Within the standard model, matter is still assumed to be composed of molecules, such as water (H2O) and carbon dioxide (CO2). Each molecule, in turn, contains atoms, the smallest identifiable physical unit of a chemical element, such as hydrogen (H), helium (He), carbon (C), or oxygen (O). Each atom consists of a tiny, but massive, positively charged nucleus surrounded by a cloud of negatively charged electrons. The electrons in this cloud arrange themselves according to the allowed set of energy states described by quantum mechanics. The nucleus consists of protons and neutrons. Within the nucleus, the resident nucleons form complex relationships and assume a variety of complicated subnuclear energy states, based on the proton and neutron populations and how these particles exchange quarks.
The dynamic activities that take place deep within the atomic nucleus are complicated. To fully understand what happens inside the nucleus as neutrons and protons interact requires sophisticated treatment within the field of high-energy nuclear physics—an exciting discipline that lies, for the most part, beyond the scope of this book. However, by assuming that the atomic nucleus is a collection of neutrons and protons, we can adequately discuss the physical principles behind the vast majority of interesting nuclear technology applications. For example, our discussions of radioactivity, nuclear fission, and nuclear fusion are based on a model of the atom consisting of just three particles: orbiting electrons, and neutrons and protons in the nucleus. Furthermore, we will treat the collective behavior of neutrons and protons within the nucleus without paying attention to what is actually going on inside an individual proton or neutron.
The proton (p) carries a unit positive charge (namely, +1.60 × 10−19 coulomb) and has a mass of approximately 1.6726 × 10−27 kg. The neutron carries no electrical charge and has a mass of 1.6749 × 10−27 kg—slightly larger than that of the proton. The neutron has many interesting properties, one of which is the fact that once outside the atomic nucleus, a free neutron no longer remains a stable particle but, rather, transforms into a proton, a negative electron, and an antineutrino in a radioactive decay process that takes, on average, about 10.2 minutes. The neutrino and its mirror matter antiparticle, the antineutrino, are electrically neutral elementary particles with no (or extremely little) mass. The tendency for free neutrons to decay does not influence the operation of nuclear reactors or explosive nuclear weapons, because these nuclear devices employ fission chain reactions with characteristic neutron generation times ranging from a few thousandths of a second in typical thermal neutron spectrum modern reactors to less than a millionth of a second in nuclear fission weapons.
Scientists call the number of protons in the nucleus the atomic number and give this important quantity a special symbol, Z. The atomic number is different for each chemical element. For example, the element carbon has the atomic number Z = 6, and the element uranium has the atomic number Z = 92. For carbon, this means that each atom has six protons in its nucleus. For any atom to maintain its electric neutrality, the number of protons in the nucleus must equal the number of electrons in orbit around the nucleus. As shown in Figure 4.4, an electrically neutral carbon atom has six orbiting electrons. Scientists often refer to the carbon atom illustrated in Figure 4.4 as carbon-12, or C-12. This simplified designation is nuclear science shorthand that implies two very important concepts: the presence of a number of neutrons in the nucleus, and the existence of isotopes.
Characteristics of the Cuban Revolution
