Opposites Do Attract Ionic Bonds

In this chapter, I introduce you to ionic bonding, the type of bonding that holds salts together. I discuss simple ions and polyatomic ions how they form and how they combine. 1 also show you how to predict the formulas of ionic compounds and how chemists detect ionic bonds. 

The Ma ic ef an Ionic Bond Sodium + Chtorine - Tahte Salt 

If you really stop and think about it, the process of creating table salt is pretty remarkable. You take two substances that are both very hazardous (chlorine was used by the Germans against Allied troops during World War 0. and from them you make a substance that s necessary for life. In this section, I show you what happens during the chemical reaction to create salt and, more importantly, why it occurs. 

Sodium is an alkali metal, a member of the lA family on the periodic table. 

The Roman numerals at the top of the A families show the number of valence electrons (s and p electrons in the outermost energy level) in the particular element (see Chapter 4 for details). So sodium has 1 valence electron and 11 total electrons because its atomic number is 11. 

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So far, we have had to do work to create the ions which will make the ionic bond it does not seem to be a very good start. However, the + and - charges attract each other and if we now bring them together, the force of attraction does work. This force is simply that between two opposite point charges ... 

When sodium reacts with chlorine to form NaCl, an electron moves from a sodium atom to a chlorine atom. The result is a compound composed of sodium ions and chloride ions, Na Ch, held together by an ionic bond. Ionic bonds do not hold molecules together by sharing electrons they hold them together because of the electrostatic attraction between the two oppositely charged ions. 

Because ions with opposite charges attract each other, an ionic bond results from the attractive force between the positively charged cation and the negatively charged anion. A typical ionic compound is a high-melting solid. In the solid crystal, several anions surround each cation. Each of these anions is attracted equally to the cation. Likewise, each anion is surrounded by several identical cations that are equally attracted to the anion. Because of these multiple interactions, we cannot say that a particular cation is bonded to a particular anion. Therefore, we do not speak of a molecule of an ionic compound, because this would imply one cation associated with one anion. 

One peculiarity of salt (and other substances with atoms locked in a crystalline structure by ionic bonds) is that individual molecules of sodium chloride do not exist at room temperature. What does exist is a lattice of oppositely charged ions, a crystal held together by the strong electrostatic attraction between ions. 

Positive-ion-to-negative-ion attractions are straightforward and strong interactions so strong, in fact, that they tend to dominate when they are present. As we pointed out in our discussion of precipitation, when opposite ions find each other, they can form an ionic bond and come out of solution. Ionic attractions do not, however, always prevail. Water molecules can keep ions apart by forming cages around the ions. These water cages are the result of dipole-to-dipole and dipole-to-ion attractions. 

These ions with their opposite charges attract each other in the same way as do the simple ions in binary ionic compounds. However, the individual polyatomic ions are held together by covalent bonds, with all of the atoms behaving as a unit. For example, in the ammonium ion, NH +, there are four N—H covalent bonds. Likewise, the nitrate ion, N03, contains three covalent N—O bonds. Thus, although ammonium nitrate is an ionic compound because it contains the NH " and N03 ions, it also contains covalent bonds in the individual polyatomic ions. When ammonium nitrate is dissolved in water, it behaves as a strong electrolyte like the binary ionic compounds sodium chloride and potassium bromide. As we saw in Chapter 8, this occurs because when an ionic solid dissolves, the ions are freed to move independently and can conduct an electric current. 

This term is usually applied only to covalently bonded atoms, inasmuch as ionic compounds do not remain together to act as a unit. For example, when a sodium atom gives up an electron or when a chlorine atom accepts an electron, the resulting Na+ and Cl" ions act independently of their original neutral atom source. Every one of the ions formed are equally attracted by any other oppositely charged ion. 

Ionic interactions result from the attraction of a positively charged ion—a cation—for a negatively charged Ion—an anion. In sodium chloride (NaCl), for example, the bonding electron contributed by the sodium atom is completely transferred to the chlorine atom. Unlike covalent bonds, Ionic interactions do not have fixed or specific geometric orientations, because the electrostatic field around an Ion— its attraction for an opposite charge—is uniform In all directions. 

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