Covalent molecule

The simplest example of a covalent molecule is the hydrogen molecule, Ha- For this molecule the electronic structure H H is written, indicating that the two electrons are shared between the two hydrogen atoms, forming the bond between them. This structure corresponds to the valence-bond structure H—H. 

The electron distribution in a hydrogen molecule and in two hydrogen atoms. The two nuclei in the molecule are 74 pm apart. 

There is a very strong tendency for atoms of the stronger metals and the nonmetals to achieve the electron number of an argonon by losing or gaining one or more electrons. It was pointed out by Lewis that the same tendency is operating in the formation of molecules containing covalent bonds, and that the electrons in a covalent bond are to be counted for each of the bonded atoms. 

Thus the hydrogen atom, with one electron, can achieve the helium structure by taking up another electron, to form the hydride anion, H , as in the salt lithium hydride, Li+H . But the hydrogen atom can also achieve the helium structure by sharing its electron with the electron of another hydrogen atom, to form a shared-electron-pair bond. Each of the two atoms thus contributes one electron to the shared electron pair. The shared electron pair is to be counted first for one hydrogen atom, and then for the other if this is done, it is seen that in the hydrogen molecule each of the atoms has the helium structure  

The covalent bond in other molecules is closely similar to that in the hydrogen molecule. For each covalent bond a pair of electrons is needed also, two orbitals are needed, one of each atom. 

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The V(IV) species are all d complexes, hence their colour. Besides the VO compounds, some halides VX4 are known, for example VCI4, a liquid with a tetrahedral, covalent molecule and properties similar to those of TiCl4, but coloured (red-brown). 

When acetic anhydride was in excess over nitric acid, acetyl nitrate and acetic acid were the only products. When the concentration of nitric acid was greater than 90 moles %, dinitrogen pentoxide, present as (N02+)(N0a ), was the major product and there were only small traces of acetyl nitrate. With lower concentrations of nitric acid the products were acetic acid, acetyl nitrate and dinitrogen pentoxide, the latter species being present as covalent molecules in this organic medium. A mixture of z moles of nitric acid and i mole of acetic anhydride has the same Raman spectrum as a solution of i mole of dinitrogen pentoxide in 2 moles of acetic acid. 

Semiconductor materials are rather unique and exceptional substances (see Semiconductors). The entire semiconductor crystal is one giant covalent molecule. In benzene molecules, the electron wave functions that describe probabiUty density ate spread over the six ting-carbon atoms in a large dye molecule, an electron might be delocalized over a series of rings, but in semiconductors, the electron wave-functions are delocalized, in principle, over an entire macroscopic crystal. Because of the size of these wave functions, no single atom can have much effect on the electron energies, ie, the electronic excitations in semiconductors are delocalized. 

If an atom or covalent molecule is placed in an electric field there will be a displacement of the light electron cloud in one direction and a considerably smaller displacement of the nucleus in the other direction (Figure 6.1 (b)). The effect of the electron cloud displacement is known as electron polarisation. In these circumstances the centres of negative and positive charge are no longer coincident. 

We have considered the weak van der Waals forces that cause the condensation of covalent molecules. The formation of an ionic lattice results from the stronger interactions among molecules with highly ionic bonds. But most molecules fall between these two extremes. Most molecules are held together by bonds that are largely covalent, but with enough charge separation to affect the properties of the molecules. These are the molecules we have, called polar molecules. 

QHsBr + OH (aq) —v- C2H6OH + Br (aq) (5) This reaction may seem similar to the reaction between aqueous HBr and NaOH but there are two important differences. The ethyl bromide reaction is very slow (about one hour is needed for the reaction) and it occurs between a covalent molecule (C2H5Br) and an ion (OH-). In contrast, the reaction between HBr and NaOH in water occurs in a fraction of a second and it involves ions only, as shown in reaction (6). 

Coordination compounds of alkali and alkaline earth metals with covalent molecules. P. N. Kapoor and R. C. Mehrotra, Coord. Chem. Rev., 1974, 14,1-27 (120). 

Reversible activation of covalent molecules by transition metal complexes. The role of the covalent molecule. L. Vaska, Acc. Chem. Res., 1968,1, 335-344 (51). 

Linus Pauling, Interatomic Distances in Covalent Molecules and Resonance between... 

It is perhaps desirable to point out that the bond type has no direct connection with ease of electrolytic dissociation in aqueous solution. Thus the nearly normal covalent molecule HI ionizes completely in water, whereas the largely ionic HF is only partially ionized. 

In the discussion of metallic radii we may make a choice between two immediate alternative procedures. The first, which I shall adopt, is to consider the dependence of the radius on the type of the bond, defined as the number (which may be fractional) of shared electron pairs involved (corresponding to the single, double, and triple bonds in ordinary covalent molecules and crystals), and then to consider separately the effect of resonance in stabilizing the crystal and decreasing the interatomic distance. This procedure is similar to that which we have used in the discussion of interatomic distances in resonating molecules.7 The alternative procedure would be to assign to each bond a number, the bond order, to represent the strength of the bond with inclusion of the resonance effect as well as of the bond type.8... 

