Rutherford this is Transmutation!

by Nicholas Mee on August 26, 2020

Chemistry has its roots in the alchemists’ vain quest to transform base metal into gold. Throughout centuries of investigation this avaricious goal remained forever elusive, the alchemists’ failure hidden within a web of arcane lore and mystical symbols. Although the alchemists’ art has long since been discredited, the idea that transmutation of metals might be possible is not as ridiculous as it might seem today. 

After all, gold must have been created out of something. And even simple chemical transformations are often quite startling. It has been known for over seven thousand years, for instance, that when a bright green rock—malachite—is crushed and heated with charcoal it transforms into a brilliant shiny metal—copper.

Some alchemists believed that gold formed in an almost organic process as the endpoint of a sequence of increasingly noble metals. If only they could discover a way to speed up the process they would be rich. Unfortunately, alchemy was more often the route to poverty rather than unlimited wealth.

The New Alchemists

Ernest Rutherford (1871-1937) left Cambridge in 1898 to take up a professorship at the newly founded McGill University in Montreal, Quebec. Rutherford had been studying radioactivity in the elements uranium and thorium and would continue this research in Canada.

Rutherford found to his great surprise that the level of radioactivity displayed by his thorium sample varied depending on how well ventilated his equipment was. He described his findings in the paper A Radioactive Substance emitted from Thorium Compounds published in January 1900:

The sensitiveness of thorium oxide to slight currents of air is very remarkable. The movement of the air caused by the opening or closing of a door at the end of the room opposite to where the apparatus is placed, is often sufficient to considerably diminish the rate of discharge. In this respect thorium compounds differ from those of uranium, which are not appreciably affected by slight currents of air.

After some deep consideration and careful experimentation Rutherford concluded that thorium must be releasing a radioactive gas or emanation that was being dispersed by the air.

In 1900 Rutherford was joined at McGill by a young English chemist Frederick Soddy (1877-1956) who had taken up the position of Demonstrator in Chemistry, which made him Rutherford’s hands-on assistant. Rutherford gave Soddy the task of investigating the chemical properties of his thorium emanation, or thoron as it was tentatively named.

Ernest Rutherford (left) and Frederick Soddy (right).

Soddy found that no sooner had he removed the thoron gas from his sample than it would reappear. It was as though thorium was continually giving birth to thoron. Then he discovered that the thoron gas was generating another new radioactive substance that was deposited on the walls of the glass vessel in which the thoron was collected.

Soddy was stunned when he understood what was was happening:

Rutherford, this is transmutation!

Rutherford replied:

For Mike’s sake Soddy, don’t call it transmutation.
They’ll have our heads off as alchemists.

In the six years since Henri Becquerel had discovered radioactivity, no one had realised that when an atom emits an alpha or beta particle its identity changes. Although Marie and Pierre Curie had chemically extracted small quantities of two new elements radium and polonium from the uranium-rich ore pitchblende, they did not know that the radium and polonium atoms were ultimately derived from uranium atoms. How could they guess that atoms within a solid lump of pitchblende were gradually changing their identity? The truth was revealed because Rutherford’s thorium emanation was an inert gas. This meant that these newly-formed atoms could seep out of the solid thorium without any chemical interaction hindering their escape.

The noble gases comprise the right-most column of the Periodic Table, once known as Group VIII, now more usually known as Group 18.

Radon

Rutherford and Soddy now realised that the highly radioactive gas escaping from their thorium must be a completely new element, and only the sixth radioactive element to be identified after uranium, radium, polonium, actinium and thorium. Since 1923 it has officially been known as radon (Rn), an abbreviation of radium emanation, as radium is its immediate progenitor. Fundamental chemistry would never be the same again. The elements were not the stable immutable building blocks that chemists had long assumed.

The recognition that atoms change their identity after emitting radiation shed a whole new light on the subject. It was the key that would enable Soddy and his fellow researchers to unlock the secrets of radioactive decay over the course of the next decade.

Inert gases were the domain of the Scottish chemist William Ramsay. In 1903 Soddy returned to London to work with Ramsay. They found that the new radioactive but chemically inert gas we now know as radon, had an emission spectrum resembling those of the noble gases argon, neon, krypton and xenon that Ramsay had discovered during the previous decade. Ramsay realised that radon was another noble gas, and five years later he determined its atomic mass showing that it slots in beneath xenon in the Periodic Table.

Inhalation of naturally occurring radon can be a health hazard. Radon is produced by the slow radioactive decay of elements within the Earth that are ultimately derived from uranium and thorium. Gradually seeping out of the ground into the atmosphere and ground water it represents the biggest source of natural background radiation. But exposure to radon varies enormously depending on the mineral composition of the bedrock. Radon detectors are available and buildings should be modified if high levels of radon are discovered. Radon comes second only to smoking as a cause of lung cancer. Most of the damage is thought to comes from the radioactive decay products of radon rather than the radon gas itself.

Helium

Ramsay and Soddy also found that a sealed container of radon left standing for a few days would show the spectral lines of helium. This gave firm support to Rutherford’s contention that the alpha particles emitted by radon and other radioactive elements were somehow essentially helium atoms. Ramsay would later show that the difference in atomic mass of radium and radon is equal to the atomic mass of helium.

By 1909 Rutherford, now leading a team at the University of Manchester, was convinced the evidence was incontrovertible. With his colleague Thomas Royds, he announced they had definitive proof that alpha particles were charged helium atoms. Rutherford’s discovery of the atomic nucleus a couple of years later would establish once and for all that an alpha particle is identical to a helium nucleus. This accounted for the otherwise inexplicable presence of helium in the uranium oxide mineral cleveite, as revealed by Ramsay in 1895.

In 1908 Rutherford was awarded the Nobel Prize for Chemistry for the discovery of the transmutation of the elements or as the Nobel Prize awards committee put it: for his investigations into the disintegration of the elements, and the chemistry of radioactive substances.

Research Costs

The great scientific advances of the early twentieth century came at a heavy price. Little was known about the dangers of radioactivity and few precautions were taken by the experimenters. Like many of the pioneers of nuclear physics Ramsay paid dearly. In 1916 he died of cancer of the jaw probably as a result of handling radium and inhaling radon over the course of the previous decade.



Further Information

There is more about William Ramsay in this post: William Ramsay’s Noble Quest.

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