Essentials of Nuclear Medicine Physics, Instrumentation, and Radiation Biology. Rachel A. Powsner

has several fundamental properties. For our purposes the most important are mass and charge (electric). We recognize mass by the force gravity exerts on a material object (commonly referred to as its weight) and by the object’s inertia, which is the “resistance” we encounter when we attempt to change the position or motion of a material object.

      Similarly, we can, at least at times, recognize charge by the direct effect it can have on us, or that we can observe it to have on inanimate objects. For example, we may feel the presence of a strongly charged object when it causes our hair to move or even to stand on end. More often than not, however, we are insensitive to charge. But whether grossly detectable or not, its effects must be considered here because of the role charge plays in the structure of matter.

Charge is generally thought to have been recognized first by the ancient Greeks. They noticed that some kinds of matter, an amber rod for example, can be given an electric charge by rubbing it with a piece of cloth. Their experiments convinced them that there are two kinds of charge: opposite charges, which attract each other, and like charges, which repel. One kind of charge came to be called positive, the other negative. We now know that the negative charge is associated with electrons. The rubbing transferred some of the electrons from the atoms of the matter in the rod to the cloth. In a similar fashion, electrons can be transferred from a cat’s fur to a hand. After petting, the cat will have a net positive charge and the person a net negative charge (Figure 1.1). With these basic properties in mind, we can look at matter in more detail.

Matter is composed of molecules. In any chemically pure material, the molecules are the smallest units that retain the characteristics of the material itself. For example, if a block of salt were to be broken into successively smaller pieces, the smallest fragment with the properties of salt would be a single salt molecule (Figure 1.2). With further fragmentation, the molecule would no longer be salt. Molecules, in turn, are composed of atoms. Most molecules consist of more than one kind of atom—salt, for example, is made up of atoms of chlorine and atoms of sodium. The atoms themselves are composed of smaller particles, the subatomic particles, which are discussed later.

      The molecule is held together by the chemical bonds among its atoms. These bonds are formed by the force of electrical attraction between oppositely charged parts of the molecule. This force is often referred to as the Coulomb force after Charles A. de Coulomb, the physicist who characterized it. This is the force involved in chemical reactions such as the combining of hydrogen and oxygen to form water. The electrons of the atom are held by the electrical force between them and the positive nucleus. The nucleus of the atom is held together by another type of force—nuclear force—which is involved in the release of atomic energy. Nuclear forces are magnitudes greater than electrical forces.

      Elements

There are more than 100 species of atoms. These species are referred to as elements. Most of the known elements—for example, mercury, helium, gold, hydrogen, and oxygen—occur naturally on earth; others are not usually found in nature but are made by humans—for example, europium and americium. A reasonable explanation for the absence of some elements from nature is that if and when they were formed they proved too unstable to survive in detectable amounts into the present.

Figure 1.1 Electrostatic charge.

Figure 1.2 The NaCl molecule is the smallest unit of salt that retains the characteristics of salt.

All the elements have been assigned symbols or abbreviated chemical names: gold, Au, mercury, Hg; helium, He. Some symbols are obvious abbreviations of the English name; others are derived from the original Latin name of the element, for example, Au is from aurum, the Latin word for gold.

All of the known elements, both natural and those made by humans, are organized in the periodic table. In Figure 1.3, the elements that have a stable state are shown in white boxes; those that occur only in a radioactive form are shown in gray boxes. The number appearing above each element’s abbreviation is referred to as the atomic number, which will be discussed later in this chapter.

Figure 1.3 Periodic table.

      The elements in the periodic table are arranged in columns (called groups) and rows (called periods). In general, elements within groups demonstrate similar properties. This is because elements in a group often have similar numbers of electrons in their outer shell; outer shell electron configurations are more important in determining how an atom interacts with other elemental atoms. The lanthanides and actinides are special groups of elements, conventionally shown in rows, separated and placed below the table. These two groups have the same number of outer‐shell electrons and share many common properties.

      Atomic structure

Atoms initially were thought of as no more than small pieces of matter. Our understanding that they have an inner structure has its roots in the observations of earlier physicists that the atoms of which matter is composed contain electrons of negative charge. In as much as the atom as a whole is electrically neutral, it seemed obvious that it must also contain something with a positive charge to balance the negative charge of the electrons. Thus, early attempts to picture the atom, modeled on our solar system, showed the negatively charged electrons orbiting a central group of particles, the positively charged nucleus (Figure 1.4).

Figure 1.4 Flat atom. The standard two‐dimensional drawing of atomic structure.

       Electrons

      In our simple solar‐system model of the atom, the electrons are viewed as orbiting the nucleus at high speeds. They have a negative charge and the nucleus has a positive charge. The electrical charges of the atom are “balanced,” that is, the total negative charge of the electrons equals the positive charge of the nucleus. As we shall see in a moment, this is simply another way to point out that the number of orbital electrons equals the number of nuclear protons.

       Electron shells and binding energy:

By adding a third dimension to our model of the atom, we can depict the electron orbits as the surfaces of spheres (called shells) to suggest that, unlike the planets orbiting the sun, electrons are not confined to a circular orbit lying in a single plane but may be more widely distributed (Figure 1.5). Although it is convenient for us to talk about distances and diameters of the shells, distance on the atomic scale does not have quite the same meaning it does with