Shared-electron pair bond. See

Sex hormones, 1040—1042, 1044 Shared-electron pair bond. See Covalent bond Shielding of nuclei in NMR spectroscopy, 493—495. See also Chemical shift Sickle-cell anemia, 1089—1090, 1100 Sigma bond, 32... 

Covalent bonding The chemical bond that is formed between two atoms in which each contributes one electron. If the electrons are shared unequally it is a covalent polar bond. Also called an electron pair bond. See also Chemical bond. 

Empirically measured parameters are additional solvent properties, which have been developed through the efforts of physical chemists and physical organic chemists in somewhat different, but to some extent related, directions. They have been based largely on the Lewis acid base concept, which was defined by G. N. Lewis. The concept originally involved the theory of chemical bonding which stated that a chemical bond must involve a shared electron pair. Thus, an atom in a molecule or ion which had an incomplete octet in the early theory, or a vacant orbital in quantum mechanical terms, would act as an electron pair acceptor (an acid) from an atom in a molecule or ion which had a complete octet or a lone pair of electrons (a base). Further developments have included the concepts of partial electron transfer and a continuum of bonding from the purely electrostatic bonds of ion-ion interactions to the purely covalent bonds of atoms and molecules. The development of the concept has been extensively described (see Ref. 11 for details). 

The simplicity of the VSEPR model is one of its primary strengths. In addition, the model provides a continuity in the development of the qualitative ideas about the nature of the chemical bond and its correlation with molecular structure. Abegg s octet rule (see, e.g.. Ref. [3-71]) and Lewis s theory of the shared electron pair [3-72] may be considered as direct forerunners of the model. 

Taber (1998) reports that the explanatory principle that atoms form bonds in order to achieve full shells or octets is very common among 16-18 year old students - apparently replacing the younger students ideas about string, glue and elastic. Molecules are usually assumed to arise from discrete atoms because the atoms want or need to obtain full outer shells or octets (see the section above on the Octet framework). For many students a shared electron pair holds atoms together because it enables them to have octets of electrons, and Barker Millar (2000) have reported that the notion of valency (which is seen by students as the number of bonds needed to obtain octet structures) is imbued with an explanatory or causative power by some learners. 

The Lewis-Langmuir theory was especially convenient for organic compounds, since the bonds between one carbon atom and another or between one carbon atom and a hydrogen atom were easily explained in this fashion. Most organic molecules could therefore easily be represented by electronic formulas where, in general, the old dash of the Kekul6 formula (see page 112) was replaced by a shared electron pair. 

The characteristic chemical property of a C=C structural unit is susceptibility to attack by electrophiles. Electrons flow from the tt component of the double bond toward the electrophile and ultimately become a shared-electron pair in a covalent bond. WeTl see numerous other examples of electrophilic addition to alkenes in this chapter. First, however, we need to extend our discussion of hydrogen halide addition to alkenes of various types. 

Covalent or coordinate bonds in which one pair of electrons Is shared are electron-pair bonds and are known as single bonds. Atoms can also share two pairs of electrons to form double bonds or three pairs in triple bonds. See ORBITAL. 

We see that nitrogen is the central atom, and it has three electron pairs around it. Remember, the double bond formed by the two shared electron pairs between the nitrogen and one of the oxygens counts as only one pair in the VSEPR theory. The three pairs will form a triangle around the N. 

In practice, another simple sequence may help. First, connect aU mutually bonded atoms in your structure by single bonds (i.e., shared electron pairs) second, if there are any electrons left, distribute them as lone electron pairs to maximize the number of octets and finally, if some of the atoms lack octet structures, change as many lone electron pairs into shared electron pairs as required to complete the octet shells (see also the Worked Examples 1-23 and 1-24). 

Protonated methane (CH ) does not violate the octet rule of carbon. A bonding electron pair (responsible for covalent bonding between C and H atoms) is forced into sharing with the proton, resulting in 2 electron-3 center bonding (2e-3c) (see Chapter 10). Higher alkanes are protonated similarly. 

The UHE wave function can also apply to singlet molecules. Usually, the results are the same as for the faster RHEmethod. That is, electrons prefer to pair, with an alpha electron sharing a molecular space orbital with a beta electron. Use the UHE method for singlet states only to avoid potential energy discontinuities when a covalent bond is broken and electrons can unpair (see Bond Breaking on page 46). 

When a Lewis base donates an electron pair to a Lewis acid, the two species share the pair and become joined by a coordinate covalent bond, a bond in which both electrons come from one of the atoms (see Section 2.11). 

---