Nuclides

Nuclides are characterized by the mass number (A) and the atomic number (Z). To be regarded as distinct a nuclide must have energy content sufficient for a measurable lifetime, usually more than 10−10 second. The term nuclide is not synonymous with isotope, which is any member of a set of nuclides having the same atomic number but differing mass number.

Chlorine-37, the nucleus of which consists of 17 protons and 20 neutrons, is a different nuclide from sodium-23 (nucleus of 11 protons and 12 neutrons) or chlorine-35(nucleus of 17 protons and 18 neutrons). Nuclear isomers, which have the same number of protons and neutrons but differ in energy content and radioactivity, are also distinct nuclides.

Nuclides are commonly expressed in the form A/ZX, where A denotes the total number of protons and neutrons, Z represents the number of protons, and the difference between A and Z is the number of neutrons. Thus 37/17Cl signifies chlorine-37.

Nuclides are associated with radioactive decay and may be stable or unstable species. About 1,700 nuclides are known, of which about 300 are stable and the rest radioactive. More than 200 of the stable nuclides were discovered by the British physicist Francis William Aston using his new invention of the mass spectrograph.

The total number of protons in the nucleus of an atom is called the atomic number of the atom and is given the symbol Z. The number of electrons in an electrically-neutral atom is the same as the number of protons in the nucleus. The number of neutrons in a nucleus is known as the neutron number and is given the symbol N. The mass number of the nucleus is the total number of nucleons, that is, protons and neutrons in the nucleus. The mass number is given the symbol A and can be found by the equation Z + N = A.

Each of the chemical elements has a unique atomic number because the atoms of different elements contain a different number of protons. The atomic number of an atom identifies the particular element.

Each type of atom that contains a unique combination of protons and neutrons is called a nuclide. Not all combinations of numbers of protons and neutrons are possible, but about 2500 specific nuclides with unique combinations of neutrons and protons have been identified. Each nuclide is denoted by the chemical symbol of the element with the atomic number written as a subscript and the mass number written as a superscript, as shown in Figure 2. Because each element has a unique name, chemical symbol, and atomic number, only one of the three is necessary to identify the element. For this reason nuclides can also be identified by either the chemical name or the chemical symbol followed by the mass number (for example, U-235 or uranium-235). Another common format is to use abbreviation of the chemical element with the mass number superscripted (for example, ²³⁵U). In this handbook the format used in the text will usually be the element's name followed by the mass number. In equations and tables, the format in Figure 2 will usually be used.

Date: 2015-05-18; view: 372; Нарушение авторских прав