1.8 Nuclear Chemistry

Most of the time in chemistry, we concern ourselves with how atoms interact with each other. However, under some circumstances the actual identity of an atom changes because of changes in the nucleus. This is nuclear chemistryChanges in an atom's identity due to changes in its nucleus.. Here we will consider a short introduction to nuclear chemistry introduction to nuclear chemistry, the impacts of which may be more common than you realize (refer to Figure 1.12).

In Chapter 1, Section 4 “Nuclei of Atoms”, we learned that we can indicate a specific isotope of an element with its element symbol and two numbers indicating its atomic number (number of protons) and mass number (number of protons + number of neutrons):

Sometimes the atomic number is omitted: ⁵⁶Fe. Up to now we have assumed that any isotope written is stable, but in reality this is not the case.  Some atoms have such large nuclei, or have such an unusual mix of protons and neutrons, that the nucleus is unstable. Such nuclei can spontaneously decompose by giving off small pieces of itself.  In doing so, it changes to an atom of a different element. For example, the nucleus in ²³⁸U is unstable, spontaneously giving off a particle consisting of two protons and two neutrons (which is identical to a helium atom nucleus, an unusually-stable nucleus). What is leftover has a mass number of 234 and has 90 protons: an atom of the element thorium:

This reaction is a nuclear reaction, and it demonstrates a requirement of the law of conservation of matter: the total number of nuclear particles (protons and neutrons) must be the same on both sides of the reaction. This is verified by showing that the sum of the superscripts (the mass numbers) are the same on both sides, and the sum of the subscripts (the number of protons) is the same on both sides. Isotopes whose nuclei can spontaneously decompose by giving off smaller particles are called radioactiveA term that identifies an isotope whose nucleus spontaneously decomposes by giving off smaller particles..  Whether or not an isotope is radioactive is characteristic of the isotope: ²³⁸U is radioactive, but ⁵⁶Fe is not.

Scientists now recognize that there are several types of radioactivity. The primary ones are:

When radioactive isotopes decay, energy is also given off: this is the source of nuclear energyEnergy given off by a nucleus during radioactive decay..  While each atom gives off only a tiny amount of energy, for macroscopic quantities of isotopes the total energy can be hundreds, thousands, even millions of times more energy than chemical reactions (energy changes in chemical reactions will be presented in Chapter 6 “Energy and Chemical Processes”). This energy can be rather destructive (in the case of nuclear weapons), or if controlled can be a useful source of energy for societal use (refer to Figure 1.13).  

Some radioactive isotopes decay quickly, while others take a long time to decay. The half-life The amount of time it takes for one-half of an initial amount of radioactive isotope to decay. of a radioactive isotope, t_1/2, is the amount of time it takes for a given amount of the isotope to decay to half of its initial value. It has a characteristic value for any given radioactive isotope, and can range from nanoseconds to billions of years. Table 1.5 lists the half-lives of various radioactive isotopes.

Half-lives of spontaneous radioactive isotopes are not affected by external environmental factors and so are constant. This makes them useful as tags in a variety of chemical and physical processes. Examples include tracing underground water, using radioactive isotopes in compounds to study chemical reactions (for example, the reactions that occur during photosynthesis were studied using ¹⁴C-labeled compounds), in dating archeological and geological artifacts, and in certain medical techniques that use radioactive isotopes for diagnosis or treatment.

Exposure to radiation is measured in a variety of ways. The most fundamental unit of radioactivity is the decay of one radioactive atom per second, a unit defined as a becquerelA unit of radioactivity defined as the decay of one atom per second. (named after Henri Becquerel, who discovered radioactivity; abbreviation Bq). Because this is such a tiny unit, a more useful unit is the curieA unit of radioactivity defined as 3.7 x 10¹⁰ decays per second. (named after Pierre Curie and not, as many think, after his wife Marie; its abbreviation is Ci), which is 3.7 × 10¹⁰ Bq; it was so defined because it is the radioactivity of 1.0 g of pure radium, an element discovered by the Curies.

Other units of radiation are based on the amount of energy emitted or absorbed, or the impact that energy has on bodily tissues. The radAn amount of radioactivity exposure that is equivalent to 0.01 joules per kilogram of body tissue. is defined as the absorption of 0.01 joules of energy (a very small amount of energy) per kilogram of body tissue. This is not a lot of energy:  it would raise the temperature of a cup of coffee by about 2 millionths of a degree! However, even that small amount of energy can disrupt a lot of molecules in a human body: a radiation dose of about 400 rad would have a mortality rate of 50%. Another unit of energy absorption is the grayA unit of radiation energy exposure equal to 100 rad. (symbol Gy), which is equal to 100 rad. 

The unit remA unit of radiation exposure that is a combination of the rad value times a factor dependent on the type of body tissue exposed. is an acronym for “radiation equivalent, man” and is the dosage in rad times a numerical factor that takes into account the type of radiation and the body tissue exposed: beta decay on muscle tissue has a factor of 1, while eye tissue exposed to alpha particles has a factor of about 30. A dental x-ray gives an exposure of about 1 millrem (mrem), while the average environmental exposure over an entire year is about 600 mrem (0.6 rem). Another unit of the biological impact of radioactivity is the sievert (SV)A unit of radiation exposure equal to 100 rem but indicative of the impact on biological tissue., which is also equal to 100 rem but is an indication of the impact on biological tissue, so like rem is dependent on the type of tissue being exposed. Both gray and sievert are SI units, but rad and rem are still in common use.

Some people think that any exposure to radioactivity is harmful, but in truth, it is all around us. Fully 80% of the annual exposure to radioactivity is natural and inescapable: radon in the soil, naturally-occurring radioactive isotopes, x-rays at the hospital or in the airport, and pollutants in the atmosphere are all sources of tiny amounts of radioactivity that we cannot escape. And due to the naturally-occurring radioactive isotopes of carbon, potassium, rubidium, and other elements, an average human body has a radioactivity of about 9,000 Bq!