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.
