Valency Oxidation State And Bonding

A knowledge of valency and bonding theory serves as the key to correct formulas. In general, the writing of formulas with elements and radicals that have a fixed valance (or oxidation state) is easy, if a knowledge of electrostatics is applied. The real difficulty stems from elements that can assume several oxidation states, from which a variety of ions, molecules, and radicals can result, and a lack of knowledge of nomenclature, which is not always consistent.

Molecules, some ions, and radicals consist of two or more atoms bonded together in some definite manner. In general, the bonds may be ionic or covalent. An ionic bond is formed by the transfer of electrons from one atom to the other. One atom then takes on a positive charge (the cation) and the other a negative charge (the anion). The ion pair that results is held together loosely by electrostatic attraction. In other cases, electrons are not transferred, but are shared between atoms. In elementary molecules with identical atoms, such as Cl2, N2, and 02, the electrons are shared equally to form a covalent bond. On the other hand, in heteronuclear molecules which consist of unlike atoms, the electrons forming the bond are shared unequally. For this case the bonding is termed polar covalent.

The valency or oxidation number of an atom is determined by the number of electrons that it can take on, give up, or share with other atoms. According to valency theory, most atoms consist of neutrons, protons (+), and electrons (-). The neutrons and protons are contained within the nucleus, and a number of electrons, corresponding to the number of protons (atomic number) in the nucleus, are arranged in orderly shells outside. The outer shell contains the valence electrons. If electrons are lost, the atom becomes a positively charged ion, and if electrons are gained, the atom becomes a negatively charged ion. Except for inert elements (such as argon) that already have complete shells, atoms tend to gain or lose electrons so as to assume or approach complete shells. To do this, they must team up with another atom in some

Sodium Sodium atom ion

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Chlorine Chloride atom ion

Figure 2.11

Electron transfer during a chemical reaction, producing a sodium ion with an oxidation state of +1 and a chloride ion with an oxidation state of -1.

manner. In the formation of ions, atoms of two elements undergo reduction and oxidation: one gains electrons and the other loses electrons. In the exchange, the metal or metal-like element loses electrons to gain or approach a stable condition with no electrons in its outer shell. The nonmetal steals electrons from the metal to complete it outer shell to eight electrons, a stable configuration. This exchange is normally accomplished by the release of a great deal of energy. This simple type of reaction is well illustrated by the one between sodium and chlorine, as shown in Fig. 2.1.

The chlorine atom also serves as an example of polar covalent bonding in its various possible combinations with oxygen. The chlorine atom contains several electrons in its outer shell. Oxygen has six electrons in its outer shell and needs two more to complete the shell. These it can obtain in various ways by sharing electrons with the chlorine atom, forming various molecular species which may or may not be charged as illustrated in Fig. 2.2. The electrons contained in the outer shells are represented by dots for simplicity. With oxygen, chlorine tends to share one, three, four, five, or seven of its electrons, to form C120, CIO J, C102, CIO3, and C1207. The oxides from which CIO2 and ClOJ are derived have never been isolated. However, compounds of chlorine with oxidation numbers of +1, +3, +4, -¡-5, and +7 are well defined. Sulfur, nitrogen, and the halogens are nonmetals that are capable of exhibiting a wide range of oxidation numbers because of their ability to take on or share electrons to complete the outer shell to eight or to give up one or more electrons to reach a stable configuration. Manganese, chromium, copper, and iron are examples of metals that can obtain several oxidation states by yielding or sharing one or more electrons. Manganese is an extreme case in that it can yield or share two, three, four, six, or seven electrons. The oxidation numbers and valences of some important elements are given in Table 2.2. Such information is useful in balancing oxidation-reduction reactions (Sec. 2.7).

Chlorine Oxygen atoms atoms s CI* Ö:

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