Читать книгу Earth Materials - John O'Brien - Страница 39

The nucleus of atoms is composed of positively charged protons and uncharged neutrons bound together by a strong force. Ninety‐two fundamentally different kinds of atoms called elements have been discovered in the natural world. More than 25 additional elements have been created synthetically in laboratory experiments during the past century. Each element is characterized by the number of protons in its nucleus. The number of protons in the nucleus is called the atomic number (Z). The atomic number is typically represented by a subscript number to the lower left of the element symbol. The 92 naturally occurring elements range from hydrogen (Z = 1) through uranium (Z = 92). Hydrogen (₁H) is characterized by having one proton in its nucleus. Every atom of uranium (₉₂U) contains 92 protons in its nucleus. The atomic number is the unique property that distinguishes the atoms of each element from atoms of all other elements.

Every atom also possesses mass that largely results from the protons and neutrons in its nucleus. The mass of a particular atom is called its atomic mass number, and is expressed in atomic mass units (amu). As the mass of both protons and neutrons is ~1 amu, the atomic mass number is closely related to the total number of protons plus neutrons in its nucleus. The simple formula for atomic mass number is: atomic mass number = number of protons plus number of neutrons (#p⁺ + #n⁰). The atomic mass number is indicated by a superscript number to the upper left of the element symbol. For example, most oxygen atoms have eight protons and eight neutrons so their atomic mass number is written¹⁶O.

Although each element has a unique atomic number, many elements are characterized by atoms with different atomic mass numbers. Atoms of the same element that possess different atomic mass numbers are called isotopes. For example, three different isotopes of hydrogen exist (Figure 2.2a). All hydrogen (₁H) atoms have an atomic number of 1. The common form of hydrogen atom, sometimes called protium, has one proton and no neutrons in the nucleus; therefore protium has an atomic mass number of 1, symbolized as¹H. A less common form of hydrogen called deuterium, used in some nuclear reactors, has an atomic mass number of 2, symbolized by²H. This implies that it contains one proton and one neutron in its nucleus (1p⁺ + 1n⁰). A rarer isotope of hydrogen called tritium has an atomic mass number of 3, symbolized by³H. The nucleus of tritium has one proton and two neutrons. Similarly oxygen occurs in three different isotopes:¹⁶O,¹⁷O, and¹⁸O. All oxygen atoms contain eight protons but neutron numbers vary between¹⁶O,¹⁷O, and¹⁸O, which contain eight, nine, and ten neutrons, respectively (Figure 2.2a). The average atomic mass for each element is the weighted average for all the isotopes of that element. This helps to explain why the listed atomic masses for each element do not always approximate the whole numbers produced when one adds the number of protons and neutrons in the nucleus of a particular isotope.

Figure 2.2 (a) Nuclear configurations of the three common isotopes of hydrogen. (b) Nuclear configurations of the three common isotopes of oxygen.

The general isotope symbol for the nucleus of an atom expresses its atomic number to the lower left of its symbol, the number of neutrons to the lower right and the atomic mass number (number of protons + number of neutrons) to the upper left. For example, the most common isotope of uranium has the symbolic nuclear configuration of 92 protons + 146 neutrons and an atomic mass number of 238:

Stable isotopes have stable nuclear configurations that tend to remain unchanged; they retain the same number of protons and neutrons over time. On the other hand, radioactive isotopes have unstable nuclear configurations (numbers of protons and neutrons) that spontaneously change over time via radioactive decay processes, until they achieve stable nuclear configurations and become stable isotopes of another element. Both types of isotopes are extremely useful in solving geological and environmental problems, as discussed in Chapter 3. Radioactive isotopes are used in many medical treatments, but also present serious environmental hazards (Chapter 19).