Part 2

By 1925, all but four of the slots in the 92-place file had been filled. The vacancies were at 43, 61, 85, and 87.

Workers using traditional analytical techniques continued to search for these elements, but their efforts were foredoomed to failure. None of the nuclei of the isotopes of elements 43, 61, 85, and 87 are stable; hence weighable quantities of them do not exist in nature, and new techniques had to be developed before we could really say we had "discovered" them.

In 1919, Rutherford accomplished scientifically what medieval alchemists had failed to do with "magic" experiments and other less sophisticated techniques. It wasn't gold (the goal of the alchemists) he found but something more valuable with even greater potential for good and evil: a method of transmuting one element into another. By bombarding nitrogen nuclei with alpha particles from radium, he found that nitrogen was changed into oxygen.

The process for radioactive transmutation is somewhat like a common chemical reaction. An alpha particle, which has the same charge (+2) and atomic mass (4) as a helium nucleus, penetrates the repulsive forces of the nitrogen nucleus and deposits one proton and one neutron; this changes the nitrogen atom into an oxygen atom. The reaction is written

{7}N<14> + {2}He<4> --> {1}H<1> + {8}O<17>.

The number at the lower left of each element symbol in the above reaction is the proton number. This number determines the basic chemical identity of an atom, and it is this number scientists must change before one element can be transformed into another. The common way to accomplish this artificially is by bombarding nuclei with nuclear projectiles.

Rutherford used naturally occurring alpha particles from radium as his projectiles because they were the most effective he could then find. But these natural alpha particles have several drawbacks: they are positively charged, like the nucleus itself, and are therefore more or less repulsed depending on the proton number of the element being bombarded; they do not move fast enough to penetrate the nuclei of heavier elements (those with many protons); and, for various other reasons (some of them unexplained), are inefficient in breaking up the nucleus. It is estimated that only 1 out of 300,000 of these alpha particles will react with nitrogen.

Physicists immediately began the search for artificial means to accelerate a wider variety of nuclear particles to high energies.

Protons, because they have a +1 charge rather than the +2 charge of the alpha particles, are repulsed less strongly by the positive charge on the nucleus, and are therefore more useful as bombarding projectiles. In 1929, E. T. S. Walton and J. D. Cockcroft passed an electric discharge through hydrogen gas, thereby removing electrons from the hydrogen atom; this left a beam of protons (i. e., hydrogen ions), which was then accelerated by high voltages. This Cockcroft-Walton voltage multiplier accelerated the protons to fairly high energies (about 800,000 electron volts), but the protons still had a plus charge and their energies were still not high enough to overcome the repulsive forces (Coulombic repulsion) of the heavier nuclei.

A later development, the Van de Graaff electrostatic generator, produced a beam of hydrogen ions and other positively charged ions, and electrons at even higher energies. An early model of the linear accelerator also gave a beam of heavy positive ions at high energies. These were the next two instruments devised in the search for efficient bombarding projectiles. However, the impasse continued: neither instrument allowed scientists to crack the nuclei of the heavier elements.

Ernest O. Lawrence's cyclotron, built in 1931, was the first device capable of accelerating positive ions to the very high energies needed. Its basic principle of operation is not difficult to understand. A charged particle accelerated in a cyclotron is analogous to a ball being whirled on a string fastened to the top of a pole. A negative electric field attracts the positively charged particle (ball) towards it and then switches off until the particle swings halfway around; the field then becomes negative in front of the particle again, and again attracts it. As the particle moves faster and faster it spirals outward in an ever increasing circle, something like a tether ball unwinding from a pole. The energies achieved would have seemed fantastic to earlier scientists. The Bevatron, a modern offspring of the first cyclotron, accelerates protons to 99.13% the speed of light, thereby giving them 6.2 billion electron volts (BeV).

Another instrument, the heavy-ion linear accelerator (Hilac), accelerates ions as heavy as neon to about 15% the speed of light. It is called a linear accelerator because it accelerates particles in a straight line. Stanford University is currently (1963) in the process of building a linear accelerator approximately two miles long which will accelerate charged particles to 99.9% the speed of light.

But highly accelerated charged particles did not solve all of science's questions about the inner workings of the nucleus.

In 1932, during the early search for more efficient ways to bombard nuclei, James Chadwick discovered the neutron. This particle, which is neutral in charge and is approximately the same mass as a proton, has the remarkable quality of efficiently producing nuclear reactions even at very low energies. No one exactly knowns why. At low energies, protons, alpha particles, or other charged particles do not interact with nuclei because they cannot penetrate the electrostatic energy barriers. For example, slow positive particles pick up electrons, become neutral, and lose their ability to cause nuclear transformations. Slow neutrons, on the other hand, can enter nearly all atomic nuclei and induce fission of certain of the heavier ones. It is, in fact, these properties of the neutron which have made possible the utilization of atomic energy.

With these tools, researchers were not long in accurately identifying the missing elements 43, 61, 85, and 87 and more--indeed, the list of new elements, isotopes, and particles now seems endless.

Element 43 was "made" for the first time as a result of bombarding molybdenum with deuterons in the Berkeley cyclotron. The chemical work of identifying the element was done by Emilio Segrè and others then working at Palermo, Sicily, and they chose to call it technetium, because it was the element first made by artificial technical methods.

Element 61 was made for the first time from the fission disintegration products of uranium in the Clinton (Oak Ridge) reactor. Marinsky and Glendenin, who did the chemical work of identification, chose to call it promethium because they wished to point out that just as Prometheus stole fire (a great force for good or evil) from the hidden storehouse of the gods and presented it to man, so their newly assembled reactor delivered to mankind an even greater force, nuclear energy.

