Ionic bonds, 134 unequal electron

We shall return to this topic in Chapter 9. Remember that the unequal distribution of electrons within bonds results in a continuous variation from covalent to ionic bonding. 

The possible formation of a dipole is a feature of covalent bonding but it is obvious that an ionic bond results in a definite unequal distribution of electrons within a molecule and such molecules (or ions) are extremely polar. However, the fact that they carry a definite charge enables additional separation techniques to be applied. The rate of migration in an electric field (electrophoresis) and the affinity for ions of opposite charge (ion-exchange chromatography) are extremely valuable techniques in the separation of ionic species. 

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.)... 

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. 

VIA nonmetals) very easily gain new valence electrons. So metals and nonmetals tend to form bonds in which the metal atoms entirely surrender valence electrons to the nonmetals. Bonds with extremely unequal electron-sharing are called ionic bonds. 

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. 

Intermediate in properties between lhe eleclrovalent and covalent bonds discussed above is the semi-covalent hond (also called dative or polarized ionic bond). It is formed when both electrons that constitute the bonding pair are supplied by one of Ihe atoms An example is the formation of amine oxides between tertiary amines and oxygen, in which both electrons are donated by the nitrogen atom. Such bonds naturally exhibit electrical polarity. They are members ol ihe large class of heleropolar bonds characieriz.ed by an unequal distribution of charge due to a displacement of... 

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. 

Although 6 means a positive charge, and 6 means a negative charge, these symbols do not mean that the bond between hydrogen and fluorine is ionic. An electron is not transferred completely from hydrogen to fluorine, as in an ionic bond. Instead, the atoms share a pair of electrons, which makes the bond covalent. However, the shared pair of electrons is more likely to be found nearer to the fluorine atom. This unequal distribution of charge makes the bond polar covalent. 

Electron pairs shared between two atoms of the same element are shared equally (a non-polar bond). At the other extreme, for ionic bonding there is no electron sharing because the electron is transferred completely from one atom to the other. Most bonds fall somewhere between these two extremes, and the electrons are shared unequally (a polar bond). 

Now consider a tug-of-war in which a 300-pound sumo wrestler is on one end of the rope and a 400-pound wrestler is on the other end. The unequal pull on the ends of the rope brings the middle of the rope closer to the 400-pound competitor. This is an analogy to a bond in which electrons are shared unequally. This type of bond falls between the two extremes of electron sharing equal sharing (a covalent bond) and completely unequal sharing (an ionic bond). Bonds in which the pair of electrons is shared unequally have electronegativity differences between 0.5 and 2.0. 

Figure 9.19 The prevalence of electron sharing. The extremes of pure ionic (top) and pure covalent (bottom) bonding occur rarely, if ever. Much more common is the unequal electron sharing seen in polar covalent bonding.

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... 

An ionic bond results when the affinity of two atoms for their shared electrons is so unequal that it results in actual transfer of one or more electrons from the atom with the lowest attraction, the electropositive atom, to the atom with the highest attraction, the electronegative atom. This transfer results in the formation of a positive ion and a negative ion. Oppositely charged ions in the substance are then attracted to each other by electrostatic attractions. Ionic bonds are common in many substances (salts, for example) but are rarely found in plastics. They are present in the side chains in certain polymers, however, and convey some unique and useful properties. For example, Na " and when used to neutralize carboxylic groups, -COO", and bonded to hydrocarbon chains such as polyethylene, produce polymers called ionomers. 

Bond polarity is a measure of how equally or unequally the electrons in any covalent bond are shared. A nonpolar covalent bond is one in which the electrons are shared equally, as in CI2 and N2. In a polar covalent bond, one of the atoms exerts a greater attraction for the bonding electrons than the other. If the difference in relative ability to attract electrons is large enough, an ionic bond is formed. 

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). 

The ionic bond and the covalent bond represent two extremes. In ionic bonding the atoms are so different that electrons are transferred between them, forming a charged pair of ions. In covalent bonding, two identical atoms share electrons equally. The bond is the mutual attraction of the two nuclei for the shared electrons. Between these extremes lie many cases in which the atoms are not different enough for a transfer of electrons but are different enough that the electron pair cannot be shared equally. This unequal sharing of electrons results in the formation of a polar covalent bond. 

Chemical bonds hold groups of atoms together. They can be classified into several types. An ionic bond is formed when a transfer of electrons occurs to form ions in a purely covalent bond, electrons are shared equally between identical atoms. Between these extremes lies the polar covalent bond, in which electrons are shared unequally between atoms with different electronegativities. 

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