The Formation of Covalent Compounds

Covalent compounds form when elements share electrons, which usually occurs between nonmetals. Even though relatively few nonmetals exist, they interact in many combinations to form a very large number of covalent compounds. 

Atoms of different elements share electrons to form the molecules of a covalent compound. A sample of hydrogen fluoride, for example, consists of molecules in which one H atom forms a covalent bond with one F atom water consists of molecules in which one O atom forms covalent bonds with two H atoms  

Distinguishing the Entities in Covalent and Ionic Substances There is a key distinction between the chemical entities in covalent and ionic substances. Most covalent substances consist of molecules. A cup of water, for example, consists of individual water molecules lying near each other. In contrast, under ordinary 

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The driving force for double substitution reactions is formation of insoluble ionic compounds or covalent compounds from ions in solution. However, if an equation has an insoluble compound on one side and a covalent compound on the other, which way does the reaction go In many cases like this, the formation of covalent compounds is more important than the formation of insoluble ionic compounds, as shown by the reaction of Ba(OH)2 with HCl. Acids usually react with insoluble bases to produce salts and water ... 

How does the octet rule apply in the formation of covalent compounds ... 

The electrons of atoms are involved in forming compounds. In the formation of ionic compounds, metal atoms transfer electrons to nonmetal atoms, and the resulting charged particles (ions) attract each other into solid arrays. In the formation of covalent compounds, nonmetal atoms share electrons and usually (but not always) form individual molecules. Each compound has a unique name, formula, and mass based on its component elements. (Sections 2.7 and 2.8)... 

The Formation of Ionic Compounds 49 The Formation of Covalent Compounds 51... 

Ranking metal borides as refractory compounds results from the formation of covalent B — B bonds by the electron-deficient B atoms ". As a result the metal lattice may be changed drastically, even for low B contents. 

While sharing of electrons, i.e., covalent bonding, is the major component of the cohesive force in intermetallics, rationalization of their structure formation based on such chemical bonding is not trivial, because of the failure of the common electron counting rules that chemists have developed over the years from the studies of covalent compounds. The origin of the problem is the well-delo-... 

In the formation of certain compounds a covalent bond can be formed in which both of the shared electrons come from only one of the atoms. These bonds are called coordinate covalent bonds. Let s examine the formation and bond structure of the NH ion which contains a coordinate covalent bond. 

To an increasing weight of the chemical bond factor (ionic and/or covalent bonding) will correspond, as an extreme case, the formation of valence compounds. According to Parthe (1980), a compound CmAn can be called a normal valence compound if the number of valence electrons of cations (ec) and anions (eA) correspond to the relation... 

By definition, alkylating agents are compounds that are capable of introducing alkyl groups into nucleophilic sites on other molecules through the formation of covalent bonds. These nucleophiUc targets for alkylation include the sulfhydryl, amino, phosphate, hydroxyl. 

Lewis and many other chemists had recognized the shortcomings of the ionic bond. When diatomic molecules, such as or Cl, were considered, there was no reason why one atom should lose an electron and an identical atom should gain an electron. There had to be another explanation for how diatomic molecules formed. We have seen how the octet rule applies to the formation of ionic compounds by the transfer of electrons. This rule also helps explain the formation of covalent bonds when molecules (covalent compounds) form. Covalent bonds result when atoms share electrons. Using fluorine, F, as a representative halogen, we can see how the octet rule applies to the formation of the molecule. Each fluorine atom has seven valence electrons and needs one more electron to achieve the stable octet valence configuration. If two fluorines share a pair of electrons, then the stable octet configuration is achieved ... 

We conclude this section by noting that sorption of charged species to NOM is generally fast and reversible, provided that no real chemical reactions take place that lead to the formation of covalent bonds (i.e., to bound residues see chapter 14). This conclusion is based on experimental data and on the assumption that in aqueous solution the more polar NOM sites are more easily accessible as compared to the more hydrophobic domains. For charged species, we may, therefore, assume that equilibrium is established within relatively short time periods. Hence, for example, in the case of TBT and TPT, contaminated sediments may represent an important source for these highly toxic compounds in the overlying water column (Berg et al., 2001). 

