Working through this chapter of the study guide will enable you to:

1. Explain the basic structure of an atom and of its nucleus.
2. Show why certain nuclei are stable, whereas others are radioactive.
3. Describe the three most common modes of radioactive decay and understand the concept of radioactive half-life.
4. Write equations for nuclear reactions and calculate the energy released in these reactions.
5. Distinguish between the processes of nuclear fission and nuclear fusion and discuss the advantages, disadvantages, and dangers of each as commercial power sources.

Discussion

The fundamental structure of matter has been studied by scientists for centuries. Several early Greeks thought that an atom was the smallest part of any substance and, as the basic part of all substances, was not able to be subdivided any further. This has proved not to be the case. Chapter 10 deals with the structure of the central portion of an atom, that area containing all of the positive electrical charge and most of the mass of the atom. This region, known as the nucleus, is quite well understood despite its extremely small size.

Today, we realize that the nucleus of an atom can change structure on its own in processes related to nuclear radioactivity and can also be restructured by certain processes controlled by scientists. These changes cause atoms to convert from one naturally occurring element into another and sometimes even into new elements not found in nature at all. It has also been found that nuclear reactions can release large amounts of energy and that this energy can be used for either peaceful or hostile purposes. It is, therefore, extremely important that all responsible citizens understand the basic concepts relating to nuclear reactions, both those that occur naturally and those that are the result of experimentation and manipulation. Reactions involving the atomic nucleus promise to be of great importance in deciding the future of our very existence on this planet.


Section  10.1Symbols of the Elements

Man has always been interested in the structure of the world around him. The early Greek scholars tried to explain the structure of the world around them using four basic classifications known as "elements." These were earth, air, fire, and water. We now know that this was much too simplistic a system, and did not describe the world well. Our current definition of the term element is based on the structure of the nucleus of the atom, where the number of protons in that nucleus determines the type of element. Since the early 1800s, the various elements have been named using the first one or two letters of the Latin, or more recently the English, names of these elements. For example, S is the symbol used for the element sulfur and Fe is the symbol used for iron, the latter being taken from the Latin name for this element, ferrum.


Section  10.2The Atomic Nucleus

The discovery of the electron and the fact that most of the mass of an atom is concentrated in a very small volume at its center has led us to the modern concept of an atom as being composed of a heavy, positive nucleus surrounded by orbiting electrons. The nucleus contains neutral particles known as neutrons and positively charged particles known as protons. Because both the positive protons and the uncharged neutrons are present in the nucleus of an atom, they are collectively called nucleons. Because matter as a whole carries no net charge and the charge on the electron and proton are identical in magnitude but opposite in sign, the number of negative electrons in a neutral atom must exactly equal the number of positive protons. The number of protons in the nucleus uniquely determines what element that atom represents. The number of neutrons plus the number of protons gives the atom a specific mass number (A) that specifies the isotope of the particular element. Adding or taking away electrons does not change the identity of the element, but it does make the atom into a charged entity called an ion.

The notation used to describe the nuclei in the atoms of a particular element requires the use of at least two parameters. One parameter represents the number of protons in the nucleus. This is uniquely indicated by the chemical symbol for that element, or it can be specifically written as the atomic number (Z).

The second required parameter indicates the number of neutrons in the nucleus. This can be shown by using the neutron number (N), but it is more common to specify the mass number (A), which indicates the total number of nucleons in the atom's nucleus. Either A or N must be specified or we will not know which particular isotope of the element the designation signifies.

When the atomic number and the mass number are known, the neutron number can be found by simple subtraction (N = A - Z). Remember that the proton number is the same for all atoms of a particular element and that this number is an indication of the position of that element in the periodic table. In other words, the atomic number of the element is equal to its proton number. Once you know the atomic number, it is quite simple to locate that element when you are consulting a periodic table.

This section  in the textbook shows how the mass number and the atomic number are arranged around the chemical symbol in what has become a standardized scientific notation. Learn this notation now because it is used extensively throughout this chapter. Even when all of these numbers are not given, the position of any number that is specified with respect to the chemical symbol tells what that number represents.

At first glance, the nucleus of an atom might appear to be unstable. Packing positively charged protons into the small volume occupied by the nucleus produces large electrostatic repulsive forces that would tear the nucleus apart if it were not for an extremely powerful attractive force that exists between all nucleons—protons and neutrons alike. Even so, the size and configuration of stable nuclei have definite limits. Only the first 83 elements in the periodic table have stable isotopes (i.e., stable nuclear configurations), and each of these elements occurs as only a limited number of stable isotopes (usually 2 or 3). All other combinations of protons and neutrons form an unstable nucleus. A naturally unstable nucleus is said to be radioactive. Any radionuclide will decay spontaneously and give off energy, as discussed in the next section.


