Rueben Bromfield, of Manawatu College, asks :-

Why are scientists making artificial/new atoms? Of what use are they?

John Campbell, a physicist at the University of Canterbury, responded.

The short answers are curiosity and who knows. Elements are characterised chemically by the number of orbital electrons each has in the neutral atom, which is the same as the number of protons (the atomic mass A) in the nucleus. Firstly we will look at the stability of chemical elements because that is easier to explain. In nature we find 90 different elements. When we build up electron shells one electron at a time we find that as each shell gets filled the resulting element is chemically stable, eg when A = 2 (helium), A = 10 (neon), A = 18 (argon), A = 36 (krypton), A = 54 (xenon), and A = 86 (radon).

When we go above A = 83 (Bismuth) the nuclei of these atoms only exists because their radioactive half-life, the time for half of a large number of this type of nuclei to decay away, is about as long as the age of the universe and thus some are still left. For example, uranium nuclei (A = 92) decay through many slightly lighter nuclei until they become stable lead (A = 82).

There are no naturally occuring nuclei above plutonium (A = 94), because their half-lives are not long enough for any to remain after being created several billion years ago. But they can be manufactured. For example, one tonne of spent nuclear reactor fuel contains about 100 grams of americium (A = 95) through bombardment of uranium and plutonium nuclei by alpha particles. Americium is used as the alpha particle emitter in smoke detectors. The only element named for a New Zealander, rutherfordium (Rf A = 104), is manufactured using particle accelerators to bombard californium (A = 98) with high speed carbon (A = 6) nuclei. Not a great honour because probably only a thousand of these nuclei have ever been made, and the longest lasted for only 70 seconds.

Heavy nuclei are unstable and decay at the lower mass end by alpha particle decay, and at the higher mass end by spontaneous fission. There are combinations of protons and neutrons (magic numbers where the nuclear particle "shells" are full) that are expected to yield stable nuclei. That is what drives physicists to attempt to produce even heavier nuclei hoping to reach these islands of stability.

We have a reasonable way to go yet. The heaviest nuclei produced currently is about A = 118, whereas the best possibility for heavy stable nuclei is for A = 126 together with 184 neutrons, (for which both proton and neutron numbers coincide with magic numbers).