Figure 3.6 A Sodium Chloride Crystal. A crystal contains a three-dimensional array of alternating positive and negative ions. The precise pattern depends on the compound. A crystal of sodium chloride, shown here, is a collection of alternating sodium and chlorine ions. Show NoteIn Section 3.2 “Ions”, we encountered LiBr and MgO, which are formulas for other ionic compounds. The formula for an ionic compound follows several conventions. First, the cation is written before the anion. Because most metals form cations and most nonmetals form anions, formulas typically list the metal first and then the nonmetal. Second, charges are not written in a formula. Remember that in an ionic compound, the component species are ions, not neutral atoms, even though the formula does not contain charges. Finally, the proper formula for an ionic compound always obeys the following rule: the total positive charge must equal the total negative charge. To determine the proper formula of any combination of ions, determine how many of each ion is needed to balance the total positive and negative charges in the compound. NoteThis rule is ultimately based on the fact that matter is, overall, electrically neutral. NoteBy convention, assume that there is only one atom if a subscript is not present. We do not use 1 as a subscript. If we look at the ionic compound consisting of lithium ions and bromide ions, we see that the lithium ion has a 1+ charge and the bromide ion has a 1− charge. Only one ion of each is needed to balance these charges. The formula for lithium bromide is LiBr. When an ionic compound is formed from magnesium and oxygen, the magnesium ion has a 2+ charge, and the oxygen atom has a 2− charge. Although both of these ions have higher charges than the ions in lithium bromide, they still balance each other in a one-to-one ratio. Therefore, the proper formula for this ionic compound is MgO. Now consider the ionic compound formed by magnesium and chlorine. A magnesium ion has a 2+ charge, while a chlorine ion has a 1− charge: Mg2+ Cl− Combining one ion of each does not completely balance the positive and negative charges. The easiest way to balance these charges is to assume the presence of two chloride ions for each magnesium ion: Mg2+ Cl− Cl− Now the positive and negative charges are balanced. We could write the chemical formula for this ionic compound as MgClCl, but the convention is to use a numerical subscript when there is more than one ion of a given type—MgCl2. This chemical formula says that there are one magnesium ion and two chloride ions in this formula. (Do not read the “Cl2” part of the formula as a molecule of the diatomic elemental chlorine. Chlorine does not exist as a diatomic element in this compound. Rather, it exists as two individual chloride ions.) By convention, the lowest whole number ratio is used in the formulas of ionic compounds. The formula Mg2Cl4 has balanced charges with the ions in a 1:2 ratio, but it is not the lowest whole number ratio. NoteBy convention, the lowest whole-number ratio of the ions is used in ionic formulas. There are exceptions for certain ions, such as Hg22+. Example 3Write the chemical formula for an ionic compound composed of each pair of ions.
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Skill-Building ExerciseWrite the chemical formula for an ionic compound composed of each pair of ions.
Polyatomic IonsSome ions consist of groups of atoms bonded together and have an overall electric charge. Because these ions contain more than one atom, they are called polyatomic ions. Polyatomic ions have characteristic formulas, names, and charges that should be memorized. For example, NO3− is the nitrate ion; it has one nitrogen atom and three oxygen atoms and an overall 1− charge. Table 3.1 “Some Polyatomic Ions” lists the most common polyatomic ions. Table 3.1 Some Polyatomic IonsNameFormulaammonium ionNH4+acetate ionC2H3O2− (also written CH3CO2−)carbonate ionCO32−chromate ionCrO42−dichromate ionCr2O72−hydrogen carbonate ion (bicarbonate ion)HCO3−cyanide ionCN−hydroxide ionOH−nitrate ionNO3−nitrite ionNO2−permanganate ionMnO4−phosphate ionPO43−hydrogen phosphate ionHPO42−dihydrogen phosphate ionH2PO4−sulfate ionSO42−hydrogen sulfate ion (bisulfate ion)HSO4−sulfite ionSO32− The rule for constructing formulas for ionic compounds containing polyatomic ions is the same as for formulas containing monatomic (single-atom) ions: the positive and negative charges must balance. If more than one of a particular polyatomic ion is needed to balance the charge, the entire formula for the polyatomic ion must be enclosed in parentheses, and the numerical subscript is placed outside the parentheses. This is to show that the subscript applies to the entire polyatomic ion. An example is Ba(NO3)2. Example 4Write the chemical formula for an ionic compound composed of each pair of ions.
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Skill-Building ExerciseWrite the chemical formula for an ionic compound composed of each pair of ions.
Recognizing Ionic CompoundsThere are two ways to recognize ionic compounds. First, compounds between metal and nonmetal elements are usually ionic. For example, CaBr2 contains a metallic element (calcium, a group 2A metal) and a nonmetallic element (bromine, a group 7A nonmetal). Therefore, it is most likely an ionic compound. (In fact, it is ionic.) In contrast, the compound NO2 contains two elements that are both nonmetals (nitrogen, from group 5A, and oxygen, from group 6A). It is not an ionic compound; it belongs to the category of covalent compounds that we will study in Chapter 4 “Covalent Bonding and Simple Molecular Compounds”. Also note that this combination of nitrogen and oxygen has no electric charge specified, so it is not the nitrite ion. Second, if you recognize the formula of a polyatomic ion in a compound, the compound is ionic. For example, if you see the formula Ba(NO3)2, you may recognize the “NO3” part as the nitrate ion, NO3−. (Remember that the convention for writing formulas for ionic compounds is not to include the ionic charge.) This is a clue that the other part of the formula, Ba, is actually the Ba2+ ion, with the 2+ charge balancing the overall 2− charge from the two nitrate ions. Thus, this compound is also ionic. Example 5Identify each compound as ionic or not ionic.
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Skill-Building ExerciseIdentify each compound as ionic or not ionic.
Looking Closer: Blood and SeawaterScience has long recognized that blood and seawater have similar compositions. After all, both liquids have ionic compounds dissolved in them. The similarity may be more than mere coincidence; many scientists think that the first forms of life on Earth arose in the oceans. A closer look, however, shows that blood and seawater are quite different. A 0.9% solution of sodium chloride approximates the salt concentration found in blood. In contrast, seawater is principally a 3% sodium chloride solution, over three times the concentration in blood. Here is a comparison of the amounts of ions in blood and seawater: IonPercent in SeawaterPercent in BloodNa+2.360.322Cl−1.940.366Mg2+0.130.002SO42−0.09—K+0.040.016Ca2+0.040.0096HCO3−0.0020.165HPO42−, H2PO4−—0.01 Most ions are more abundant in seawater than they are in blood, with some important exceptions. There are far more hydrogen carbonate ions (HCO3−) in blood than in seawater. This difference is significant because the hydrogen carbonate ion and some related ions have a crucial role in controlling the acid-base properties of blood. (For more information on the acid-base properties of blood, see Chapter 10 “Acids and Bases”, Section 10.5 “Buffers”.) The amount of hydrogen phosphate ions—HPO42− and H2PO4−—in seawater is very low, but they are present in higher amounts in blood, where they also affect acid-base properties. Another notable difference is that blood does not have significant amounts of the sulfate ion (SO42−), but this ion is present in seawater. Concept Review Exercises
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