Nearly equal electron sharing

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... 

Chemists consider bonded atoms with AEN between 0 and 0.4 as being mostly covalent electrons are shared equally or nearly equally. 

Nonpolar bond (Section 1.9) A bond between atoms of similar electronegativities in which the electrons are shared nearly equally. 

Elements with nearly equal electronegativities would be expected to form covalent bonds and share their electrons nearly equally. A molecule with this combination would be expected to be nonpolar. 

We have termed the chemical bond in the hydrogen molecule, Hf, a covalent bond. This indicates that electrons are shared so that they are simultaneously and, on the average, equally near two nuclei. This makes the system more stable and a chemical bond results. 

We have seen that the pure elements may solidify in the form of molecular solids, network solids, or metals. Compounds also may condense to molecular solids, network solids, or metallic solids. In addition, there is a new effect that does not occur with the pure elements. In a pure element the ionization energies of all atoms are identical and electrons are shared equally. In compounds, where the most stable electron distribution need not involve equal sharing, electric dipoles may result. Since two bonded atoms may have different ionization energies, the electrons may spend more time near one of the positive nuclei than near the other. This charge separation may give rise to strong intermolecular forces of a type not found in the pure elements. 

In homonuclear diatomic molecules (in which both atoms are the same, as in H2 and CI2), the electrons are shared equally between the two atoms, and the covalency is nearly ideal for such molecules. 

You have seen how atoms can share electrons and that they become more stable by doing so, but do they always share electrons equally The answer is no. Some atoms have a greater attraction for electrons than others do. Chlorine, for example, attracts electrons more strongly than hydrogen does. When a covalent bond forms between hydrogen and chlorine, the shared pair of electrons tends to spend more time near the chlorine atom than the hydrogen atom. 

Covalent bonds may be either polar or nonpolar. In a nonpolar bond such as that in the hydrogen molecule, H2, (H H or H—H) the electron pair is shared equally between the two hydrogen nuclei. We defined electronegativity as the tendency of an atom to attract electrons to itself in a chemical bond (see Section 6-6). Both H atoms have the same electronegativity. This means that the shared electrons are equally attracted to both hydrogen nuclei and therefore spend equal amounts of time near each nucleus. In this nonpolar covalent bond, the electron density is s)mimetrical about a plane that is perpendicular to a line between the two nuclei. This is true for all homonuclear diatomic molecules, such as H2, O2, N2, F2, and CI2, because the two identical atoms have identical electronegativities. We can generalize ... 

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