be made. But again, the estimated rate of forming a superheavy element this way is between 1 and 10 in a million million reactions which again is below levels of experimental detection.

Detecting the new element is incredibly difficult. You have to show that the atomic number of the new species is different from the atomic number of all previously known elements. Elements 95 to 101 were identified chemically by preparing salts of the element and separating ions out using an ion exchange resin. The order in which the ions filter out of the separating apparatus gives the atomic number, in the same way as molecules of different weight and reactivity can be separated using chromatography. This technique does not work for elements where the half-life is only a few seconds, so it has not been used to identify new elements beyond 101. Here we rely on subtler methods whereby we identify the products of radioactive decay and work backwards.

The principal way in which the surviving heaviest nuclei decay is by losing alpha particles (helium nuclei) or by spontaneous fission. Elements that decay by losing alpha particles produce so-called daughter nuclei with an atomic number two units and a mass number four units lower (the mass of the alpha particle). The daughters will also decay by emitting alpha particles but their half-life will already be known so the daughter elements can be identified. This automatically tells us what the atomic number of the new element is. Elements 104 to 109 were identified by this ‘genetic’ method. We expect element 110 to have a reasonable half-life disintegrating through alpha decay. But the superheavy elements, in other words, those in the neighbourhood of element 114, may also decay by spontaneous fission. In these cases it will be much more difficult to determine whether or not we have a new element because we do not yet have an efficient device that can measure the atomic numbers of the fission fragments simultaneously.

The Soviet physicist Vitaly Ginzburg recently said that the synthesis of superheavy elements was one of the 20 or so ‘especially important and interesting problems in physics’. Certainly, determining the nuclear properties of the superheavy nuclei and the nuclear reactions to synthesise them represents one of the hardest problems in nuclear physics. Nevertheless we are confident that superheavy nuclei should exist. Their half-lives should be long enough to be detected in the laboratory but much too short to exist in nature. The main thing we need to do now is improve the sensitivity of our detectors. We should focus on detecting the fragments from spontaneous fission more efficiently and measuring their atomic numbers more accurately.

Glenn Seaborg won the Nobel Prize for Chemistry in 1951. He and his research group at the Lawrence Berkeley Laboratory have discovered 10 of the transuranium elements. Walter Loveland is professor of chemistry at Oregon State University.

Further reading The Elements beyond Uranium, Walter Loveland and Glenn Seaborg, Wiley, 1990.