Название: Earth Materials
Автор: John O'Brien
Издательство: John Wiley & Sons Limited
Жанр: География
isbn: 9781119512219
isbn:
Protons (p+) and neutrons (n0) each have a mass of ~1 atomic mass unit (amu) and are clustered together in a small, positively charged, central region of the atom called the nucleus (Figure 2.1). Protons possess a positive electric charge and neutrons are electrically neutral. The nucleus is surrounded by a vastly larger, mostly “empty”, region called the electron cloud. The electron cloud represents the area in which the electrons (e−) in the atom move about the nucleus in patterns called orbitals (Figure 2.1). Electrons have a negative electric charge and an almost negligible mass of 0.000 005 amu. Knowledge of these three fundamental particles in atoms is essential to understanding how minerals and other materials form, how they can be used as resources and how we can deal with their hazardous effects that can be seen occasionally.
Table 2.1 Major properties of electrons, protons, and neutrons.
Particle type | Electric charge | Atomic mass (amu)a |
---|---|---|
Proton (p+) | +1 | 1.00 728 |
Neutron (n0) | 0 | 1.00 867 |
Electron (e−) | −1 | 0.0 000 054 |
a amu = atomic mass unit = 1/12 mass of an average carbon atom.
Figure 2.1 Simplified model atom with nucleus that contains positively charged protons (dark blue) and electrically neutral neutrons (light blue) surrounded by an electron cloud (red shades) in which negatively charged electrons move in orbitals about the nucleus.
2.1.1 The nucleus, atomic number, atomic mass number, and isotopes
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).
2.1.2 The electron cloud
Electrons СКАЧАТЬ