Rutherford’s Protons and Prout’s Protyles

by Nicholas Mee on November 27, 2020

John Dalton (1766-1844) grew up in the English Lake District and spent most of his working life in Manchester. In 1808 he published A New System of Chemical Philosophy where he argued that each chemical element is composed of a distinct type of atom. Dalton listed the elements in a table (shown on the right), assigning a chemical symbol to each and giving its atomic mass as a multiple of that of hydrogen. His figures were, however, rather inaccurate.

Dalton believed, quite correctly, that compounds consist of combinations of atoms and that these combinations are rearranged in chemical reactions. He illustrated his ideas with small wooden balls of varying sizes, similar to the much later ball and stick models used to represent molecules.

Prout’s Hypothesis

William Prout (1785-1850)

William Prout (1785-1850) was a contemporary of Dalton’s. He was a medical doctor and accomplished chemist who studied organic materials and was the first to classify foodstuffs as fats, carbohydrates and proteins.

Prout published two papers in 1815 and 1816 where he observed that the atomic mass of every element so far measured was a multiple of the atomic mass of hydrogen. He suggested that this might be because there was just one fundamental unit or protyle, as he called it, from which all the elements were formed. This idea became known as Prout’s hypothesis. It implied that an atom of hydrogen was formed of a single protyle, whereas each atom of the heavier elements was composed of several protyles that were somehow bound together. This was a great idea. Unfortunately, the experimental basis on which it was constructed seemed to be flawed.

Jöns Jacob Berzelius

Berzelius

The Swedish chemist Jöns Jacob Berzelius (1779–1848) was one of the founders of modern chemistry and made wide-ranging contributions throughout the subject. One of his achievements was to replace the old alchemical symbols with the much more flexible modern chemical notation. He also conclusively demonstrated that elements combine in fixed proportions thereby lending support to Dalton’s atomic hypothesis. But Berzelius’s measurements were far more accurate than Dalton’s. And when he published a table of atomic masses in his Textbook of Chemistry in 1826 he showed that they could not be strict integer multiples of the atomic mass of hydrogen. Berzelius’s results discredited Prout’s hypothesis and it fell out of favour with chemists. 

Lord Rayleigh

But Prout’s hypothesis was never completely forgotten. The simplicity of the idea appealed especially to physicists. In 1904 when Lord Rayleigh accepted the Nobel Prize for Physics he recalled that Prout’s hypothesis had inspired his research.

The subject of the densities of gases has engaged a large part of my attention for over 20 years. In 1882 in an address to the British Association I suggested that the time had come for a re-determination of these densities, being interested in the question of Prout’s law. At that time the best results were those of Regnault, according to whom the density of oxygen was 15.96 times that of hydrogen. The deviation of this number from the integer 16 seemed not to be outside the limits of experimental error.

Rayleigh attempted to resolve the issue by investigating the atomic mass of nitrogen, and this led to the discovery of the noble gas argon and Rayleigh’s Nobel Prize, as discussed here: William Ramsay’s Noble Quest.


Modern values for the atomic masses of many lighter elements (shown in the table above) are indeed remarkably close to multiples of hydrogen’s. Hydrogen is quoted as 1.008, with the atomic masses of helium, carbon, nitrogen and oxygen quoted as 4.003, 12.011, 14.007 and 15.999 respectively. These masses may not be exact multiples of hydrogen’s, but as Rayleigh pointed out, they do seem suspiciously close. There are exceptions, however, most notably chlorine with an atomic mass of 35.453. So was Prout correct or not?

Canal Ray Conundrum

Joseph John Thomson (1856-1940), or J.J. Thomson as he is usually known, succeeded Rayleigh as Director of the Cavendish Laboratories in Cambridge in 1884. During the 1890s Thomson investigated the nature of cathode rays and is credited with the discovery of the first sub-atomic particle—the electron. By 1912, Thomson had turned his attention to streams of positively charged ions, then known as canal rays. Thomson and his research assistant Francis Aston developed a way to measure their mass by firing beams of ions through a magnetic field and recording their trajectories on a photographic plate. The mass of each ion was indicated by the degree to which its beam was deflected by the magnetic field. To Thomson and Aston’s surprise singly ionised neon (Ne) atoms produced two separate tracks, as labelled in the image above. They initially assumed that neon must be composed of two distinct gases neon and meta-neon whose atoms had the same properties apart from their mass. In a sense this is true.