Element 85 is called astatine, from the Greek astatos, meaning "unstable," because astatine is unstable (of course all other elements having a nuclear charge number greater than 84 are unstable, too). Astatine was first made at Berkeley by bombarding bismuth with alpha particles, which produced astatine and released two neutrons. The element has since been found in nature as a small constituent of the natural decay of actinium.

The last of the original 92 elements to be discovered was element 87, francium. It was identified in 1939 by French scientist Marguerite Perey.

Children have a game in which they pile blocks up to see how high they can go before they topple over. In medieval times, petty rulers in their Italian states vied with one another to see who could build the tallest tower. Some beautiful results of this game still remain in Florence, Siena, and other Italian hill cities. Currently, Americans vie in a similar way with the wheelbase and overall length of their cars. After 1934, the game among scientists took the form of seeing who could extend the length of the periodic system of the elements; as with medieval towers, it was Italy that again began with the most enthusiasm and activity under the leadership of Enrico Fermi.

Merely adding neutrons would not be enough; that would make only a heavier isotope of the already known heaviest elements, uranium. However, if the incoming neutron caused some rearrangement within the nucleus and if it were accompanied by expulsion of electrons, that would make a new element. Trials by Fermi and his co-workers with various elements led to unmistakeable evidence of the expulsion of electrons (beta activity) with at least four different rates of decay (half-lives). Claims were advanced for the creation of elements 93 and 94 and possibly further (the transuranium elements, Table I). Much difficulty was experienced, however, in proving that the activity really was due to the formation of elements 93 and 94. As more people became interested and extended the scope of the experiments, the picture became more confused rather than clarified. Careful studies soon showed that the activities did not decay logarithmically--which means that they were caused by mixtures, not individual pure substances--and the original four activities reported by Fermi grew to at least nine.

As a matter of fact, the way out of the difficulty had been indicated soon after Fermi's original announcement. Dr. Ida Noddack pointed out that no one had searched among the products of Fermi's experiment for elements lighter than lead, but no one paid any attention to her suggestion at the time. The matter was finally cleared up by Dr. Otto Hahn and F. Strassmann. They were able to show that instead of uranium having small pieces like helium nuclei, fast electrons, and super-hard x-rays, knocked off as expected, the atom had split into two roughly equal pieces, together with some excess neutrons. This process is called nuclear fission. The two large pieces were unstable and decayed further with the loss of electrons, hence the activity. This process is so complicated that there are not, as originally reported, only four half-lives, but at least 200 different varieties of at least 35 different elements. The discovery of fission attended by the release of enormous amounts of energy led to feverish activity on the part of physicists and chemists everywhere in the world. In June 1940, McMillan and Abelson presented definite proof that element 93 had been found in uranium penetrated by neutrons during deuteron bombardment in the cyclotron at the University of California Radiation Laboratory.

The California scientists called the newly discovered element neptunium, because it lies beyond the element uranium just as the planet Neptune lies beyond Uranus. The particular isotope formed in those first experiments was {93}Np<239>; this is read neptunium having a nuclear charge of 93 and an atomic mass number of 239. It has a half-life of 2.3 days, during which it gives up another electron ( particle) and becomes element 94, or plutonium (so called after Pluto, the next planet beyond Neptune). This particular form of plutonium ({94}Pu<239>) has such a long half-life (24,000 years) that it could not be detected. The first isotope of element 94 to be discovered was Pu<238>, made by direct deuteron bombardment in the Berkeley 60-inch cyclotron by Radiation Laboratory scientists Seaborg, McMillan, Kennedy, and Wahl; it had an [alpha]-decay half-life of 86.4 years, which gave it sufficient radioactivity so that its chemistry could be studied.

Having found these chemical properties in Pu<238>, experimenters knew {94}Pu<239> would behave similarly. It was soon shown that the nucleus of {94}Pu<239> would undergo fission in the same way as {92}U<235> when bombarded with slow neutrons and that it could be produced in the newly assembled atomic pile. Researchers wished to learn as much as possible about its chemistry; therefore, during the summer of 1942 two large cyclotrons at St. Louis and Berkeley bombarded hundreds of pounds of uranium almost continuously. This resulted in the formation of 200 micrograms of plutonium. From this small amount, enough of the chemical properties of the element were learned to permit correct design of the huge plutonium-recovery plant at Hanford, Washington. In the course of these investigations, balances that would weigh up to 10.5 mg with a sensitivity of 0.02 microgram were developed. The "test tubes" and "beakers" used had internal diameters of 0.1 to 1 mm and could measure volumes of 1/10 to 1/10,000 ml with an accuracy of 1%. The fact that there was no intermediate stage of experimentation, but a direct scale-up at Hanford of ten billion times, required truly heroic skill and courage.

By 1944 sufficient plutonium was available from uranium piles (reactors) so that it was available as target material for cyclotrons. At Berkeley it was bombarded with 32-MeV doubly charged helium ions, and the following reactions took place:

{94}Pu<239> ([alpha], n) {96}Cm<242> [alpha] / 150 days --> {94}Pu<238>.

This is to be read: plutonium having an atomic number of 94 (94 positively charged protons in the nucleus) and a mass number of 239 (the whole atom weighs approximately 239 times as much as a proton), when bombarded with alpha particles (positively charged helium nuclei) reacts to give off a neutron and a new element, curium, that has atomic number 96 and mass number 242. This gives off alpha particles at such a rate that half of it has decomposed in 150 days, leaving plutonium with atomic number 94 and mass number 238. The radiochemical work leading to the isolation and identification of the atoms of element 96 was done at the metallurgical laboratory of the University of Chicago.

The intense neutron flux available in modern reactors led to a new element, americium (Am), as follows: