Covalent compounds defined

The covalent compounds of graphite differ markedly from the crystal compounds. They are white or lightly colored electrical insulators, have Hi-defined formulas and occur in but one form, unlike the series typical of the crystal compounds. In the covalent compounds, the carbon network is deformed and the carbon atoms rearrange tetrahedraHy as in diamond. Often they are formed with explosive violence. 

Lewis acids are defined as molecules that act as electron-pair acceptors. The proton is an important special case, but many other species can play an important role in the catalysis of organic reactions. The most important in organic reactions are metal cations and covalent compounds of metals. Metal cations that play prominent roles as catalysts include the alkali-metal monocations Li+, Na+, K+, Cs+, and Rb+, divalent ions such as Mg +, Ca +, and Zn, marry of the transition-metal cations, and certain lanthanides. The most commonly employed of the covalent compounds include boron trifluoride, aluminum chloride, titanium tetrachloride, and tin tetrachloride. Various other derivatives of boron, aluminum, and titanium also are employed as Lewis acid catalysts. 

We learned to write formulas of ionic compounds in Chaps. 5 and 6. We balanced the charges to determine the number of each ion to use in the formula. We could not do the same thing for atoms of elements in covalent compounds, because in these compounds the atoms do not have charges. In order to overcome this difficulty, we define oxidation numbers, also called oxidation states. 

More generally, in many cases of intermetallic compounds, unlike a high number of covalent compounds (compare for instance with the illustrative example of a carbon atom in the diamond structure), we cannot speak of bonds of an atom directed to (and saturated with) a well-defined group of atoms. 

Ionization of a covalent compound may be defined as the process leading to the formation of solvated ions independent of their presence as associated ions or as free entities (Fig. 6). In a medium of low dielectric constant the formation of associated ions is favored. It is therefore conceivable to consider the overall process of ionization as consisting of two steps, i.e., the formation of associated ions due to cation-coordination and anion-solvation and the dissociation of the associated ions in solution as a dielectric effect. 

The energy equation 3-3 may be used to define the ion energy level of ionic compound molecules. Fmthermore, this energy equation may also be used to define the ion energy level in covalently bonded compoimd molecules, though the physical meaning of the ion level in covalent compounds is a matter of discussion. 

Acids react with HgO to produce corresponding Hg(II) compounds. Two classes of Hg(II) compounds maybe defined covalent and ionic. The covalent compounds HgCl2, HgBr2, HgD, and Hg(CN)2 go into HOH solution chiefly as undissociated molecules, which undergo little hydrolysis. The ionic compounds which include HgF2, Hg(N03)2, HgS04, and Hg(C104)2 go into... 

A substance composed of atoms held together by covalent bonds is a covalent compound. The fundamental unit of most covalent compounds is a molecule, which we can now formally define as any group of atoms held together by covalent bonds. Figure 6.13 uses the element fluorine to illustrate this principle. 

Some atoms, even in covalent compounds, carry a formal charge, defined as the number of valence electrons in the neutral atom minus the sum of the number of unshared electrons and half the number of shared electrons. Resonance occurs when we can write two or more structures for a molecule or ion with the same arrangement of atoms but different arrangements of the electrons. The correct structure of the molecule or ion is a resonance hybrid of the contributing structures, which are drawn with a double-headed arrow () between them. Organic chemists use a curved arrow (O) to show the movement of an electron pair. 

In section 3.2, you learned about the strong bonds that hold ions in clearly-defined lattice patterns. You learned that these bonds are responsible for the properties of ionic compounds. You also learned how to describe the properties of compounds that are made up of molecules with covalent bonds. In this section, you discovered that the properties of compounds with polar covalent bonds depend on their shape. The following Concept Organizer summarizes some of the properties of covalent compounds that are made up of polar and non-polar molecules. 

The left column of Scheme 35 repeats the ionization-dissociation scheme discussed in Section II.G. If carbon is connected to an electronegative element, one speaks of a covalent compound with a polarized C—X bond. This treatment is justified as there is an approximately tetrahedral environment of the corresponding carbon center. Diphenylmethyl chloride, for example, is never termed a contact ion pair. A well-defined ionization step, which was discussed in Section II.G, generates a carbocation... 

Carbenium ions react with neutral nucleophiles to produce onium ions. A favorable equilibrium between active carbenium ions and temporarily inactive onium ions can be used to produce well-defined polymers (Chapter 4). However, rather than reacting directly with carbenium ions, nucleophiles may also react with Lewis acids to form strong complexes, thereby reducing their activity and ability to ionize covalent compounds. A third reaction that basic nucleophiles may be involved in is /3-proton elimination this transfer reaction may subsequently result in termination if it involves a strong base [Eq. (131)]. 

Covalent bonds are different from ionic bonds, in that they are directional in nature. Furthermore, each covalent bond has a particular length. The consequence of this is that when one atom is covalently bonded to another atom, the relative position in space of these two atoms is fixed. This means that in a molecule held together by covalent bonds, each and every atom has a defined, and predictable, geometric relationship to every other atom within the molecule. Furthermore, in the case of covalent compounds, it is now the norm for every atom to be considered individually in respect of its geometric relationship to every other constituent atom. 

The subscripts in the formula of a compound give the ratio of the number of atoms of each element to the number of atoms of each other element in the formula. The collection of atoms written to represent the compound is defined as one formula imit. That is, the formula unit of ammonium sulfide, (NH4)2S, contains two atoms of nitrogen, eight atoms of hydrogen, and one atom of sulfur. The term formula mass (sometimes called formula weight) refers to the sum of the atomic masses of every atom (not merely every element) in a formula unit. There are several names for formula masses corresponding to different kinds of formulas. For uncombined atoms, the formula mass is the atomic mass. For covalent compounds, which consist of molecules, the formula mass can be called the molecular mass. For ionic compounds, there is no special name for formula mass. These terms are summarized in Table 4-1. 

The differences in ionic and covalent bonding result in markedly different properties for ionic and covalent compounds. Because covalent molecules are distinct units, they have less tendency to form an extended structure in the solid state. Ionic compounds, with ions joined by electrostatic attraction, do not have definable units but form a crystal lattice composed of enormous numbers of positive and negative ions in an extended three-dimensional network. The effects of this basic structural difference are summarized in this section. 

We define the oxidation states (or oxidation numbers) of the atoms in a covalent compound as the imaginary charges the atoms would have if the shared electrons were divided equally between identical atoms bonded to each other or, for different atoms, were all assigned to the atom in each bond that has the greater attraction for electrons. Of course, for ionic compounds containing monatomic ions, the oxidation states of the ions are equal to the ion charges. 

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