Further light was shed on the conundrum the following year when Frederick Soddy showed that a radioactive species referred to as ionium was chemically identical to thorium, but with atoms of a slightly different mass. Soddy soon realised that many radioactive elements have atoms with several distinct masses. He described them as different isotopes of the element. Although all Soddy’s examples were found amongst the heaviest known elements, most of which were radioactive, he believed that even the lighter elements might be mixtures of different isotopes.

The Mass Spectrometer

Francis Aston

In 1918 J.J. Thomson became Master of Trinity College, Cambridge, and the following year Ernest Rutherford took over his position as Director of the Cavendish Laboratories.

Aston had spent the war years working for the newly formed Royal Air Force at Farnborough. On his return to the Cavendish, Rutherford encouraged him to further develop the apparatus he had used as Thomson’s assistant prior to the war. By November 1919 he had constructed the mass spectrograph, a device that would evolve into the mass spectrometer. By now Soddy’s proposal of isotopes was generally accepted and the meta-neon atoms were recognised as a second isotope of neon. 

With the mass spectrograph Aston could determine atomic masses with greater accuracy than ever before. His measurements showed that the atomic masses of the two neon isotopes were almost exactly twenty and twenty-two times the mass of hydrogen. He then found that chlorine had two isotopes with masses thirty-five and thirty-seven times that of hydrogen. This explained why the atomic mass of chlorine was not close to an integer—it was a mixture of two isotopes.

The Whole Number Rule

During the early months of 1920 Aston systemically analysed various elements and discovered that many were composed of more than one isotope. (Over his career he would discover more than 200 isotopes.) Aston found that every isotope had an atomic mass that was almost exactly a whole number multiple of hydrogen’s. He referred to this observation as the Whole Number Rule. It implied that whenever previous measurements had assigned an element a fractional atomic mass it was because the element was a mixture of isotopes. 

Francis Aston with his mass spectrograph.

Aston’s Whole Number Rule resurrected Prout’s hypothesis of a century earlier. But now the picture was looking decidedly clearer. In 1917 Rutherford had discovered one of the constituents of the atomic nucleus, a positively charged particle identical to the nucleus of a hydrogen atom. In 1920, and partly as a tribute to Prout, he named this particle the proton, taking its stem from Prout’s fundamental unit—the protyle. Rutherford speculated that the nucleus might have a second electrically neutral constituent with a similar mass to the proton. He named it the neutron.

Nobel Prizes

In 1922 Soddy and Aston both received a Nobel Prize for Chemistry. Soddy was given the 1921 award and Aston the 1922 award. Soddy’s award was

for his contributions to our knowledge of the chemistry of radioactive substances, and his investigations into the origin and nature of isotopes.

Aston’s award was

for his discovery, by means of his mass spectrograph, of isotopes, in a large number of non-radioactive elements, and for his enunciation of the whole-number rule.

The Neutron

Rutherford’s team at the Cavendish searched for the neutron throughout the 1920s, but without success. Finally in 1932, James Chadwick saw an opportunity that others had missed and tracked down the elusive particle. This discovery resolved the issue of isotopes. The nucleus of an atom is composed of two types of particle—protons and neutrons. The number of protons determines the chemical identity of the atom and the element to which it belongs, but the number of neutrons can vary and this is why some elements have more than one isotope. For instance, neon’s isotopes are neon-20 whose nucleus contains ten protons and ten neutrons, and neon-22 whose nucleus contains ten protons and twelve neutrons.

Nevertheless, Aston’s Whole Number Rule cannot be exactly true. The nucleus of a hydrogen atom is a single proton, whereas other atoms have neutrons and protons in their nucleus. But the neutron’s mass is 0.1% greater than the proton’s mass. Also, binding energy is released when a nucleus forms, so it has slightly less mass than an equivalent number of free neutrons and protons. Therefore the mass of a nucleus cannot simply be a multiple of the proton’s mass.

Replica of Francis Aston’s third mass spectrometer. Credit: Jeff Dahl/Wikimedia.

The mass spectrometer is now a standard piece of laboratory equipment with widespread applications in physics, chemistry, biology and medical research. Carbon isotope separation for radiocarbon dating is just one example where this technology has had an enormous impact.

During World War II separating the isotopes of uranium became a top priority and we are still living with the repercussions today. But that is another story.



Further Information

There is more information about Frederick Soddy and his role in the discovery of radioactive isotopes here: Frederick Soddy and the World Set Free.

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Twenty Thousand Leagues Under the Seas

by Nicholas Mee on November 21, 2020

In 1870 the French science fiction writer Jules Verne published his epic adventure Twenty Thousand Leagues Under the Seas—the story of a modern day Odysseus named Captain Nemo. Nemo is commander of a submarine The Nautilus constructed at a secret location to his own design. Its lavish interior includes a library and an elegant dining room furnished with valuable artworks and a grand pipe organ that Nemo plays while cruising the ocean deeps.

The Nautilus

The Nautilus is a cigar-shaped vessel, seventy metres long and eight metres wide. It is propelled by electricity from sodium-mercury batteries, and has a top speed of eighty kilometres an hour. The crew has plentiful supplies of water produced by distilling sea water, but there is no way to replenish the air, so the ship has a limit of five days beneath the waves.

The technology of the fictional Nautilus was beyond anything feasible in the nineteenth century. The reality was rather different. Although numerous enterprising individuals and several navies had experimented with underwater craft on and off for centuries, there was a long history of disappointment. Optimism was regularly followed by the swift abandonment of each project as the underwater craft’s impracticalities became apparent. The main issue was the source of propulsion. In Verne’s day power was usually supplied by a steam engine fuelled by coal. Unfortunately, burning coal would rapidly exhaust the onboard oxygen supply.

By the end of the First World War submarines were becoming more practical. Diesel or kerosene engines would propel the boat on the surface whilst replenishing the batteries that powered the vessel when submerged. Even with very large batteries, however, the underwater range and speed of such craft was limited.

The Coffin Service

Hyman G. Rickover (1900-1986) joined the U.S. Navy in 1918 and trained as a marine engineer. In 1929 he volunteered for submarine duty and commanded the submarines S-9 and S-48. It was a dangerous assignment. The submarine corps was referred to as the coffin service—accidents were frequent and casualties were not uncommon. From 1933 Rickover served on surface vessels, but his first-hand experience of life beneath the waves in a vulnerable tin can remained at the forefront of his mind. He was determined to improve the welfare of submariners.

Rickover rose through the ranks during the Second World War and in late 1945 he was appointed Inspector General of the 19th Fleet on the west coast of the United States. This was the dawn of the nuclear age and the new technology seemed to offer unlimited possibilities. Rickover was tasked with supervising a nuclear propulsion system that General Electric were developing for naval destroyers. But he could see a much better application for the nuclear reactor—submarine propulsion.

Nuclear power could solve the submarine’s biggest problems. It was a reliable power source that did not consume oxygen and would run for long periods on very small amounts of fuel. For several years Rickover struggled to convince the naval bureaucracy of the huge potential of nuclear submarines. The Navy wasn’t interested, responding rather like a battleship whose course had been set. But Rickover was persistent and eventually the battleship was turned around. In July 1951 Congress approved construction of the world’s first nuclear powered submarine. Like a twentieth century Nemo, Captain Rickover insisted that it be called the USS Nautilus.

