第21章核化学

Nuclear chemistry is the study of reactions that involve changes in nuclear structure. The chapter on atoms, molecules, and ions introduced the basic idea of nuclear structure, that the nucleus of an atom is composed of protons and, with the exception of ${}_1^1\text{H},$ neutrons. Recall that the number of protons in the nucleus is called the atomic number (Z) of the element, and the sum of the number of protons and the number of neutrons is the mass number (A). Atoms with the same atomic number but different mass numbers are isotopes of the same element. When referring to a single type of nucleus, we often use the term nuclide and identify it by the notation ${}_{\text{Z}}^{\text{A}}\text{X},$ where X is the symbol for the element, A is the mass number, and Z is the atomic number (for example, ${}_6^{14}\text{C}).$ Often a nuclide is referenced by the name of the element followed by a hyphen and the mass number. For example, ${}_6^{14}\text{C}$ is called “carbon-14.”

Protons and neutrons, collectively called nucleons, are packed together tightly in a nucleus. With a radius of about 10⁻¹⁵ meters, a nucleus is quite small compared to the radius of the entire atom, which is about 10⁻¹⁰ meters. Nuclei are extremely dense compared to bulk matter, averaging 1.8 $\times$ 10¹⁴ grams per cubic centimeter. For example, water has a density of 1 gram per cubic centimeter, and iridium, one of the densest elements known, has a density of 22.6 g/cm³. If the earth’s density were equal to the average nuclear density, the earth’s radius would be only about 200 meters (earth’s actual radius is approximately 6.4 $\times$ 10⁶ meters, 30,000 times larger). Example 21.1 demonstrates just how great nuclear densities can be in the natural world.

To hold positively charged protons together in the very small volume of a nucleus requires very strong attractive forces because the positively charged protons repel one another strongly at such short distances. The force of attraction that holds the nucleus together is the strong nuclear force. (The strong force is one of the four fundamental forces that are known to exist. The others are the electromagnetic force, the gravitational force, and the nuclear weak force.) This force acts between protons, between neutrons, and between protons and neutrons. It is very different from the electrostatic force that holds negatively charged electrons around a positively charged nucleus (the attraction between opposite charges). Over distances less than 10⁻¹⁵ meters and within the nucleus, the strong nuclear force is much stronger than electrostatic repulsions between protons; over larger distances and outside the nucleus, it is essentially nonexistent.