Section  10.3Radioactivity

A radioactive nucleus can decay into a more energetically stable form in several ways. The most important of these are the alpha, beta, and gamma decay processes. In alpha decay the nucleus releases an alpha particle consisting of two protons and two neutrons. An alpha particle is itself very stable and is the same as the nucleus of the most common isotope of helium, ⁴₂He. Beta decay involves the emission of an electron from the nucleus, and gamma decay occurs when a high-energy photon of electromagnetic radiation called a gamma ray is ejected from the nucleus.

The decay of radioactive nuclei is a precise activity that has specific time intervals related to the decay rate of any particular radionuclide. The decay rate is indicated by the radioactive half-life of the radionuclide. The half-life is the time interval in which one-half of a sample of the radionuclide will decay into a new form. Each radionuclide has its own unique half-life. This decay rate leads to an exponential decay pattern for each radioactive material because only one-half of the remaining radioactive sample will decay in each subsequent half-life. An example of such a decay rate is shown as a graph in Fig. 10.10 of the textbook.


Section  10.4Nuclear Reactions

Some nuclear transformations do not occur spontaneously, as do those involving radioactive isotopes. These processes are initiated by the bombardment of the nucleus of a normally stable atom by an energetic particle or photon. Such processes are called artificial transmutations and are generally accomplished only by the use of large nuclear reactors or devices known as particle accelerators. Several types of nuclear reactions are discussed in the textbook. Note that the equations for these reactions must be balanced so that the same total mass number and total atomic number appear on each side of the reaction arrow.


Section  10.5Nuclear Fission

Two nuclear reactions are of particular interest to us because when they occur, they liberate large amounts of energy. One of these processes, fission, is currently used as a commercial power source for generating electricity in many countries, including the United States. Some people believe that serious environmental and safety problems inherent in the fission process make it an unattractive alternative to other energy sources such as fossil fuels. Unfortunately, many of our other energy sources are being depleted rapidly, so it appears that our dependence on fission as a commercial power source is quite likely to continue for many years to come.

Nuclear fission occurs in a process known as a chain reaction in which a single incident neutron can initiate a reaction in which a heavy nucleus splits apart into lighter reaction fragments and 2 or more additional neutrons are released along with a considerable amount of energy. These newly created neutrons can then go on to produce additional reactions in which even more neutrons are released. The subsequent fissions of other nuclei can lead to either a rapidly accelerating reaction resulting in an atomic explosion or, in properly designed nuclear reactors where the reaction rate can be controlled, to produce a steady release of nuclear energy.


Section  10.6Nuclear Fusion

The other energy-producing nuclear reaction, fusion, does not appear to have as many environmental drawbacks; however, as yet the fusion reaction cannot be controlled adequately. Energy produced so far on Earth by this process has been primarily in the form of the hydrogen bomb, which does not allow a continuous and controlled release of energy. Recent advances in magnetic and inertial confinement techniques have brought us very close to a continuous, self-sustaining fusion reaction, but it will be many years before such a process is available as a commercial power source. Fusion is the energy process that powers our Sun, and thus fusion does supply a large portion of all direct and indirect energy used on Earth today, but one dream of mankind still remains the controlled operation of fusion reactors here on Earth itself.

Most scientists believe that both fission and fusion will play important parts in the future development of our society. There is no question that decisions affecting the use of nuclear reactors, as commercial power sources to satisfy our ever-increasing electricity needs, will be crucial to our continued existence. Information contained in this chapter may be extremely important in helping you to evaluate the policies that will have to be implemented relative to our use of these energy sources in future years.


Section  10.7Biological Effects of Radiation

Modern medicine uses radiation for many purposes, such as both for a diagnostic tool and for the actual treatment of cancer and other forms of disease. There are also ways in which mankind can become exposed to large doses of natural and man-made radiation. It is, therefore, important to understand the effects of radiation on the human body and the limits to which exposure to radiation can be tolerated. This is becoming even more critical since we now have the capability of sending astronauts above the protective layer of atmosphere that surrounds our Earth and into regions where solar radiation is much stronger and more potentially dangerous than it is by the time it reaches Earth's surface. The expansion of nuclear power generating plants also increases the possibility of radioactive accidents and other radiation related injuries. This Section gives a general explanation of both the somatic and the genetic effects that ionizing radiation can have on living cells.

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