The major synthetic routes to transition metal silyls fall into four main classes (1) salt elimination, (2) the mercurial route, a modification of (1), (3) elimination of a covalent molecule (Hj, HHal, or RjNH), and (4) oxidative addition or elimination. Additionally, (5) there are syntheses from Si—M precursors. Reactions (1), (2), and (4), but not (3), have precedence in C—M chemistry. Insertion reactions of Si(II) species (silylenes) have not yet been used to form Si—M bonds, although work may be stimulated by recent reports of MejSi 147) and FjSi (185). A new development has been the use of a strained silicon heterocycle as starting material (Section II,E,4). 

It is obvious from the graphical presentation in Fig. 7 that the cation [2S ] and the anion [2 ] exist in equilibrium with the covalent hydrocarbon [28-2] as well as the radical [28-] and [2-] as formulated in (35). In other words, the THF solution is a unique system in which one can observe four elemental species of organic compounds, i.e. covalent molecule, cation, anion and radical, at the same time. 

To study covalent molecules, chemists find the use of models and drawings of structures helpful. In models, colored wooden or plastic balls are used to represent atoms. These balls have holes drilled in them according to the number of covalent bonds they will form. The holes are bored at angles that approximate the accepted bond angles. 

The structures used to show the bonding in covalent molecules are called Lewis structures. When bonding, atoms tend to achieve a noble gas configuration. By sharing electrons, individual atoms can complete the outer energy level. In a covalent bond, an octet of electrons is formed around each atom (except hydrogen.)... 

Since chemical hardness is related to the gaps in the bonding energy spectra of covalent molecules and solids, the band gap density (Eg/Vm) may be substituted for it. When the shear moduli of the III-V compound crystals (isoelec-tronic with the Group IV elements) are plotted versus the gap density there is again a simple linear correlation. 

The covalent nature of the asphaltene molecules and the complex nature of the corresponding environment results in agglomeration. Based on the formulated structural models, asphaltenes are seen as an aromatic core which aggregates in concentrated solutions, > 1 %, comprising high-MW covalent molecules, which are surrounded by varying numbers of smaller ones held together by various intermolecular bonds [408], Such molecules are considered to be overlapped/stacked over each other in oil mainly due to... 

Figure 9.53 DPA derivatives have been used as potent enhancers of lanthanide luminescence. Three DPA groups can coordinate with a terbium ion. The iodoacetate derivative of DPA has been used to label covalently molecules for lanthanide luminescence.

Of course, HF is actually a polar covalent molecule, but from the extent of the polarity, it behaves as if it were composed of the two structures shown above. A similar analysis can be carried out for all of the hydrogen halides, and the results are shown in Table 3.2. 

This reaction represents a neutralization reaction in liquid sulfur dioxide. It makes no difference that the solvent does not ionize or that SOCl2 is a covalent molecule. The utility of the solvent concept is not that it correctly predicts that solvents undergo some autoionization. The value of the solvent concept is that it allows us to correctly predict how reactions would take place if the solvent ionized. Note that in this case SOCl2 does not ionize, but if it did it would produce S02+ (the acidic species characteristic of the solvent) and Cl-. 

The chemical behavior of ions, ion pairs, and polarizable molecules partakes of the same indistinctness as the definitions of these species. Any attempt to make a complete catalog of the reactions of ions will almost certainly include borderline reactions whose intermediates are in fact ion-pairs or even covalent molecules. For many purposes the identification of a reaction as carbonium ion-like, or what the Germans would call Krypto-ionenreaktion, is as useful as the certain knowledge that the intermediate is actually a carbonium ion. Many of the ionic reaction mechanisms in the literature do not represent actual free ions and were not so intended by their authors. The ionic representation is often merely a convenient simplification if it is an oversimplification it is one that is easily rectified when the pertinent data become available. The value of such approximate mechanisms is that... 

An explanation not easily distinguishable from the one involving resonance with a carbonium ion structure in the transition state is that the reactive species is an ion pair in equilibrium with the covalent molecule. This is quite likely in a solvent insufficiently polar to cause dissociation of the ion pairs. Examples of second order nucleophilic displacements accelerated by the sort of structural change that would stabilize a carbonium ion are of fairly frequent occurrence. Allyl chloride reacts with potassium iodide in acetone at 50° seventy-nine times as fast as does -butyl chloride.209 Another example is the reaction of 3,4-epoxy-1 -butene with methoxide ion.210... 

In a purely covalent molecule, the bonding pair of electrons is evenly shared between the two nuclei. 

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