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Introduction to Nuclear Chemistry

Nuclear science and chemistry

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1 Nuclear chemistry and radiochemistry

All beginning Chemistry students learn that the atomic nucleus accounts for almost the entire mass of the atom, and that its positive electric charge (the atomic number) determines the identity of the particular element. More importantly, the nuclear charge, through its influence on the occupancies, energies, and shapes of the electron orbitals, determines the ordinary chemical properties of the element. 

Beyond this, however, we tend to regard the nucleus as something that is "just there", and whose detailed study belongs more in the realm of physics than chemistry.

But there are a number of properties and processes involving the nucleus that have direct chemical consequences, and these will be the subject of this unit. Foremost among these is radioactive decay; this process not only transforms the nucleus of one atom into that of another element, but the particles emitted can cause further chemical changes when they strike nearby molecules.

The scope of nuclear chemistry

The term nuclear chemistry can refer to any of a number of processes that relate to the atomic nucleus and have chemical relevance. Examples are

  • Nuclear reactions, in which a nuclide collides with another particle to form a different element.
  • Radioactive decay, in which an unstable nucleus spontaneously emits a particle and is transformed into a nuclide of a different element.
  • Techniques such as nuclear magnetic resonance (NMR) that is widely used to help determine the structures of molecules.

Several sub-fields of nuclear chemistry have become so important that they have been given their own names. The most important ones are:

  • Radiochemistry refers to the study of radioactive isotopes, including their isolation by chemical means, the study of their decay processes, and their many applications as "tracers" in the study of the mechanisms of chemical reactions.  Environmental radiochemistry has recently become an important sub-field.
  • Radiation chemistry is the study of the chemical effects of ionizing radiation on matter. Since the particles emitted in radioactive decay constitute this kind of radiation, this field is easily confused with radiochemistry.  But because other sources of ionizing radiation (such as beams of fast-moving electrons or protons) that produce the same kinds of chemical effects, radiation chemistry is a more general term that encompasses the chemical effects of radioactive decay but extends somewhat beyond the scope of "pure" nuclear chemistry.

2  How it all began: the origins of nuclear chemistry

Nuclear chemistry had its beginning with the discovery in 1896 by the French physicist Antoine Henri Becquerel (1852-1908) that uranium salts produce radiation which, like X-radiation (which had been discovered by Röntgen in the previous year), could be recorded photographically and could also produce ions when passed through air.

Becquerel was mainly interested in the phosphorscence that some minerals emit after having been exposed to light. Roentgen's discovery led him to wonder if a fluorescing mineral also might emit X-rays, so he undertook an experiment in which he would expose crystals of such minerals the to sunlight.  He reasoned that any X-rays coming from the material would penetrate the black paper wrapping of an unexposed photographic film and leave an image similar to that created by exposure to X-rays.

Serendipity, n: the act of making a desirable, but unsought-for discovery by accident. [From a fairy tale, The Three Princes of Serendip]

Serendipty is not uncommon in science, and is, in fact, one of its frequent rewards. The unlikely conjunction of three chance occurrences that resulted in Becqueral's discovery of radioactivity remains one its most famous examples.

Becquerel's discovery in his own words (English translation)

For his first study, Becquerel selected crystals of potassium uranyl sulfate K(UO)SO, a well-known fluorescent material. He placed the material over an unexposed photographic plate wrapped in black paper; in between, he inserted a sheet of aluminum that X-rays would not penetrate, the idea being that any X-rays that reached the film would be revealed by the shadow of the metal, which happend to be in the shape of a Maltese cross.

The winter day on which he had planned to expose the crystals to the sun happened to be overcast, so Becquerel had to postpone the experiment.  He stored the assembled sample, aluminum cross and protected film in a drawer.  After several more days of dull Paris weather, he decided to develop the film anyway.  To his great surprise, the film had recorded the shadow of the aluminum cross, suggesting that the crystalline sample was indeed emitting penetrating radiation of some kind which, unlike phosphorescence, did not require previous exposure to bright light.

Following this up, Becquerel then found that other uranium salts that are not phosphorescent and had not been exposed to light also emit "Becquerel rays" (as they were later known.)  His conclusion was that the rays had nothing to do with phosphorescence and were continuously emitted by uranium atoms.

Enter the Curies

Because most universities of the time did not accept women students, Polish-born Maria Sklawodowsa (1867-1934) emigrated to Paris where she became the first woman to be accepted at the Sorbonne. There she later took degrees in physics and mathematics. In 1895, shortly after beginning her doctoral work, she met and married Pierre Curie, a physicist who had devised instruments for making electrical and magnetic measurements. Marie (having adopted the French version of her name) was intrigued by Becquerel's 1894 discovery, and decided to continue his work. 

