Nuclear chemistry

Definition: Of the three subatomic particles (protons, electrons, and neutrons), much of chemistry is concerned with electrons, which are responsible for the chemical bonds that join atoms into molecules. In contrast, nuclear chemistry is the study of the chemistry of nuclei—the protons and neutrons that form the dense core of atoms. More specifically, nuclear chemistry can include the following: the chemistry of atoms with high atomic numbers, in which the nucleus is larger and therefore plays a greater role; the chemical relevance of the basic properties of nuclei; the chemistry of large systems that owe their behavior to nuclear phenomena; and the use of scientific and medical techniques based on nuclei.

Basic Principles

The field of nuclear chemistry emerged largely from the work of Marie Curie, a physicist and chemist and the winner of two Nobel Prizes. Curie discovered radioactivity toward the end of the nineteenth century, during her studies of radium, polonium, and other radioactive elements. The first use of radiation in medicine also dates to this time period. The second major event in nuclear chemistry was the development of quantum mechanics, which provides a physical model for describing how nuclei and other very small particles behave. World War II saw the deployment of the first atomic bombs, which were developed by applying nuclear chemistry to release large amounts of energy from very little matter. The first nuclear reactor was used to generate electricity in 1951, and the first application of magnetic resonance imaging occurred shortly thereafter. Given its youth relative to other fields—organic chemistry or materials science, for example—nuclear chemistry has already come a long way and permanently changed the course of medicine, science, and human history.

The word nuclear has negative connotations due to its association with atomic bombs and the devastation caused by the Three Mile Island, Chernobyl, and Fukushima nuclear meltdowns. In fact, a technique known to chemists and physicists as nuclear magnetic resonance (NMR) is called magnetic resonance imaging (MRI) when used as a procedure in the medical field to avoid the implication that it involves exposure to nuclear radiation, which can cause radiation sickness and cancer. The word nuclear in NMR actually refers to what is being studied: the nuclei of atoms in the body. By measuring the differences in how nuclei respond to a magnetic field, scientists and medical specialists can produce an image of the scanned area that differentiates between and among organs, tissues, and bones. Nuclear is a broad term, referring to phenomena in which atomic nuclei play a predominant role.

Core Concepts

Nuclei in Atoms. Atoms are composed of electrons, protons, and neutrons; nuclei contain the latter two in a very small, very dense core at the center of an atom. The number of protons and neutrons in the atom determines the element and isotope of the atom, respectively. For example, an atom with a single proton is a hydrogen atom, one with thirty-seven protons is rubidium, and one with ninety-two protons is uranium. The isotope of a given element is determined by the number of neutrons in the nucleus. Generally, the most stable form of each element is that in which the number of neutrons is equal to the number of protons, but some other arrangements can also be quite stable. For example, the element carbon usually refers to carbon-12, with the numeral indicating the sum of the number of protons and neutrons. However, carbon-13 is another stable form of carbon, and carbon-14 is quite well known due to its use in carbon dating. Because carbon-14 decays into nitrogen-14 with a known half-life (the amount of time it takes for half of a given quantity to decay) of approximately 5,700 years, comparing the prevalence of carbon-14 in biological material (a fossil, for example) with the amount of nitrogen-14 yields the age of the biological material.

Radioactivity. Some isotopes are unstable, meaning that they have too much energy and will eventually release some of the excess energy. The identity of the final, more stable nucleus (the daughter nucleus) depends on the way in which this excess energy is released. In alpha decay, unstable atoms emit a particle comprising two protons and two neutrons—that is, a helium nucleus. Thus, the daughter nucleus is actually a different element from the parent nucleus; for example, uranium can undergo alpha decay into thorium. In beta decay, the parent nucleus emits an electron and another subatomic particle called an electron antineutrino. Whereas alpha decay decreases the atomic number by two, beta decay actually increases it by one. Thus, uranium could undergo beta decay to neptunium. The third and final of the main decay mechanisms is gamma decay, in which the parent nucleus releases high-energy photons. In this process, the daughter nucleus is the same as the parent, because no nuclear matter is lost or gained. Depending on the context, discussions of radioactivity can focus on the change in the parent nucleus (in a physics context) or the radiation emitted (in medical and health contexts).

