Scientists follow a magic trail

Why are there only 92 naturally-occurring elements? It's a question that has occurred to children studying chemistry for years. And scientists wonder, too - so much so that they are trying to create another, dubbed the "magic element". And some believe it is in sight.

Why are there only 92 naturally-occurring elements? It's a question that has occurred to children studying chemistry for years. And scientists wonder, too - so much so that they are trying to create another, dubbed the "magic element". And some believe it is in sight.

Everything on earth, including our bodies, is made from a combination of those elements - ranging from the simplest, hydrogen (whose nucleus consists of a lone proton) to uranium, whose nucleus holds 92 protons and 146 neutrons.

In the 1940s, scientists tried to make larger elements by fusing smaller nuclei together. The result was neptunium, with 93 protons. A spate of elements - each with different chemical properties - followed until scientists reached a nuclear size limit. For a while it seemed impossible to go beyond element 100. But theories persisted that an element might exist which would be very stable, despite having a large nucleus. How? Inside the nucleus, the nucleons (the protons and neutrons) are arranged in "shells" of different energy levels. When a shell is full, a new one must be started, which means adding a more energy, making the nucleus more unstable.

Rather like the electron shells that surround the nucleus, the lowest energy situation - and therefore the most stable - occurs when a shell is full. The magic numbers are simply the numbers of nucleons that complete a shell: two for the first, eight for the next, then 20, 28, 50, 82 and so on. Nuclei possessing a "magic number" of protons or neutrons are surprisingly long-lived for their size. The "doubly magic" isotope lead-208, for example, with 82 protons and 126 neutrons, lasts forever.

Most scientists agree with the Polish physicist Adam Sobiczewski, who predicted in the 1960s that the doubly magic nucleus would have 114 protons and 184 neutrons. The elements close to the magic would also be relatively stable, existing on a so-called "Island of Stability" amidst the sea of unstable isotopes.

How could you "cross" that Sea of Instability to the magic element? Bigger nuclei are normally made by bombarding smaller nuclei with nucleons in the hope that they fuse. However, the products of virtually all these interactions are incredibly unstable. Fusion needs nuclei with exactly the right energies in a head-on collision.

Early in 1999, a team of scientists from the Joint Institute for Nuclear Research in Dubna, Russia, and the Lawrence Livermore National Laboratory in California, announced they had reached the Island of Stability, after 40 days bombarding a lump of plutonium with high-energy beams of a rare isotope of calcium. And for a magical 30 seconds, element 114 - with 175 neutrons - lived and died. Thirty seconds might not sound much, but at the rarefied level of such elements it is an age; element 104 has a half-life of 0.3 seconds.

Six months later, a team at the Lawrence Berkeley National Laboratory, California, created element 118 - and then, when it decayed, element 116.

According to the predictions, elements 118 and 116 shouldn't have been on the "island" at all. A life-time of one millisecond had exceeded expectations; even if it had been created, it should have been far too unstable to hold together.

Recently, Professor Adam Sobiczewski, of the Soltan Institute for Nuclear Studies in Warsaw, Poland, has forecast some surprising characteristics for super-heavy nuclei. Some of them should be hollow, or bizarrely deformed, with the protons and neutrons squashed into ellipsoids, or bearing strange lumps and bumps. For some combinations of protons and neutrons, a sphere is no longer the lowest energy shape.

This doesn't mean that the magic element is less magic, but the theories surrounding it are changed. The hopes of finding one with a lifetime of a year have faded. Some say it may only last a few hours.