Description
Atomic Structure
Matter is composed of atoms. An atom consists of a nucleus containing protons (Z) and neutrons (N), collectively called nucleons, and electrons rotating around the nucleus. The sum of neutrons and protons (total number of nucleons) is the mass number denoted by A. The properties of neutrons, protons, and electrons are listed in Table 1.1. The number of electrons in an atom is equal to the number of protons (atomic number Z) in the nucleus. The electrons rotate along different energy shells designated as K-shell, L-shell, M-shell, etc. (Fig. 1.1). Each shell further consists of subshells or orbitals, e.g., the K-shell has s orbital; the L-shell has s and p orbitals; the M-shell has s, p, and d orbitals, and the N-shell has s, p, d, and f orbitals. Each orbital can accommodate only a limited number of electrons. For example, the s orbital contains up to 2 electrons; the p orbital, 6 electrons; the d orbital, 10 electrons; and the f orbital, 14 electrons. The capacity number of electrons in each orbital adds up to give the maximum number of electrons that each energy shell can hold. Thus, the K-shell contains 2 electrons; the L-shell 8 electrons, the M-shell 18 electrons, and so forth.
A combination of a given number of protons and neutrons in a nucleus leads to an atom called the nuclide. A nuclide X is represented by XN. Some nuclides (280 or so) are stable, while others (more than 3400) are unstable. The unstable nuclides are termed the radionuclides, most of which are artificially produced in the cyclotron or reactor, with a few naturally occurring. The nuclides having the same number of protons are called the isotopes, e.g., 126C6 and 136C7; the nuclides having the same number of neutrons are called the isotones, e.g., 168O8 and 157N8; the nuclides having the same mass number are called the isobars, e.g., 131I and 131Xe; and the nuclides with the same mass number but differing in energy are called the isomers, e.g., 99mTc and 99Tc.