>> Key Terms (indicated in blue within the text below):

decay alpha particle antimatter decay belt of stability Bequerel beta particle binding energy chain reaction critical mass Curie

dose electron electron capture fission fusion gamma ray half-life ionizing radiation mass defect neutron neutron capture

nucleon penetrating radiation positron positron emission proton rad radioactive decay radioactivity relative biological effectiveness rem roentgen

Nuclear chemistry differs from traditional chemistry because elements are changed from one type into another. To describe the details of this process, isotopic notation is used. In isotopic notation (seen in Chapter 1), the mass number (A) is written as a superscript before the element symbol (E). Writing nuclear reactions requires that the atomic number, Z, and the mass number, A, add up to the same value on each side. Therefore it is convenient in nuclear chemistry to include the atomic number (Z) as a subscript directly under the mass number. In Chapter 1, Z stood for the number of protons. In nuclear chemistry it is more general and stands for the charge. (Note: Since protons have a +1 charge, this generally does not change anything.) The mass number, A, was described in Chapter 1 as the sum of protons and neutrons. It is used more generally here as the approximate mass in atomic mass units. The mass of a proton or a neutron is about 1 amu. Each particle of an atom can be written in isotopic notation as follows:

proton = 11p A proton is the same as a hydrogen nucleus = 11H

neutron = 10n

electron = 0–1e

                              >> Explore: Balancing Nuclear Reactions Tutorial

Protons and neutrons, the particles in the nucleus, are called nucleons. Other particles are commonly used in nuclear chemistry. These include

alpha = 42 An alpha particle is the same as a helium nucleus = 42He

beta = 0–1 A beta particle is the same as an electron = 0–1e

gamma = 00 A gamma ray is a very energetic photon.

Particles with the same mass as the particles typically found in an atom but opposite in charge are called antimatter. An example of antimatter is a positron, 01, which can be described as a positive electron. When antimatter collides with its opposite matter, both particles are annihilated, and energy, often in the form of gamma rays, is produced.

The force that holds the protons and neutrons together as a nucleus can be calculated as binding energy. The sum of the masses of the protons and neutrons in a nucleus is always more than the actual mass of the nucleus. This difference, called the mass defect, has been converted to binding energy. The relationship between mass and energy was described by Einstein as E = mc². By using the mass defect as m in this equation, the binding energy can be calculated. A stronger binding energy corresponds to a stronger force holding the nucleons together. However, if there are more nucleons, there must be more energy. Consequently, the best measure of how effectively the nucleons are held together is the ratio of binding energy to nucleons. Atoms with a higher binding energy per nucleon are more stable.

Nuclear reactions can occur by fusion, where nuclear particles and nuclei combine to form larger nuclei, or by fission, where nuclei fall apart to smaller nuclei and particles. For fusion to occur, the particles must be accelerated to high velocities to overcome the repulsion between the particles.

                              >> Explore: Fusion of Hydrogen Tutorial

Large nuclei are created by particles accelerated by the extreme heat of stars and supernovas. On earth, devices such as linear accelerators or cyclotrons are often used to accelerate particles. Many synthetic (not naturally occurring) isotopes are created this way. If a neutron is the bombarding particle and it is absorbed by the nucleus, this process is called neutron capture.

                              >> Explore: Synthesis of Elements Tutorial

Bombarding nuclei with smaller particles can also induce fission reactions. If the fission reaction creates more of the bombarding particles and sufficient energy to accelerate them, more fission reactions will occur. This is called a chain reaction. For a chain reaction to occur, not only must the bombarding particles be created, but accessible targets must exist for the particles to hit. Therefore a chain reaction requires not only the appropriate type of reaction, but also a critical mass of target nuclei.

Many nuclear reactions occur without bombarding particles. These reactions, where a nucleus falls apart into a different nucleus and a particle, are called decays. The type of particle produced from a nuclear decay will depend on the type of isotope decaying. There is a ratio of neutrons to protons that is stable (will not decay). This ratio is called the belt of stability (Figure 2.9 in the textbook). For small (low Z) isotopes, the stable ratio is about 1. As atomic number increases, more neutrons are required for stability.

Isotopes that are neutron-rich will undergo decay. The isotope will produce an element with the same mass number but an atomic number one higher and a particle. Isotopes that are neutron-poor will undergo either positron emission or electron capture. A positron is the antiparticle of an electron. It has no significant mass and a positive charge. In electron capture an electron outside the nucleus is captured by the nucleus. In both cases the atomic number increases and the mass number remains the same. The belt of stability can be used to determine whether isotopes are neutron-rich or neutron-poor. Large isotopes (Z > 83) undergo decay to emit an particle. In this type of decay, both the mass number and the atomic number decrease.

                              >> Explore: Modes of Radioactive Decay Tutorial

The emission of , , or particles due to nuclear decay is called radioactivity. The decay is easily measured with photographic film or a Geiger counter. The unit for the rate of decay is called a Bequerel (Bq) = 1 decay/second. Another common unit for the rate of decay is a Curie (Ci), where 1 Ci = 3.70 x 10¹⁰ Bq. Measurement of nuclear decay is very useful for the following reasons:

1. When , , or particles of sufficient energy collide with atoms, they can tear apart the atom, and the particles can be a health hazard. Such collisions are called ionizing radiation.
2. It is not difficult to detect even one , , or particle; it is a very sensitive method of detection.
3. These decays occur at very specific rates. Therefore isotopic ratios can be used to date objects.

The effect of ionizing radiation depends on the dose, the quantity of ionizing radiation absorbed by the body; the relative biological effect of the radiation; and the penetrating ability of the radiation. The dose is measured in rads. One rad is 0.01 J radiation/kg tissue. In most cases a rad is the same as a roentgen (R) = 2 x 10⁹ units of electrical charge/cm³ dry air. However, the actual effect the radiation has on the tissue depends on its relative biological effectiveness (RBE). The unit that takes relative biological effectiveness into account is the rem = radRBE. For and radiation, RBE = 1. For radiation, RBE = 20. However, radiation is not necessarily damaging because it is not penetrating. That is, it can be easily blocked. radiation is more difficult to block, and radiation requires lead shielding to block. Table 2.4 lists health effects of various radiation exposure. Since radiation has a larger effect on growing cells, it will have a larger effect on cancer cells than on normal cells. Consequently, it can be used in cancer treatment.

Since radiation is very easy to detect, it is possible to use the emitted radiation as a tracer. The element will have the same chemical properties regardless of whether it is radioactive. Therefore it is possible to follow an element throughout the body (or any chemical process) by following the emission of radiation. Only a small (and therefore safe) quantity of the radioactive material is required.

Radioactive decay occurs at a very-well-defined rate. This rate is normally expressed as a half-life, which is the time required for half the material to decay. This is used in radiocarbon dating of organic materials. The assumption is that the radioactive carbon-14 (¹⁴C) was only incorporated into the material when it was alive. As the carbon-14 undergoes decay to nitrogen-14, the ratio of carbon-14 to carbon-12 decreases. The amount of the decrease will depend on time. This time can be calculated from the equation

t =

t1/2 0.693

ln

A0 At

(Equation 2.28)

where t is the time, t_1/2 is the half-life, A₀/A_t is the ratio of the initial amount of carbon-14 to the current amount of carbon-14. The half-life of carbon-14 is 5730 years.

                              >> Explore: Half-Life Tutorial