Equal electron sharing

Both representations (e) or (f) of unsaturated rings are used in this text. In the case of linear molecules containing formal multiple bonds, equal electron sharing is sometimes implied by using broken lines as (d) rather than localised (c) (see Chapter 3). 

Equal electron sharing in a covalent bond and complete electron separation in an ionic bond represent the two extreme cases. Most bonds show some charge separation and some electron sharing. These are called polar covalent bonds. 

Covalent — refers to a chemical bond in which there is an equal/even sharing of bonding electron pairs between atoms. This is typical of the bonding between carbon atoms and between carbon and hydrogen atoms in organic compounds. 

All chemical bonds occur because electrons can be placed simultaneously near two nuclei. Yet it is often true that the electron-sharing which permits this is not exactly equal sharing. Sometimes the electrons, though close to both nuclei, tend to distribute nearer to one nucleus... 

If two atoms have the same electronegativity, then the bond between them is purely covalent. Hydrogen, for instance, occurs as two joined atoms, H-H. Since both atoms in the molecule have the same electronegativity, they form a pure covalent bond with two electrons shared equally by the atoms. 

If an ion contains two or more atoms covalently bonded to each other, the total charge on the ion must equal the total core charge less the total number of electrons, shared and unshared ... 

Figure 4.43 Energy- and angle-resolved triple-differential cross section for direct double photoionization in helium at 99 eV photon energy. The diagram shows the polar plot of relative intensity values for one electron (ea) kept at a fixed position while the angle of the coincident electron (eb) is varied. The data refer to electron emission in a plane perpendicular to the photon beam direction for partially linearly polarized light (Stokes parameter = 0.554) and for equal energy sharing of the excess energy, i.e., a = b = 10 eV. Experimental data are given by points with error bars, theoretical data by the solid curve.

Electron pairs shared between two atoms of the same element are shared equally. At the other extreme, for ionic bonding there is no electron sharing... 

The right-hand column of Figure 15-11 shows the electronic structure for the allyl radical, with three pi electrons in the lowest available molecular orbitals. Two electrons are in the all-bonding MO (iri), representing the pi bond shared between the Cl—C2 bond and the C2—C3 bond. The unpaired electron goes into tt2 with zero electron density on the center carbon atom (C2). This MO representation agrees with the resonance picture showing the radical electron shared equally by Cl and C3, but not C2. Both... 

A bond with the electrons shared equally between the two atoms is called a nonpolar covalent bond. The bond in H2 and the C — C bond in ethane are nonpolar covalent bonds. In most bonds between two different elements, the bonding electrons are attracted more strongly to one of the two nuclei. An unequally shared pair of bonding electrons is called a polar covalent bond. 

The relationship between electronegativity and bond type is shown in Table 13.1. For identical atoms (electronegativity difference of zero) the electrons in the bond are shared equally and no polarity occurs. When two atoms with widely differing electronegativities interact, electron transfer usually occurs, producing ions—an ionic substance is formed. Intermediate cases give polar covalent bonds with unequal electron sharing. 

As we said above, bonds aren t always purely ionic or purely covalent. The best models for ionic bonds include some electron sharing. And covalent bonds don t always share electrons completely evenly. Any difference in electronegativity between bonding nuclei means that the electrons will be attracted more to one nucleus than the other. However, a bond could be considered purely covalent if it is between two identical nuclei, as in hydrogen gas, H2, or oxygen gas, O2. An extreme example of equality in electron sharing is the metallic bond. In a metal, all the atoms are identical and share electrons so readily that the metal can be thought of as one big molecule. 

Consider the formation of a hydrogen molecule, which is shown in Figure 9-2. Each covalently bonded atom equally attracts one pair of shared electrons. Thus, two electrons shared by two hydrogen nuclei belong to each atom simultaneously. 

The charge on any given atom, whether isolated or in a molecule, can be calculated by the following formula subtract the number of electrons that are actually present in the valence shell from the number normally present in the neutral uncharged atomic species. Then, add half the number of electrons which that atom shares with any other atom in the molecule. The result is the charge on that atom. Thus, the charge on an atom equals (the number of normal valence electrons) minus (the number of electrons actually present in the valence shell) plus half (the number of electrons shared by that atom). 

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