Although Becquerel did not follow up on this work, others soon did, most notably Marie and Pierre Curie.  Marie began her Ph.D. research by investigating the Becquerel rays in more detail. Her first major discovery was that a uranium source, kept in dry air, slowly acquired a positive electric charge, implying that the rays emitted by the uranium consisted of negatively-charged particles.  Using a sensitive device for measuring minute electric currents that Pierre and his brother had invented some years previously, she found that these particles caused the air near the sample to become electrically conductive.  This could only mean that the particles, on colliding with the molecules in the air, caused them to acquire electtrical charges of their own — that is, to become ionized.

Ionizing radiation

This general term can refer to both

  • electromagnetic radiation ("light") of sufficiently short wavelength (normally in the UV, X-ray or gamma-ray range) that can be absorbed by neutral molecules and is sufficiently energetic to decompose them into ions;
  • fast-moving charged particles such as ions (including α particles) or electrons (β particles) that collide with neutral molecules and convert them into ions. Even neutral particles such as artifically-produced fast neutrons can be "ionizing" if they strike a molecule with sufficient kinetic energy.
When ionizing radiation passes through air, the gaseous ions that result render the air electrically conductive.

Some years before they met, Pierre Curie had invented a sensitive "electrometer" that could measure the conductivity of air. The Curies found that the air near a radioactive material becomes conductive, and its observation provides a simple and accurate way of detecting and measuring radioactivity.

Later, Marie Curie showed that compounds not only of uranium (92U) but also of thorium (90Th ) could ionize air.

Curie also found that the radioactive mineral pitchblende produced more ionizing radiation than uranium, suggesting that it contains one or more elements that is more radioactive than any then known. In order to discover this new element, she began the painstaking chemical fractionation of pitchblende. Working with crude equipment and under difficult conditions in an unheated, leaky shed, she finally discovered the metallic element polonium (84Po) in 1898 and radium (88Ra) in 1902. (A total of 0.1 gram of RaCl3 was isolated from more than one tonne of pitchblende.)

Rutherford

Rutherford's Nuclear World is an excellent, multi-part scientific biography of Rutherford's remarkable career. Highly recommended!

Ernest Rutherford (1871-1937), who delighted in referring to himself as "a New Zealand farmer", is the other giant of the early days of nuclear science and is often referred to as "The father of nuclear physics". His early discovery that radioactive decay could be accompanied by the transmutation of one element into another won him the 1908 Nobel Prize in Chemistry, but this was just the beginning of his remarkable career. By that time he had moved from McGill University in Canada to the University of Manchester in the UK where, in 1907, Rutherford and Royds, in an ingeniously simple experiment, showed that alpha particles are identical with helium ions, 4He2+.

Rutherford's most famous triumf was the "gold foil" alpha-scattering experiment described farther below.

 

 

 

Alpha, beta and gamma radiation

Studies by Becquerel, Rutherford, the Curies and others revealed that different radioactive substances produce different types of "rays" (particles of radiation). By 1900, Ernest Rutherford and others had shown these to consist of three kinds, namely alpha (α), beta (β) and gamma (γ).  Rutherford assigned them these names based on their increasing power to penetrate materials.

In 1903, Frederick Soddy and William Ramsay showed that helium gas is formed when radium decays. A few years later, Rutherford and Royds found that this helium is formed from He2+ ions, thus identifying these as the alpha particles.

In experiments published in 1899, Rutherford identified a second kind of radiation which he found to have 100 times the penetrating power of alpha particles. In the following year, Becquerel found that these beta particles had the same charge-to-mass ratio that J.J. Thompson had previously measured for "cathode rays" which turned out to be electrons.

Gamma rays, discovered by Villard in 1900, exhibit extreme penetrating power and lack electric charge (revealed by their non-deflection by a magnetic field); these facts suggested that they are qualitatively different from the two other kinds of radiation. By 1914 it was known that gamma radiation had a wave-like nature, and is thus a form of electromagnetic radiation similar to X-rays. 

Discovery of the nucleus

To refresh your understanding of the alpha-scattering experiment that led to the discovery of the nucleus, watch the following excellent video:

The discovery of the atomic nucleus (UK SciTech Council, 3½ min)

In 1909, at Rutherford's suggestion, his assistants Hans Geiger (1882-1945) and Edward Marsden (1889-1970) used a beam of alpha particles in their famous "gold foil" experiments (published in 1911), demonstrating that the atom is mostly empty space, with the positively-charged nucleus occupying only a tiny fraction (about 10–4) of the volume of the atom. Rutherford later remarked:

"I remember ...later Geiger coming to me in great excitement and saying, 'We have been able to get some of the α-particles coming backwards...' It was quite the most incredible event that has ever happened to me in my life. It was almost incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."

