CHEMISTRY THE CENTRAL SCIENCE

21 NUCLEAR CHEMISTRY

CHAPTER SUMMARY AND KEY TERMS

INTRODUCTION AND SECTION 21.1 The nucleus of an atom contains protons and neutrons, both of which are called nucleons. Reactions that involve changes in atomic nuclei are called nuclear reactions. Nuclei that spontaneously change by emitting radiation are said to be radioactive. Radioactive nuclei are called radionuclides, and the atoms containing them are called radioisotopes. Radionuclides spontaneously change through a process called radioactive decay. The three most important types of radiation given off as a result of radioactive decay are alpha (α) particles beta (β) particles , and gamma (γ) radiation Positrons , which are particles with the same mass as an electron but the opposite charge, can also be produced when a radioisotope decays.

In nuclear equations, reactant and product nuclei are represented by giving their mass numbers and atomic numbers, as well as their chemical symbol. The totals of the mass numbers on both sides of the equation are equal; the totals of the atomic numbers on both sides are also equal. There are four common modes of radioactive decay: alpha decay, which reduces the atomic number by 2 and the mass number by 4, beta emission, which increases the atomic number by 1 and leaves the mass number unchanged, positron emission and electron capture, both of which reduce the atomic number by 1 and leave the mass number unchanged.

SECTION 21.2 The neutron-to-proton ratio is an important factor determining nuclear stability. By comparing a nuclide's neutron-to-proton ratio with those in the band of stability, we can predict the mode of radioactive decay. In general, neutron-rich nuclei tend to emit beta particles; proton-rich nuclei tend to either emit positrons or undergo electron capture; and heavy nuclei tend to emit alpha particles. The presence of magic numbers of nucleons and an even number of protons and neutrons also help determine the stability of a nucleus. A nuclide may undergo a series of decay steps before a stable nuclide forms. This series of steps is called a radioactive series or a nuclear disintegration series.

SECTION 21.3 Nuclear transmutations, induced conversions of one nucleus into another, can be brought about by bombarding nuclei with either charged particles or neutrons. Particle accelerators increase the kinetic energies of positively charged particles, allowing these particles to overcome their electrostatic repulsion by the nucleus. Nuclear transmutations are used to produce the transuranium elements, those elements with atomic numbers greater than that of uranium.

SECTIONS 21.4 AND 21.5 The SI unit for the activity of a radioactive source is the becquerel (Bq), defined as one nuclear disintegration per second. A related unit, the curie (Ci), corresponds to 3.7 × 10¹⁰ disintegrations per second. Nuclear decay is a first-order process. The decay rate (activity) is therefore proportional to the number of radioactive nuclei. The half-life of a radionuclide, which is a constant, is the time needed for one-half of the nuclei to decay. Some radioisotopes can be used to date objects; ¹⁴C, for example, is used to date organic objects. Geiger counters and scintillation counters count the emissions from radioactive samples. The ease of detection of radioisotopes also permits their use as radiotracers to follow elements through reactions.

SECTION 21.6 The energy produced in nuclear reactions is accompanied by measurable changes of mass in accordance with Einstein's relationship, ΔE = c² Δm. The difference in mass between nuclei and the nucleons of which they are composed is known as the mass defect. The mass defect of a nuclide makes it possible to calculate its nuclear binding energy, the energy required to separate the nucleus into individual nucleons. Energy is produced when heavy nuclei split (fission) and when light nuclei fuse (fusion).

SECTIONS 21.7 AND 21.8 Uranium-235, uranium-233, and plutonium-239 undergo fission when they capture a neutron, splitting into lighter nuclei and releasing more neutrons. The neutrons produced in one fission can cause further fission reactions, which can lead to a nuclear chain reaction. A reaction that maintains a constant rate is said to be critical, and the mass necessary to maintain this constant rate is called a critical mass. A mass in excess of the critical mass is termed a supercritical mass.

In nuclear reactors the fission rate is controlled to generate a constant power. The reactor core consists of fuel elements containing fissionable nuclei, control rods, a moderator, and a primary coolant. A nuclear power plant resembles a conventional power plant except that the reactor core replaces the fuel burner. There is concern about the disposal of highly radioactive nuclear wastes that are generated in nuclear power plants.

Nuclear fusion requires high temperatures because nuclei must have large kinetic energies to overcome their mutual repulsions. Fusion reactions are therefore called thermonuclear reactions. It is not yet possible to generate power on Earth through a controlled fusion process.

SECTION 21.9 Ionizing radiation is energetic enough to remove an electron from a water molecule; radiation with less energy is called nonionizing radiation. Ionizing radiation generates free radicals, reactive substances with one or more unpaired electrons. The effects of long-term exposure to low levels of radiation are not completely understood, but it is usually assumed that the extent of biological damage varies in direct proportion to the level of exposure.

The amount of energy deposited in biological tissue by radiation is called the radiation dose and is measured in units of gray or rad. One gray (Gy) corresponds to a dose of 1 J/kg of tissue. The rad is a smaller unit; 100 rad = 1 Gy. The effective dose, which measures the biological damage created by the deposited energy, is measured in units of rem or sievert (Sv). The rem is obtained by multiplying the number of rad by the relative biological effectiveness (RBE); 100 rem = 1 Sv.

KEY SKILLS

• Write balanced nuclear equations. (Section 21.1)

• Predict nuclear stability and expected type of nuclear decay from the neutron-to-proton ratio of an isotope. (Section 21.2)

• Write balanced nuclear equations for nuclear transmutations. (Section 21.3)

• Calculate ages of objects and/or the amount of a radionuclide remaining after a given period of time using the half-life of the radionuclide in question. (Section 21.4)

• Calculate mass and energy changes for nuclear reactions. (Section 21.6)

• Calculate the binding energies for nuclei. (Section 21.6)

• Describe the difference between fission and fusion. (Sections 21.7 and 21.8)

• Understand how a nuclear power plant operates and know the differences among various types of nuclear power plants. (Section 21.7)

• Understand the meaning of radiation dosage terms. (Section 21.9)

• Understand the biological effects of different kinds of radiation. (Section 21.9)

KEY EQUATIONS