Quantum Mechanics. Nuclei are small; the diameter of a single nucleus is on the order of 2 to 15 femtometers, tens and hundreds of times smaller than the diameters of an atom. Whereas our everyday world is described adequately by classical, or Newtonian, mechanics, phenomena on this small of a scale are best described by quantum mechanics, a field that was developed in the early twentieth century. The world of quantum mechanics is a strange one: the difference between particles and waves disappears, and everything becomes probabilistic rather than deterministic, meaning that a researcher can compute the probability that a particle is at a given location at a given time but can never actually pin down exactly where it is. These results and more follow from the discovery that energy (in addition to other quantities) only exists in discrete units, called quanta. That is, a particle can have 1 quantum of energy, 2 quanta of energy, and so forth, but it cannot have 1.2 quanta of energy. This principle completely revolutionized the understanding of physics, thanks to work by many famous scientists, such as Albert Einstein, Niels Bohr, and Erwin Schrödinger.

Nuclear Spin. In addition to being a crucial concept for understanding nuclear chemistry, spin is also a good example of a purely quantum-mechanical concept with no analogue in the Newtonian world. Despite its name, the spin of a particle has nothing to do with actual rotation, but this fact was only discovered after the name had caught on. For most purposes, spin can be imagined as the rotation of a particle without sacrificing accuracy; readers interested in learning more about spin should consider further study of quantum mechanics and particle physics. Imagining nuclei as spinning tops capable of spinning in different directions, where those directions correspond to different spin numbers, is sufficient for this overview.

Nuclear Magnetic Resonance Spectroscopy.Nuclear Magnetic Resonance spectroscopy is a technique used by scientists to elucidate the structures of molecules. A magnetic field is used to align the nuclear spins (the spinning tops described above) in one direction. Radio waves are then used to knock the spinning tops out of alignment, and the time it takes for the spins to realign is recorded. This realignment time varies based on each nucleus’s local environment. After some calculations, scientists can determine which atoms are bonded to one another, how they are bonded (for example, with a single bond or a double bond), and the exact distance between them. This information can then be used to identify the overall structure of the molecule being studied.

Magnetic Resonance Imaging. MRI operates in the same fashion as NMR spectroscopy but is applied to medical imaging rather than identification of molecular structure. Instead of focusing on the responses of each individual atom, MRI looks at relatively large areas, identifying trends in how nuclei in the body respond to the magnetic field and radio waves. This information can be used to differentiate between organs, tissues, fluids, and bone, providing doctors with a useful image of a person’s internal anatomy without harming the patient.

Applications Past and Present

Research. NMR spectroscopy allows chemists, physicists, and biologists to determine the structure of, and therefore identity, an unknown compound. Chemists and biologists might use NMR to identify a newly synthesized compound or the conformation of a protein, whereas physicists might use the information about the strength of a chemical bond and the spin of the nuclei to draw conclusions about the physical behavior of the electrons and nuclei. Electron paramagnetic resonance (EPR) spectroscopy is the analogous technique for electrons rather than nuclei. However, because few compounds have electron configurations with a net spin—in most cases, the spins cancel one another out—it is a much less common technique, used primarily to study free radicals and some magnetic compounds.

Medicine—Magnetic Resonance Imaging. MRI is a popular imaging technique due to its ability to provide information about structures in the body that is difficult to obtain using other techniques while being essentially harmless. An MRI can be used to detect swelling, inflammation, tumors, blockages, organ damage, or fluid leakages. It can also help to diagnosis nervous-system problems, such as Alzheimer’s disease, dementia, or multiple sclerosis. Most negative effects experienced during or after an MRI are due to interactions between the strong magnetic field used for imaging and any metal that might be imbedded in the patient, such as iron present in some tattoos. Before recommending an MRI, doctors determine whether the patient has a pacemaker, metal joint, or anything else that might cause discomfort or injury in a high magnetic field.

Medicine—Radiation Therapy. Cancer is caused by tumorous cells—cells that duplicate and grow without restraint, forming tumors. One method of treating cancer is to kill these cells by subjecting them to radiation. Radiation can be applied externally, using instruments that send high-energy x-rays through the tumorous region of the body, or internally, by having the patient ingest radioactive materials that release radiation as they decay inside the body. The efficacy of radiation therapy lies in applying the radiation such that maximum damage is done to cancerous cells and minimum damage is done to healthy cells.

