All matter is composed of a limited number of elements (118 so far, see Periodic Table, Table 1.1), which in turn are made of atoms. An atom is the smallest part of an element that retains all its chemical properties. In general, atoms are electrically neutral; that is, they do not show any electric charge. However, atoms are not indivisible, as once was thought, but are composed of three elementary particles: electrons, protons, and neutrons.

An electron is a tiny particle that possesses a negative charge of 1.6022 × 10^-19 coulomb (unit of charge) and a mass of 9.109 × 10^-31 kg. A proton is a particle with a positive charge equal in amount to that of an electron. A neutron does not have any electric charge and weighs slightly more than a proton. Protons and neutrons have masses of 1.6726 × 10^-27 and 1.6749 × 10^-27 kg, respectively; hence, they are about 2,000 times heavier than an electron.

An atom is generally neutral because it contains the same number of electrons and protons. The number of protons in an atom is also known as the atomic number Z. It specifies the position of that element in the periodic table (Table 1.1), and therefore its chemical identity. The electrons, protons, and neutrons in an atom are arranged in a planetary structure; that is, the protons and neutrons (the sun) are located at the center, and the electrons (planets) are revolving over the surface of spherical shells (or orbits) of different radii. The center in which the protons and neutrons are located is known as the nucleus and is similar to a packed sphere. The size of atoms of different elements varies greatly but is in the range of 1 to 2 × 10^-10 m. The nucleus is really small in comparison to the atom (about 10⁵ times smaller or 10^-15 m in size).

The attractive coulomb (electrical) force between the positively charged nucleus (on account of the protons) and the negatively charged electrons provides stability to the electrons revolving in the spherical shells. The first shell (having the smallest radius) is known as the K shell, the second shell as L, the third shell as M, and so on. There is a limit to the number of electrons that can occupy a given shell. The K shell can be occupied by a maximum of 2 electrons, the L shell by a maximum of 8 electrons, the M shell by a maximum of 18 electrons, and the N shell by a maximum of 32 electrons. However, the outermost shell in a given atom cannot be occupied by more than eight electrons. In a simple atom such as hydrogen, there is only one electron that under normal circumstances occupies the K shell. In a complex atom such as iodine, there are 53 electrons that are arranged

in the K, L, M, N, and O orbits in numbers of 2, 8, 18, 18, and 7, respectively. The arrangement of electrons in various shells for hydrogen and three other typical atoms is shown in Figure 1.1. This is a simplified description of the atomic structure, which, in reality, is more complex because each shell is further divided into subshells. For our purpose, however, it is more than sufficient.

Molecules are formed by the combination of two or more atoms (e.g., a molecule of water, H₂O, is formed by the combination of two hydrogen atoms and one oxygen atom). The combination of atoms is accomplished through the interaction of electrons (also known as valent electrons) in the outermost orbits of the atoms. Valent electrons participate in the formation of the molecules in several ways—for example, in ionic binding, covalent binding, and hydrogen binding. In theory, most chemical reactions and chemical properties of atoms or molecules can be explained on the basis of the interaction of the valent electrons.

Each electron in a given shell is bound to the nucleus with a fixed amount of energy. Therefore, if one wishes to remove an electron from a particular shell to make it free and no longer associated with that atom, energy will have to be provided to the electron from outside the atom. The minimum amount of energy necessary to free an electron from an atom is known as the binding energy of the electron in that atom. The unit in which energy is measured on the atomic scale is known as an electron volt (eV), which is the energy acquired by an electron accelerated through 1 V of potential difference. The electrons in the K shell are the most tightly bound electrons in an atom and therefore require the most energy to be removed from the atom. Electrons in the outermost shell, on the other hand, are the least tightly bound electrons and therefore require the least amount of energy for their removal from the atom. The binding energy of electrons in various shells increases rapidly with the atomic number Z. Table 1.2 lists the K- and L-shell average binding energies of electrons in the atoms of various elements.

Under normal conditions, electrons occupy the lowest possible shells (those closest to the nucleus) consistent with the maximum number of electrons by which a given shell can be occupied. However, electrons can be made to move into higher shells (unoccupied shells) temporarily by the absorption of energy. This absorption can take place in various ways—for example, by heating a substance, by subjecting matter to high electric fields, by passage of a charged particle through matter, or even by a high mechanical impact. When an electron absorbs sufficient energy for its removal from the atom, the process is called ionization and the remaining atom, an ion. When the electron absorbs amounts of energy that are just sufficient to move it into a higher unoccupied shell, the process is known as excitation and the atom as an excited atom. Excited atoms are, in general, unstable and acquire their normal configuration by emitting electromagnetic radiation (light, ultraviolet light, or x-rays), generally within 10^-9 seconds.

For example, let us consider a sodium atom, which has an atomic number of 11 and therefore 11 electrons and 11 protons. The electrons are arranged in K, L, and M shells in numbers of 2, 8, and 1, respectively. The energies of these electrons in the K, L, and M shells are approximately -1,072, -63, and -1 eV, respectively. To remove an electron from the K shell of a sodium atom, it is necessary to provide an amount of energy equal to 1072 eV, whereas from the M shell only 1 eV of energy is necessary. An electron from the L shell can move to the M shell by absorbing 62 eV of energy, thereby producing an excited atom of sodium. When this excited atom decays (i.e., when the electron jumps back into the L shell), an electromagnetic radiation of 62 eV will be emitted.