Irreversible inhibition, which is much more important toxicologically, can arise from various causes. In most cases the formation of covalent or other stable bonds or the disruption of the enzyme structure is involved. In these cases the effect cannot be readily reversed in vitro by either dialysis or dilution. The formation of stable inhibitory complexes may involve the prior formation of a reactive intermediate that then interacts with the enzyme. An excellent example of this type of inhibition is the effect of the insecticide synergist piperonyl butoxide (Figure 9.6) on hepatic microsomal monooxygenase activity. This methylenedioxyphenyl compound can form a stable inhibitory complex that blocks CO binding to P450 and also prevents substrate oxidation. This complex results from the formation of a reactive intermediate, which is shown by the fact that the type of inhibition changes from competitive to irreversible as metabolism, in the... 

The chemistry and physical properties of various -triazines are summarized in Tables A1 and A2 of the Appendix. The triazines are Lewis bases as the ring nitrogen (N) atoms may donate electron pairs for the formation of covalent bonds. In aqueous systems, triazines exist as either neutral or protonated (cationic) forms depending on the pKa of the compound and the pH of the system. The most basic ring N and the most likely site of protonation are located in the 5 position between the electron-rich alkylamino side chains (Figure 21.1). 

All cellular life today incorporates two processes we will refer to as self-assembly and directed assembly (Fig. 1). The latter involves the formation of covalent bonds by energy-dependent synthetic reactions and requires that a coded sequence in one type of polymer in some way direct the sequence of monomer addition in a second polymeric species. On the other hand, spontaneous self-assembly occurs when certain compounds associate through noncovalent hydrogen bonds, electrostatic forces, and nonpolar interactions that stabilize orderly arrangements of small and large molecules. Three well-known examples include the self-assembly of water molecules into ice, DNA... 

The tendency to form covalent rather than ionic bonds is crucial in the formation of hypervalent compounds. Incorporation of fluoride as a ligand usually stabilizes covalent... 

With regard to the formation of ionic compounds, it is not too relevant whether the 8p or 7d shell is occupied in the neutral atom, as studied in extenso by Mann and Wdber (50). Instead, the significant question for more ionic compounds is whether in the ions, after all outer s, p and d electrons are removed, some g or f electrons will be in frontier orbitals or whether they might be easily excited to an outer electron shell so that they can be removed as well. Prince and Waber (103) showed that even in the divalent state of element 126 one g electron has changed to an / electronic state. However, the 8s electrons are not the first to be removed. Thus, the divalent ions will be expected to act as soft Lewis acids and possibly form covalent complex ions readily. Crystal or ligand fields influence the nature of the hybridization. Details such as directionality of bonds... 

Directional natiire oj bonds involving d orbitals— The atoms of the transition elements may take part in both c and n bonds in the formation of a compound. The number of a bonds formed by the central atom cannot exceed the number of surrounding atoms or groups since between any pair of atoms, only one a bond is permissible, but the number of less than the number of neighbours, since bonds other than the covalent are possible, e.g, bonds due to ion-dipole and dispersion forces. 

Natural rubber comes from tropical trees. It is soft and sticky, so it has little practical use. However, while experimenting with rubber in 1839, Charles Goodyear dropped a mixture of sulfur and natural rubber on a hot stove by mistake. The heated rubber became tough and elastic because of the formation of covalent bonds. The resulting compound was vulcanized rubber, which is strong enough to make up a basketball that can take a lot of hard bounces. 

In other situations, the neutral oxygen atom may share electrons with one or more other atoms, in order to act as though it has a complete valence shell part of the time. These shared electrons represent covalent bonds and result in the formation of molecular compounds, as shown here ... 

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