Electron unequal sharing

In a Lewis structure a shared pair denoted by a bond line counts as contributing to the valence shell of both atoms, so that both atoms acquire an octet of electrons. Once we have introduced the concepts of a polar bond and unequal sharing of a pair of electrons, the meaning of the octet rule becomes less clear. The conventional Lewis structure of CF4 (6) obeys the octet rule, but structures 7 and 8, which would be used to describe the polarity of the bonds, do not. 

When two atoms share electrons unequally, it means that the bond between them is polar. Another way to describe this is to say that the bond has partial ionic character. For the molecule AB, this is equivalent to drawing two structures, one of which is covalent and the other ionic. However, there are actually three structures that can be drawn... 

If the electrons are shared equally, the bond is a nonpolar covalent bond, but unequal sharing results in a polar covalent bond. 

We must also modify our thinking of bonding as being simply ionic (where there is transfer of electrons between atoms) or covalent (where there is equal sharing of electrons). These represent two extremes, but bond polarity now provides a middle ground where there is sharing of electrons, but an unequal sharing. 

We are familiar with the concept that the reactivity of a carbonyl group can be ascribed to the difference in electronegativity between carbon and oxygen, and the resultant unequal sharing of electrons. The polarization 8-I-/8— can be considered as a contribution from the resonance form having full charge separation. 

Due to the unequal sharing of the bonding electrons, the 2 atoms have fractional electrical charges, represented by the Greek letter delta (A), in Figure 5-12. 

The two atoms in the hydrogen molecule are identical, so we have a covalent bond with equal sharing of the electrons by the two nuclei. When the two atoms are not identical, such as in HCl, we have a partially ionic bond with unequal sharing of electrons—which are clustered more around the Cl nucleus than the H nucleus. 

It is easy to see from the examples in the previous section how two identical atoms can share electrons to achieve an octet and form diatomic molecules. Because each of our examples dealt with identical atoms, the electrons can be considered to be shared equally by each atom. The bond formed when the atoms are equally shared can be thought of as a pure covalent bond. But what happens in covalent compounds Remember, a compound contains two different elements. When atoms of two different elements are held together by covalent bonds, there is an unequal sharing of the electrons. The sharing of electrons in a covalent bond may be compared to you and a friend sharing a flashlight while walking down a dark street. If you and your friend both held the... 

In the previous section, HCl was described as a polar molecule containing a polar covalent bond. The unequal sharing of electrons in a bond leads to what is referred to as a dipole. A dipole is symbolized using a modified arrow pointing toward the negative end of the dipole. For HCl, this can be represented as... 

It can be anticipated that, even in covalent bonds, the electrons will often be unequally shared by the bound atoms, giving rise to a certain degree of ionic character or polarity to the bond, whenever the electronegativities (x) of the atoms are different. 

Unequal sharing of electrons in a covalent bond is by no means restricted to molecules where the dot structure would indicate formal charges. Take, for example, the hydrogen halides, HX, where X represents a halogen atom. 

Polarity The tendency of a molecule to have positive and negative regions because of the unequal sharing of a pair of electrons. 

Polar Bonded H and I atoms share electrons unequally. 

---