Rutherford coined the term "nucleus" from the Latin for "little nut".

This key discovery established the general picture of the atom, but the details of the composition and structure of the nucleus would not emerge until somtime later.

Discovery of the proton

Over the next few years, Rutherford conducted experiments showing that hjydrogen nuclei can be emitted when alpha particles are shot at the nuclei of nitrogen and other elements. In  1917, he was able to conclude that hydrogen nuclei are present in the nuclei of all elements. Recognizing that this lightest of all nuclei must constitute a fundamental particle of nature, in 1920 he assigned it the name "proton", from the Greek word for "first".

By this time, thanks to the work of Moseley, the "element numbers" that Mendeleev had assigned (not always correctly) on the basis of their atomic weights had been refined to atomic numbers, which correspond to the numbers of protons in the nuclei of each element.

Discovery of isotopes

The name most closely associated with the isotope concept is Frederick Soddy (England, 1877-1956).  During his early years as a lecturer at the University of Glasgow, he frequently suggested that the work of others implied the existence of isotopes, and he replicated some of the crucial experiments and carried out his own investigations. 

Soddy first used the term "isotope" in 1913. It was actually coined by a friend at a dinner party, based on the Greek isos ("same") and topos ("place".)

The idea that two or more elements having identical chemical properties (as evidenced by their inseperability by procedures such as precipitation or fractional crystallization) could have different atomic masses was never really "discovered" in the usual sense. Instead, it was built on the work of a large number of investigators, and evolved gradually over a period of slightly more than ten years, beginning in 1902.

In that year, shortly after the Curies had discovered radioactivity, Rutherford and his associate Frederick Soddy (1877-1956), working together that year at McGill University in Montreal, sought to investigate the fate of nuclei such as uranium and thorium following their emission of alpha particles, which had not yet been identified as helium nuclei.  They suspected that the decay products might be different elements, but at the time were unable to prove this. Other investigators found that these decay products were indeed nuclei of different elements, but their masses did not correspond to those of any that were then known.

Even more puzzling was the lack of any empty spaces in the periodic table to accommodate these new elements, that had been given names such as ionium and mesothorium.

It must be remembered that it was not until 1913 that Moseley's work showing that the different elements are distinguished by their atomic numbers, rather than by their atomic weights, which has been the basis of Mendeleev's placement of the elements in the periodic table.

In 1909, the Swedish chemists Daniel Strömholm and The Svedberg proposed that the "new" elements formed in radioactive decay processes could be properly accommodated in the spaces of the periodic table already occupied by known elements.  They also suggested the same principle might apply to non-radioactive elements; that is, there could be more than one chemically identical form of these elements, and that this could solve another mystery, namely the fact that measured atomic weights of many elements are non-integral.   Soon after that, Soddy replicated the experiments of Strömholm and Svedberg, and stated that their work constituted the first convincing evidence of isotopes.

Three years later, the first example of a non-radioactive element exhibiting more than one mass number was discovered by J.J. Thompson who showed that neon, which consistes mostly of Ne20, also contains traces of Ne22.

In 1914 Soddy and his co-worker Ada Hitchens (1891-1972) showed that 92U235, which can undergo both alpha and beta decay, forms atoms chemically identical to those already known, but having different masses:

alpha decay 92U23590Th2314He2  
beta decay 92U235 → 93Np235  + 0e–1  

 

Discovery of the neutron

All nuclei heavier than hydrogen have atomic weights (and thus atomic masses) that are roughly twice their atomic numbers (proton numbers.)  For example, the alpha-particle 4He2+ has a mass number twice as large at its atomic number. This suggested that nuclei contain another kind of particle similar in mass to the proton, but lacking its electrical charge.

Rutherford, noting that many radioactive elements emit electrons, first thought that this neutral particle might be another proton that was bound together with an electron, but this can be unlikely for several reasons. In 1920, he concluded that the additional mass is due to a separate kind of fundamental particle which he called the neutron.

For details of Chadwick's discovery, see this understandable summary from Cambridge University.

But the fact that these particles carry no electric charge made them very difficult to detect; they could not be deflected by electrostatic or magnetic fields, and they interacted only weakly with materials they would pass through.  It was not until James Chadwick (1891-1974), a former PhD student of Rutherford, was able to identify the neutron and measure its mass.

 

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