The Pressurized Water Reactor

Many designs for nuclear reactors had been dreamt up by physicists such as Enrico Fermi and Leo Szilard in the years since Fermi’s first reactor. These ranged from those using heavy water or graphite as the moderator to Szilard’s much more exotic fast reactor cooled with liquid sodium. But Rickover needed a safe and reliable design that was compact and easy to operate. It would have to function for long periods without refuelling or maintenance and without the attention of highly-trained technicians.

Rickover settled on a design that matched all his requirements. It is known as a pressurized water reactor (PWR). In these reactors water acts as both the moderator and the coolant, and the water is pressurized to 150 atmospheres so it remains liquid within the reactor core at temperatures of around 300° C. In Rickover’s submarine the fuel would be highly enriched weapons-grade uranium composed of 93% uranium-235. This would dramatically extend the periods between refuelling and reduce the reactor’s physical size. 

Water has a dual role in a PWR, acting as both moderator and coolant. This means that a loss of coolant, due to a leak or the formation of vapour bubbles also implies a loss of moderator, which dampens down the nuclear chain reaction. More generally, if the temperature in the core increases the water density decreases, which reduces the effect of the moderation, so the nuclear reaction slows, bringing the temperature back down. This gives the reactor an inbuilt stability. These inherent safety features have led to the widespread commercial deployment of PWRs around the world.

Rickover’s nuclear reactor for the Nautilus produced 10 MW (megawatts) of electricity, which was sufficient to provide the submarine’s propulsion and power all its vital life support systems such as maintaining air quality, regulating the temperature and distilling fresh water from sea water.

The Mightiest Motion Picture of Them All!

Disney’s Deep Sea Adventure Story.

Walt Disney played his part in selling Rickover’s revolutionary submarine to the American public. In 1954 Disney released its steampunk blockbuster 20,000 Leagues Under the Sea based on the Jules Verne masterpiece. It was the most expensive Hollywood film up to that date and starred James Mason as the dark anti-hero Captain Nemo. Although the film is set in the nineteenth century, it suggests that Nautilus is powered by a new and mysterious source of energy discovered by Nemo. The nature of this energy would be all too obvious to the cinema-goers of the 1950s who were very familiar with the nuclear ambitions of the United States.

The film was released on 23 December 1954, and less than a month later on 17 January, nuclear reality emulated science fiction with the launch of the USS Nautilus. At almost one hundred metres in length, she was somewhat bigger than her fictional namesake. Prior to her launch, all submarines might more accurately be regarded as submersibles. Most of their time was actually spent on the surface, with their dives restricted to relatively short periods. But Nautilus suffered from no such limits. By February 1957 she had logged 60,000 nautical miles, or twenty thousand leagues, under the sea. 

The USS Nautilus.

Her most famous mission came the following year. On 3 August 1958 Nautilus cruised beneath the polar icecap and became the first vessel to reach the North Pole. This was a big boost to American prestige, and ratcheted up the Cold War arms race another notch. The United States was still recovering from the shock of the previous October when the Soviet Union had launched Sputnik, the world’s first artificial satellite. Now the implied threat of Intercontinental Ballistic Missiles (ICBM) was countered by the threat of Submarine Launched Ballistic Missiles (SLBM). The nuclear-powered submarine armed with nuclear missiles would be the ultimate Cold War deterrent. Able to operate throughout the world’s oceans submerged for months at a time and essentially untraceable, it would be capable of launching a devastating nuclear strike even after its homeland had been wiped off the map.

Nuclear Proliferation

The American monopoly on seaborne nuclear propulsion did not last. Nuclear submarines are now found in the fleets of six nations: the United States, Russia, China, the United Kingdom, France and India. There are currently about 200 nuclear powered ships in the world. Most are submarines but they also include aircraft carriers and ice breakers. The reactors in these vessels typically generate around 100 MW of electricity and run on highly enriched uranium containing 20% to 93% uranium-235, which enables them to operate for decades without refuelling. Diesel generators are used as a back-up system in case of a reactor shut-down.

The only nuclear-powered submarine ever to fire on an enemy ship is the British HMS Conqueror which sunk the Argentine cruiser the General Belgrano with two torpedoes during the Falklands War in 1982. Of the almost one thousand on board the Belgrano, 323 were killed.

