Nuclear Chemistry | Research & Encyclopedia Articles

Nuclear Chemistry

In some ways, chemistry can be viewed as having nothing whatsoever to do with the nucleus of the atoms.Bonding, spectroscopy, chemical reactivity, and such are all a consequence of the electrons of which an atom is partially composed. The nucleus, in chemical terms, is merely there. It contains the mass of the atom but it does not participate in chemistry.

Of course, this is a very limited view of chemistry as a whole and, in particular, the role of the nuclear chemistry. Just saying that the nucleus contains all of an atom's mass is not saying nearly enough about its role. The concept of the "atom"--the smallest unique piece of matter for any substance--can be dated back to the Greek philosopher, Democritus, but it took another 2,500 years before the concept really captured the imagination of scientists. Even John Dalton, who put forward the theory of the atom at the beginning of the chemical revolution, did not think of them as anything more than simple, solid spheres. For most practicing scientists, atoms were a way of arranging the elements and talking about compounds, but not something to be taken too seriously.

However, as the 19th century progressed, a better understanding of the atom developed. Radiation was observed, implying that there must be some structure within the atom. The electron was discovered and determined to be smaller than the atom. If the atoms was the "smallest, most indivisible component of matter," then what could the electron be? What was this radiation that scientists were observing? These are the questions that ultimately lead to the discovery of the nucleus and the formation of the twin sciences, nuclear physics and nuclear chemistry. Indeed, they are such twins that it is often difficult to tell whether a scientist researching the nucleus is a chemist or a physicist.

Atom nuclei are composed of two particles, the proton and the neutron. These particles are not fundamental. They are made up of quarks, gluons, and such but for the purposes of constructing the elements in the periodic table, they are the basic building blocks. The proton is slightly lighter than the neutron (1.0073 amu and 1.0087 amu, respectively, where an "amu" is an atomic mass unit) but it is also positively charged. The magnitude of the charge is equal to that on the electron but of opposite sign. The electron is much smaller than either nuclear particle or "nucleon", with a mass of only 0.0005 amu. This is why it is accurate to say that the mass of an atom is principally located in the nucleus as the electrons contribute only a very small amount.

The nucleus is also incredibly small in size. Relatively speaking, it is about 100,000 times as small as the atom itself--about the equivalent of a golf ball sitting center field in the Los Angeles Coliseum, with the electron being a flea hopping around the outer concourse. But it is this tiny lump of protons and neutrons that determines the elemental nature of the atom. Each element has a unique number of protons, called its "Atomic Number." That is, all hydrogen atoms--whether here on Earth or in the farthest reaches of outer space--have only a single proton in their nucleus. All oxygen atoms have eight protons and all uranium have ninety-two. The one characteristic that determines an atoms identity is the number of protons in its nucleus, not the number of electrons nor neutrons. In a neutral atom, the number of electrons is the same as the number of protons but atoms can be ionized, either oxidized or reduced, and end up with a different electron count.

The number of neutrons, on the other hand, can actually vary within a single element. As an example, consider hydrogen. It occurs in three different forms. The vast majority of hydrogen occurs as the simplest of all elements. It has a single proton for its nucleus and a single electron occupying an orbital. The second form of hydrogen has a nucleus with a proton and a neutron and is called "deuterium." The third form has a proton and two neutrons and is called "tritium." This particular arrangement of nucleons is unstable and so tritium readily undergoes radioactive decay to yield a helium atom. This decay process involves the conversion of a neutron to a proton via the ejection of an electron, which is an example of nuclear math. Two particles can combine to make one or be made by the decay of a single particle.

Each of these different forms of hydrogen are called "isotopes" (Greek for "equal in place") and hydrogen is unique in having named isotopes. All the other elements use numbers to designate their isotopes, such as carbon-14 and uranium-235. Within every element, up to and including uranium, there is at least one arrangement of protons and neutrons that yields a natural isotope. However, not all isotopes are stable. Indeed, many isotopes spontaneously convert to other elements through radioactivity. It is the instability of the nuclear configuration that is the source of radioactivity.

