Two and a half centuries ago the science of chemistry was just emerging from the mists of alchemy. By 1820, scientists were using a variety of methods - heating substances, combining them with each other, or passing electric currents through them - to isolate materials that they could not break down any farther. Many of these, such as gold, silver, antimony, sulfur, mercury, carbon, iron, arsenic, copper, lead, and tin had been known since ancient times. Others, such as phosphorus, hydrogen, oxygen, nitrogen, chlorine, nickel, chromium, sodium, potassium and iodine, were produced and named between 1650 and 1850.

In spite of an intense study of these elements, in 1850 one big question remained unanswered: How many elements were there? The number of those known was growing steadily, and it seemed possible that there might be hundreds or even thousands yet to be discovered.

A Russian chemist, Dmitri Mendeleyev, finally brought order out of chaos by grouping the known elements according to their chemical properties and their weights. He did the basic work one winter's morning in 1869 (he had intended to visit a cheese factory, but the weather was bad so he stayed home). This grouping assigned the elements to places in what is now known as the periodic table. The table is a kind of pegboard, in which each element occupies a well-defined slot (in fact, a poster of the periodic table may be the most popular wall decoration found in physicists' and chemists' offices).

Mendeleyev's achievement is greater than it may seem at first sight, because the order of the elements in the periodic table is actually based on the number of protons in each one's nucleus - a notion that would not come along until forty years later. Others before Mendeleyev had tried the same thing, less successfully, but he had enough faith in his work to treat the periodic table as a basic law of nature. He pointed out that his table contained a number of gaps. For three of those gaps he predicted that they should be occupied by unknown elements, and stated what their properties ought to be because of their positions in his table. Sure enough, the elements were discovered, gallium in 1875, scandium in 1879, and germanium in 1886.

Everything seemed to be wrapped up neatly. There were exactly ninety-two elements, ranging from hydrogen to uranium. Moreover, there could be no "missing" elements, unless there was a place for them in the periodic table. True, a fair number of gaps remained to be filled, but chemists were confident that this was just a matter of time and more experiments. And sure enough, many were filled - fluorine in 1886, argon in 1894, helium in 1896, and krypton, neon, and xenon in 1898. But there remained a couple of irritating and inexplicable gaps. For example, what was in the periodic table between element 42, molybdenum, and element 44, ruthenium? And for the heavier elements the periodic table looked awfully ragged. There was element 83, bismuth, and elements 90 and 92, thorium and uranium. But what about the others in-between?

No one had answers until, in 1896, Henri Becquerel discovered radioactivity in uranium. Radioactive elements were found to emit "rays," some of which actually turned out to be particles (helium nuclei and electrons). When they did so, they transformed into different elements. Here was a possible answer to the gaps in the periodic table. Suppose that an element had once existed, but was radioactive? Then over time it would become some different element, and would no longer be around to be discovered.

This, in fact, is exactly what was happening. The missing element 43 is technetium. It was presumably around when the solar system was formed, but it is radioactive and in any four- million-year period half of it will have changed to something else. By now, all has gone. But we can produce technetium artificially, for example by bombarding molybdenum with other particles, and it has just the right properties to fill its place in the periodic table.

The same is true of the elements beyond bismuth. They are all radioactive, from polonium (element 84), through astatine, radon, francium, radium, actinium, thorium (already known, but almost stable and not realized to be radioactive until after Becquerel's discovery), protactinium, and uranium (element 92).

With the discovery of these elements, the periodic table was complete.

Or was it? It did not take scientists long to realize that if you bombarded a heavy element with particles, you might produce an even heavier element, beyond uranium in the periodic table. Of course, any such discovery would have to be unstable itself, and so not found naturally. The first to be found of these "transuranic" elements was neptunium, element 93, produced in 1940 by firing neutrons at a uranium target. Plutonium, element 94, was discovered the same year. Neptunium and plutonium, as predicted, are unstable and decay radioactively.

After that, the hunt was on for elements higher and higher in the periodic table. We now have americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium, lawrencium, rutherfordium, dubnium (after the Russian town Dubna, not after George W.), seaborgium, bohrium, hassium, and meitnerium. That takes us to element 109, the last one with an official name. All of these are highly unstable and have half-lives (the time for half their nuclei to transform to some other element) ranging from days to fractions of a second.

As a general rule, the transuranic elements become less stable as their element number increases. However, that is not the whole story. There are theoretical reasons why elements with numbers between 112 and 118 might be more stable, so this region of the extended periodic table is often referred to as the "island of stability," although so far with little justification. Elements 110, 111, 112, 114, 116, and 118 have been produced, but all decay rapidly (minutes or less). The form of element 114 with 184 neutrons in the nucleus, expected to be more stable than the others, has never yet been formed.

Each of these transuranic elements will have its own unique chemistry, and should produce a range of new compounds of unknown properties and value. Now for the most interesting question. Might we some day stabilize these elements? A free neutron, left to itself, can decay in a quarter of an hour to yield a proton and an electron. Bound within a nucleus, however, the neutron is a stable particle. Perhaps we could embed a transuranic nucleus from the island of stability within some larger structure, and slow its decay for an indefinitely long period.

If you were a chemist, what wouldn't you give for another twenty-five stable elements to play with?

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