Atoms for Peace

The PWR was first developed for Rickover’s nuclear submarine programme. It is now the most widely used source of commercial nuclear energy. The first, and to date only, commercial PWR nuclear power station in the UK is Sizewell B on the Suffolk coast in southern England. It came into service in 1995 and generates 1.2 GW (gigawatts) of electricity.

Sizewell B, the UK’s first PWR nuclear power station.

Out of the 441 commercial nuclear reactors around the world, 299 are PWRs. They are capable of generating a combined total of 284 GW of electricity.


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The World’s First Nuclear Reactor

November 15, 2020

The investigations by the French authorities in 1972 led to a uranium mine in the Central African country of Gabon, then a French overseas territory. The reason for the missing uranium-235 turned out to be quite incredible. Fermi’s Chicago nuclear reactor had been foreshadowed 1.7 billion years earlier by a natural nuclear reactor in a uranium deposit at Oklo near Franceville. Detailed analysis of the unusual nuclear isotopes found in the deposit has enabled physicists to construct a picture of what happened there long long ago.

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The Age of Rock

October 31, 2020

Rutherford suggested that the amount of helium trapped within a rock would reveal how much uranium has decayed since the rock formed. And by comparing the amount that has decayed to the amount that remains the age of the rock could be determined. For instance, if half the uranium has decayed then the rock’s age equals the half-life of uranium, which can be measured in the laboratory. 

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Pregnant Camels Often Sit Down Carefully

October 30, 2020

After several months of arduous work Holmes could assign an age to the rock—370 million years. This is considered to be the first accurate radiometric dating. (It agrees with modern results for the Devonian, according to which this period lasted from 419-359 million years ago.) Not bad for an undergraduate project! Holmes was just twenty years old and would graduate later that year. His project must rank as one of the most remarkable ever completed by an undergraduate.

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Why Does Antimatter Matter?

October 17, 2020

There is a very important modern technology that makes use of antimatter. It is known as PET (Positron Emission Tomography) and it is a non-invasive medical imaging technique.

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Hell Creek Apocalypse

September 27, 2020

Four decades of detective work by geologists, nuclear physicists, palaeontologists and geochronologists has given us an incredibly detailed and compelling picture of what happened one dramatic day just over sixty-six million years ago—the day that sealed the fate of the dinosaurs.

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A Goddess Spurned and the Fate of the Sun

September 25, 2020

So how will the Sun evolve and what is its ultimate fate?
Within the Sun’s core hydrogen nuclei are gradually fusing to form helium nuclei, releasing a steady stream of energy. The flow of thermal energy from the nuclear furnace supports the Sun against its tendency to collapse under gravity. For as long as the nuclear fuel holds out this balance will be maintained. In the case of the Sun this is around ten billion years. Currently, the Sun is nearly half way through its allotted time span.

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An Exceptionally Cold Case

September 19, 2020

Much has been learnt about Ötzi the Iceman. High levels of copper and arsenic were found in his body, which suggests that he may have been involved in the smelting of copper. He was about 45 years old and he suffered from a number of ailments. He had parasitic worms in his gut, gallstones, arthritic joints, hardened arteries and rotting teeth. He also had numerous tattoos produced by rubbing charcoal into lesions in the skin. From the contents of Ötzi’s stomach it seems that his last meal included fatty ibex meat and einkorn grain. The radiocarbon dating of Ötzi’s remains reveal that he lived some time around 5300 years ago.

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Frederick Soddy and the World Set Free

September 13, 2020

In the novel Holsten discovers a new element named carolinium that could be induced to release the energy locked within its atoms. This opened the door to military applications, bombs of hitherto inconceivable destructive power, atomic bombs, as Wells calls them in the first ever use of this term. Although Wells’s method of delivery for the new weapons was of its time—they were lobbed over the side of aircraft by intrepid airmen, as depicted on the front cover of the American edition—the novel would prove uncannily accurate about future developments of nuclear weapons and their military consequences.

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