Sharing Electrons—Unequally

Shared electron pairs between two identical atoms are shared equally and the covalent bond is nonpolar 

The ultimate in unequal sharing of electrons is the ionic bond, in which there is a complete transfer of electrons. 

The two common ways of showing a polar bond are illustrated below. The 8s represent partial positive and partial negative chat the point of the arrow is toward the more electronegative atom, which attracts electrons more strongly than the other atom  

The covalent bonds examined so far have consisted of electrons contributed equally from both of the atoms involved in the bond. It is possible to have covalent bonds in which only 1 of the 2 atoms that are joined together contributes both of the electrons in the bond. The bond so formed is called a coordinate covalent bond or a dative bond. A typical coordinate covalent bond forms when ammonia gas and hydrogen chloride gas react. This sometimes happens accidentally in the laboratory when these gases are evolved from beakers of concentrated ammonia solution and concentrated hydrochloric acid (a solution of HCl gas) that are accidentally left uncovered. When these two gases meet, a white chemical fog is formed. It is ammonium chloride, NH Cl, a salt that is produced by the reaction 

Site of coordinate covalent bond formation (after the bond has formed, the four N-H bonds are indistinguishable) 

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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... 

Polar Bonded H and I atoms share electrons unequally. 

It is not always clear whether atoms share electrons or transfer them. Atoms with different electronegativities can share electrons unequally without exchanging them. How unequal does the sharing have to be before the bond is considered ionic ... 

Ionic bonding exists between oppositely-charged ions. The polar covalent bond exists between atoms that share electrons unequally. The nonpolar bonding exists between atoms that share electrons equally (atoms with similar attractions for electrons). 

Electronegativity differences less than 1.7 are classified as covalent. Unequal differences in sharing electrons are known as polar covalent. 

Many substances contain bonds that are intermediate in character— between pure covalent and pure ionic bonds. Such polar bonds occur when one of the elements attracts the shared electrons more strongly than the other element. In hydrogen fluoride, for instance, the shared electrons are so much more attracted by fluorine than hydrogen that the sharing is unequal. (See Figure 5-11.)... 

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... 

Bond polarity In bonds between atoms of unequal electronegativities, shared electrons are more likely to be found near the more electronegative atom such a bond is said to be polar. 

In a polar covalent bond, the shared electrons are shared unequally, being pulled more toward the atom with the higher electronegativity. As shown in Table 9.1, an electronegativity difference of 0.3 to 1.4 characterizes a polar covalent bond. This unequal electron... 

In diatomic molecules, if the (only) bond is polar, the molecule as a whole has an asymmetrical electronic distribution. For example, HCl has an unequally shared electron pair ... 

Electronegativity values are used as a guideline to indicate whether the electrons in a bond are equally shared or unequally shared between two atoms. For example, whenever two identical atoms are bonded together, each atom attracts the electrons in the bond to the same extent. The electrons are equally shared, and the bond is nonpolar. Thus, a carbon-carbon bond is nonpolar. The same is true whenever two different atoms having similar electronegativities are bonded together. C-H bonds are considered to be nonpolar, because the electronegativity difference between C (2.5) and H (2.2) is small. 

The C-O bond has some ionic character because there is an unequal sharing of electrons between the carbon atom and the oxygen atom. The oxygen atom attracts the shared electrons more strongly than the carbon atom does. 

Figure 19 Two hydrogen atoms share electrons with one oxygen atom, but the sharing is unequal. The electrons are more likely to be closer to the oxygen than the hydrogens. The space-saving model shows how the charges are separated or polarized. Define the term polar.

When the electronegativity difference between bonding atoms is between 0.5 and 2.0, the electron sharing is not so rmequal that a complete transfer of electrons takes place. Instead, there is a partial transfer of the shared electrons to the more electronegative atom. The less electronegative atom stiU retains some attraction for the shared electrons. The bond that forms when electrons are shared unequally is called a polar covalent bond. A polar covalent bond has a significant degree of ionic character. 

Polar covalent bond is the preferred term for a bond made up of unequally shared electron pairs. One end of the bond (in this case, the F atom) is more electron rich (higher electron density), hence, more negative. The other end of the bond (in this case, the H atom) is less electron rich (lower electron density), hence, more positive. These two ends, one somewhat positive and the other somewhat negative may be described as electronic poles, hence the term polar covalent bonds. 

A). On the other hand, in covalent bonds between nonidentical atoms, the sharing is unequal one atom attracts the electron pair more strongly than the other. For reasons discussed in Chapter 9, an O atom attracts electrons more strongly than an H atom. Therefore, in each O—H bond in water, the shared electrons spend more time closer to the O atom. 

In Chapter 9, you learned that a covalent bond is polar when it joins atoms of different electronegativities because the atoms share the electrons unequally. In diatomic molecules, such as HF, where there is only one bond, the bond polarity causes the molecule itself to be polar. Molecules with a net imbalance of charge have a molecular polarity. In molecules with more than two atoms, both shape ami bond polarity detennine molecular polarity. In an electric field, polar molecules become... 

According to Lewis s octet rule, each atom should be surrounded by four pairs of electrons, either shared or free pairs. Lewis derived stmctures for halogen molecules, the ammonium ion, and oxy acids, inexplicable according to previous valence theories. He viewed polar bonds as unequally shared electron pairs. Because the complete transfer of electrons was only an extreme case of polarity, he abandoned his earlier dualistic view the polar theory was just a special case of his more general theory. 

We have seen that when a metal and a nonmetal react, one or more electrons are transferred from the metal to the nonmetal to give ionic bonding. On the other hand, two identical atoms react to form a covalent bond in which electrons are shared equally. When different nonmetals react, a bond forms in which electrons are shared unequally, giving a polar covalent bond. The unequal sharing of electrons between two atoms is described by a property called electronegativity the relative ability of an atom in a molecule to attract shared electrons to itself... 

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