Nuclear Power. One of the well-known applications of nuclear chemistry is nuclear power. Nuclear energy is generated by using the energy released during fission, or the process of splitting a heavy, unstable nucleus into two lighter nuclei. The nuclei of the element uranium are often used in fission, as they break into smaller, more stable nuclei. This energy is then used to heat water into steam, which in turn drives a turbine and generates electricity. Nuclear power has the advantages of being independent of nonrenewable energy sources (like oil) and generally being a more environmentally friendly process: it generates relatively little waste and involves far less carbon emission than burning fossil fuels. However, malfunctions can cause nuclear meltdowns, releasing radiation into the area surrounding the nuclear power plant and harming residents and the environment. The incidents at Three Mile Island (1979, United States), Chernobyl (1986, Soviet Union), and Fukushima (2011, Japan) are good examples of the dangers of nuclear power. Because the material used to generate nuclear power is the same as that used to produce nuclear weapons, nuclear power also has political implications for countries that are not thought to currently possess nuclear weapons.

To avoid some of the issues and risks associated with conventional nuclear power, which relies on fission, many researchers explored the potential use of nuclear fusion, which involves combing two light nuclei into a heavier nucleus and releasing a massive amount of energy, as an energy source. Unlike fission-based nuclear power, fusion does not produce long-lasting radioactive waste, has no potential for a nuclear meltdown, and does not rely on materials which can be used to construct nuclear weapons; it also produces far greater amounts of energy per reaction. Additionally, as with nuclear fission, nuclear fusion does not produce greenhouse gases. The field of nuclear fusion saw a number of breakthroughs during the twenty-first century; for example, in December 2022, researchers in the United States achieved a net energy gain during a nuclear fusion test, raising hopes that fusion could become a viable energy source in the future. The amount of energy produced during this first test was small, however, only 3.5 megajoules, which is enough to boil about 10 gallons of water. However, subsequent tests in 2023 created more energy, 3.88 megajoules. While scientists were hopeful, they needed to increase the amount of fusion while also bringing down the cost for this to be a viable energy source.

Nuclear Weapons. Nuclear weapons rely on the massive amount of energy stored in atomic nuclei. The sudden release of this energy not only inflicts enormous damage from the initial bombing but also releases radiation that lingers in the environment, causing deaths due to radiation poisoning in the short term and cancer in the long term. The first and, to date, only use of nuclear weapons was in the World War II bombing of Hiroshima and Nagasaki in Japan, but nuclear-weapon stockpiles exist in at least eight different countries, with the United States and Russia having almost twenty thousand warheads between them. A variety of nuclear weapons exists, and research into improvements in their design is ongoing.

Social Context and Future Prospects

Over the last century, nuclear chemistry has had a profound effect on many areas of science and technology that are still being developed today. NMR has revolutionized how chemists identify molecules, and MRI is one of the most useful medical-imaging techniques. Radiation treatment has greatly improved cancer prognoses. Additionally, a better understanding of the health effects of radiation has helped scientists understand the relationship between sun exposure and skin cancer. Nuclear power is an important energy source for many countries, and nuclear weaponry has changed the global political landscape. The wide-reaching applications of the field ensure that nuclear chemistry will continue to be an important area of study for many years to come.

Due to its breadth, the future of nuclear chemistry and nuclear science is quite promising. The aging American nuclear arsenal will need to be addressed soon; if a policy of complete nuclear disarmament were adopted, which is unlikely, disassembling and disposing of existent nuclear stockpiles would be a long, difficult project. In the more likely event of continued nuclear armament, research will be ongoing to develop better nuclear weaponry. Nuclear power faces a similar scenario; some people would like to see the use of nuclear power completely halted, but it is likely to continue to play a strong role in energy production worldwide as we search for alternatives to fossil fuels. Research into nuclear fusion has added to the potential for the nuclear power sector to grow in the future.

The prospects for the application of nuclear chemistry in medicine are even better. Magnetic resonance imaging is and will continue to be a key imaging technique; further research will include cost-reduction efforts to reduce the number of patients who must undergo potentially harmful procedures, such as X-rays, due to the prohibitive cost of an MRI. Concerning radiation therapy, future research will focus on minimizing the damage done to normal cells while maximizing the damage done to cancerous cells. For example, the development of ways to make internal radiation therapy treatments more specific to tumorous cells would reduce the negative side effects of the procedure, which would improve quality of life and the overall survival rate.

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