Covalent bond/bonding definition

Another fundamental property of chemical bonds is polarity. In general, it is to be expected that the distribution of the pair of electrons in a covalent bond will favor one of the two atoms. The tendency of an atom to attract electrons is called electronegativity. There are a number of different approaches to assigning electronegativity, and most are numerically scaled to a definition originally proposed by Pauling. Part A of Table 1.6... 

By far the majority of carbohydrate material in nature occurs in the form of polysaccharides. By our definition, polysaccharides include not only those substances composed only of glycosidically linked sugar residues but also molecules that contain polymeric saccharide structures linked via covalent bonds to amino acids, peptides, proteins, lipids, and other structures. 

The Lewis definition of acids and bases is broader and more encompassing than the Bronsted-Lowry definition because it s not limited to substances that donate or accept just protons. A Lewis acid is a substance that accepts an electron pair, and a Lewis base is a substance that donates an electron pair. The donated electron pair is shared between the acid and the base in a covalent bond. 

In 1923. Lewis published a classic book (later reprinted by Dover Publications) titled Valence and the Structure of Atoms and Molecules. Here, in Lewis s characteristically lucid style, we find many of the basic principles of covalent bonding discussed in this chapter. Included are electron-dot structures, the octet rule, and the concept of electronegativity. Here too is the Lewis definition of acids and bases (Chapter 15). That same year, Lewis published with Merle Randall a text called Thermodynamics and the Free Energy of Chemical Substances. Today, a revised edition of that text is still used in graduate courses in chemistry. 

Another definition of acids and bases is due to G. N. Lewis (1938). From the experimental point of view Lewis regarded all substances which exhibit typical acid-base properties (neutralisation, replacement, effect on indicators, catalysis), irrespective of their chemical nature and mode of action, as acids or bases. He related the properties of acids to the acceptance of electron pairs, and bases as donors of electron pairs, to form covalent bonds regardless of whether protons are involved. On the experimental side Lewis definition brings together a wide range of qualitative phenomena, e.g. solutions of BF3, BC13,... 

Questions such as, for example, whether sphalerite contains Zn++ and S= ions or has a covalent structure similar to that of diamond, and whether ionic or covalent bonds are present in complexes such as [FeF% —, [Fe(CN)e]=, etc., have been extensively discussed it has, indeed, until recently not been at all clear whether or not they could be definitely... 

In both cases the nitrogen atom uses its pair of nonbonding electrons to make a new covalent bond. This similarity led G. N. Lewis to classify ammonia as a base in its reaction with B (CH3)3 as well as in its reaction with H3 O . Whereas the Br< )nsted definition focuses on proton transfer, the Lewis definition of acids and bases focuses on electron pairs. 

A molecule is formed when two or more atoms form a covalent bond. According to this definition, which of these is NOT a molecule ... 

Am. SC I2 and (NH4),S involve covalent bonding, and therefore by definition, electron sharing. (NH4KS also exhibits ionic bonding. 

In contrast, covalent bonding involves the sharing of electron pairs between two specific atoms, and it is possible to speak of a definite bond. For example, in molecules of H, and CC14 there are one and four covalent bonds per molecule, respectively. 

All the inconsistencies can be traced back to the often incompatible requirements of electron-pair covalency as the manifestation of the ideal covalent bonding state on the one hand, and covalency as the opposite to ionicity on the other. On the basis of the ideas advocated in Section III. 1, it is comparatively easy to choose between the two alternatives and give the definition ... 

This expression indicates that there is a decreased probability (indicated by the term —2(/>A(/>B) of finding the electrons in the region between the two nuclei. In fact, there is a nodal plane between the positive and negative (with respect to algebraic sign) of the two regions of the molecular orbital. As a simple definition, we can describe a covalent bond as the increased probability of finding electrons between two nuclei or an increase in electron density between the two nuclei when compared to the probability or density that would exist simply because of the presence of two atoms. 

Let us first seek to give a more rigorous and operational ab initio characterization of such units. The important physical idea underlying the above definitions is that of the connecting covalent bonds that link the nuclei. One can therefore recognize that a molecular unit is equivalently defined by the covalent-bond network that contiguously links the nuclei included in the unit. We can re-state the definition of a molecular unit in a way that emphasizes the electronic origin of molecular connectivity. 

The short-range valence or chemical forces resonance bonding Virtually by definition, the forces between molecules cannot be ascribed to covalent bonds such as those that link atoms within molecules. Hence, such forces may be (and often are) tautologically labeled as noncovalent to distinguish them from the strongest 2c/2e covalent bonds that underlie the formation of molecules. (Nearly as uninformative is the designation weak bonding, which... 

The modem definition from IUPAC says, A covalent bond is a region of relatively high electron density between nuclei which arises (at least partially) from sharing of electrons, and gives rise to an attractive force and characteristic inter-nuclear distance . 

Lewis (1923) put forward another definition of acids and bases solely dependent on giving or taking of an electron pair. According to Lewis— an add is an electron pair acceptor, whereas a base is an electron pair donor . Therefore, it is obvious that whenever any neutralization occurs the formation of an altogether new coordinate covalent bond between the electron pair donor and acceptor atoms take place. 

The possible formation of a dipole is a feature of covalent bonding but it is obvious that an ionic bond results in a definite unequal distribution of electrons within a molecule and such molecules (or ions) are extremely polar. However, the fact that they carry a definite charge enables additional separation techniques to be applied. The rate of migration in an electric field (electrophoresis) and the affinity for ions of opposite charge (ion-exchange chromatography) are extremely valuable techniques in the separation of ionic species. 

Disulfide bridges are, of course, true covalent bonds (between the sulfurs of two cysteine side chains) and are thus considered part of the primary structure of a protein by most definitions. Experimentally they also belong there, since they can be determined as part of, or an extension of, an amino acid sequence determination. However, proteins normally can fold up correctly without or before disulfide formation, and those SS links appear to influence the structure more in the manner of secondary-structural elements, by providing local specificity and stabilization. Therefore, it seems appropriate to consider them here along with the other basic elements making up three-dimensional protein structure. 

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