As an example, consider tritium. The arrangement of two neutrons to one proton means that the nucleus is "neutron rich." Neutrons, themselves, are unstable and decompose if they are removed from the nucleus. It is only through binding to the proton that neutrons have any stability at all. It is easy to see then that while the first neutron and proton might be stable, adding a second would generate some instability. The neutron in tritium decays as a consequence. This is a very simplistic view of the reasoning behind the process. Nuclear decay is a function of the "weak force" which has been studied by nuclear physicists and is beyond the scope of this discussion. Suffice it to say, though, that a roughly one to one ratio of neutrons to protons is stable for the lighter elements. As the atomic number increases, more neutrons are required to mediate the strong repulsive forces between the positively charged protons. The ratio of neutrons to protons increases so that uranium, for example, has 92 protons in its nucleus but about 148 neutrons, depending upon the isotope.

The lifetime of a radioactive nucleus is randomly determined. That is, any single unstable atom could undergo a nuclear reaction now or five years from now or five million years from now. If the lifetime of any single atom is undetermined, what does a "half-life" for a radioactive species mean?

Chemists routinely deal in mole quantities of substances and since a mole contains 6.022 x 1023 atoms or molecules, chemists invariably deal in statistics. With that many atoms, it is possible to say that one of them will decay in such and such a period of time. Not which one but that one will. Further, using calculus, it is relatively straightforward to show that half of the atoms initially present will decay in a specified time. This is called the "half-life". For example, the half-life of tritium is 12.26 years. That means that if we had a mole of tritium atoms right now, in 12.26 years, we would have half a mole. At that point, we would be starting with half a mole, and in another 12.26 years we would have half of that quantity or a quarter of our original mole of tritium. In the following 12.26 years, we would be down to one eighth, and so on. It would take approximately 79 half-lives or 969 years before the tritium atoms actually run out.

The half-life of an isotope can vary in length from nanoseconds to millions or billions of years. This means that for some isotopes of different elements, their radioactivity is very short-lived. For others, they will still be emitting radiation so far into the future, that it is hard to imagine. This, of course, has led to many concerns about the use of nuclear power as a source of energy. How to dispose of radioactive waste is a major difficulty, particularly when that waste will still be radioactive many, many generations from now.

Nuclear medicine employs radioactive isotopes in the detection and curing of disease. Everything from brain scans to determining bone density to imaging the lungs employs radioisotopes. In essence, the process involves binding a radioisotope to a compound that has a specific affinity for an organ or region of the body. Using a variety of radiation detectors, the medical technician is able to view the soft tissue. For example, Positron Emission Tomography (PET) uses the positron (a positive electron) emitted by carbon-11 that has been incorporated into glucose to scan brain tissue for metabolic activity.

Radioactivity has also seen use in the treatment of cancer. There are two basic approaches that have been adopted. Historically, the first was to expose the whole patient to radioactive rocks or powders in the hope that the damaging radiation would preferentially kill the cancer cells. This approach has been refined so that tightly-focussed beams are now employed. The second approach relies on the chemical inclusion of the radioactive atom into a molecule that is targeted for the tumor cells. This takes the radiation into the body and allows it to be more effective. Unfortunately, it is hard to focus chemical compounds on single organs with the result that other parts of the body are affected. The latter approach is a significant area of research in nuclear chemistry, finding the right delivery compound for the right type of radioactive isotope.

Radioactivity also finds use in preserving foods in which the gamma ray emissions from cobalt-60 are used to eradicate any bacteria or other decay causing agents. This method of preservation has lead to a significant increase in shelf life for produce but also to controversy over the potential harmful side effects. These concerns arise from the fact that just as radiation can be emitted from an atom, it can also be absorbed which leads to the formation of "unnatural" compounds within the food. These, in turn, could be harmful and may lead to disease. The process of testing the proposed use of radiation for food is meant to safeguard against this possibility.

One of the more interesting uses of nuclear chemistry is the natural incorporation of carbon-14 into living cells. Carbon-14 is a naturally occurring radioactive isotope of the carbon that makes up every cell in our bodies. It is generated, via irradiation, in the upper atmosphere by the bombardment of nitrogen with neutrons from cosmic rays. In the form of carbon dioxide, it is incorporated into plant cells which pass it along to other organisms. As long as an organism is alive, it accumulates carbon-14. But when it dies, the amount of carbon-14 present is fixed and thereafter, it decreases. By detecting both carbon-14 and its decay product, nitrogen-14, scientists are able to determine how long ago something was alive. In the case of an ancient artifact made of, for example, leather, this provides a date for its death. Assuming that the leather object was made shortly thereafter, carbon-14 dating provides a good method for determining its age. This technique is one of the most important uses of nuclear chemistry for archeologists.

This is the complete article, containing 1,844 words (approx. 6 pages at 300 words per page).