Learning Objectives

Write the chemical formula for a simple ionic compound. Recognize polyatomic ions in chemical formulas.

We have currently encountered some chemical formulas for easy ionic compounds. A chemical formulaA concise list of the facets in a compound and also the ratios of these facets. is a concise list of the facets in a compound and also the ratios of these aspects. To much better understand also what a chemical formula way, we should consider exactly how an ionic compound is created from its ions.

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Ionic compounds exist as alternating positive and also negative ions in regular, three-dimensional arrays referred to as crystalsA three-dimensional selection of alternating positive and negative ions. (Figure 3.6 "A Sodium Chloride Crystal"). As you have the right to watch, there are no individual NaCl “particles” in the array; instead, tbelow is a continuous lattice of alternating sodium and also chloride ions. However before, we deserve to usage the ratio of sodium ions to chloride ions, expressed in the lowest feasible totality numbers, as a method of describing the compound. In the instance of sodium chloride, the proportion of sodium ions to chloride ions, expressed in lowest whole numbers, is 1:1, so we use NaCl (one Na symbol and also one Cl symbol) to recurrent the compound. Therefore, NaCl is the chemical formula for sodium chloride, which is a concise way of describing the relative variety of various ions in the compound. A macroscopic sample is created of myriads of NaCl pairs; each pair called a formula unitA collection of oppositely charged ions that create an ionic compound.. Although it is convenient to think that NaCl crystals are created of individual NaCl devices, Figure 3.6 "A Sodium Chloride Crystal" mirrors that no single ion is solely connected via any kind of various other single ion. Each ion is surrounded by ions of opposite charge.


Figure 3.6 A Sodium Chloride Crystal

*

A crystal has a three-dimensional variety of alternating positive and also negative ions. The precise pattern depends on the compound. A crystal of sodium chloride, presented right here, is a arsenal of alternating sodium and chlorine ions.


Note

In 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 composed before the anion. Due to the fact that most steels create cations and also most nonsteels create anions, formulas generally list the metal initially and then the nonmetal. Second, charges are not created 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 adhering to rule: the total positive charge should equal the full negative charge. To determine the proper formula of any kind of combination of ions, identify exactly how many of each ion is required to balance the full positive and negative charges in the compound.


Note

This dominion is inevitably based on the fact that issue is, as a whole, electrically neutral.


Note

By convention, assume that tright here is only one atom if a subscript is not existing. We do not use 1 as a subscript.


If we look at the ionic compound consisting of lithium ions and also bromide ions, we see that the lithium ion has a 1+ charge and also the bromide ion has actually a 1− charge. Only one ion of each is necessary to balance these charges. The formula for lithium bromide is LiBr.

When an ionic compound is created from magnesium and oxygen, the magnesium ion has actually a 2+ charge, and the oxygen atom has actually a 2− charge. Although both of these ions have actually greater charges than the ions in lithium bromide, they still balance each other in a one-to-one proportion. As such, the proper formula for this ionic compound is MgO.

Now consider the ionic compound formed by magnesium and chlorine. A magnesium ion has actually a 2+ charge, while a chlorine ion has actually a 1− charge:

Mg2+ Cl−

Combining one ion of each does not entirely balance the positive and also negative charges. The simplest way to balance these charges is to assume the existence of two chloride ions for each magnesium ion:

Mg2+ Cl− Cl−

Now the positive and also negative charges are balanced. We might create the chemical formula for this ionic compound as MgClCl, but the convention is to usage a numerical subscript once there is more than one ion of a provided type—MgCl2. This chemical formula states that tright here are one magnesium ion and also two chloride ions in this formula. (Do not review the “Cl2” part of the formula as a molecule of the diatomic elemental chlorine. Chlorine does not exist as a diatomic facet in this compound. Rather, it exists as two individual chloride ions.) By convention, the lowest entirety number ratio is used in the formulas of ionic compounds. The formula Mg2Cl4 has balanced charges via the ions in a 1:2 proportion, however it is not the lowest totality number proportion.


Note

By convention, the lowest whole-number ratio of the ions is provided in ionic formulas. Tbelow are exceptions for specific ions, such as Hg22+.


Example 3

Write the chemical formula for an ionic compound written of each pair of ions.

the sodium ion and also the sulhair ion the aluminum ion and also the fluoride ion the 3+ iron ion and the oxygen ion

Solution

To attain a valence shell octet, sodium develops an ion with a 1+ charge, while the sulhair ion has a 2− charge. Two sodium 1+ ions are required to balance the 2− charge on the sulfur ion. Rather than composing the formula as NaNaS, we shorten it by convention to Na2S. The aluminum ion has a 3+ charge, while the fluoride ion created by fluorine has a 1− charge. Three fluorine 1− ions are required to balance the 3+ charge on the aluminum ion. This combicountry is composed as AlF3. Iron have the right to form two possible ions, however the ion via a 3+ charge is specified here. The oxygen atom has actually a 2− charge as an ion. To balance the positive and also negative charges, we look to the leastern widespread multiple—6: 2 iron 3+ ions will certainly provide 6+, while 3 2− oxygen ions will provide 6−, thereby balancing the as a whole positive and also negative charges. Thus, the formula for this ionic compound is Fe2O3.

Skill-Building Exercise

Write the chemical formula for an ionic compound created of each pair of ions.


the calcium ion and the oxygen ion


the 2+ copper ion and the sulhair ion


the 1+ copper ion and the sulhair ion


Table 3.1 Some Polyatomic Ions

Name Formula
ammonium ion NH4+
acetate ion C2H3O2− (additionally composed CH3CO2−)
carbonate ion CO32−
chromate ion CrO42−
dichromate ion Cr2O72−
hydrogen carbonate ion (bicarbonate ion) HCO3−
cyanide ion CN−
hydroxide ion OH−
nitprice ion NO3−
nitrite ion NO2−
permanganate ion MnO4−
phosphate ion PO43−
hydrogen phosphate ion HPO42−
dihydrogen phosphate ion H2PO4−
sulfate ion SO42−
hydrogen sulfate ion (bisulfate ion) HSO4−
sulfite ion SO32−

The ascendancy for building formulas for ionic compounds containing polyatomic ions is the very same as for formulas containing monatomic (single-atom) ions: the positive and also negative charges must balance. If even more than among a specific polyatomic ion is essential to balance the charge, the entire formula for the polyatomic ion need to be enclosed in parentheses, and the numerical submanuscript is placed outside the parentheses. This is to show that the subscript uses to the whole polyatomic ion. An instance is Ba(NO3)2.


Example 4

Write the chemical formula for an ionic compound created of each pair of ions.

the potassium ion and the sulfate ion the calcium ion and also the nitrate ion

Solution

Potassium ions have actually a charge of 1+, while sulfate ions have actually a charge of 2−. We will need two potassium ions to balance the charge on the sulfate ion, so the correct chemical formula is K2SO4. Calcium ions have actually a charge of 2+, while nitrate ions have actually a charge of 1−. We will need two nitprice ions to balance the charge on each calcium ion. The formula for nitrate should be enclosed in parentheses. Therefore, we compose Ca(NO3)2 as the formula for this ionic compound.

Skill-Building Exercise

Write the chemical formula for an ionic compound created of each pair of ions.


the magnesium ion and also the carbonate ion


the aluminum ion and also the acetate ion


Recognizing Ionic Compounds

Tbelow are 2 means to acknowledge ionic compounds. First, compounds in between metal and also nonmetal aspects are usually ionic. For instance, CaBr2 includes a metallic facet (calcium, a team 2A metal) and a nonmetallic element (bromine, a group 7A nonmetal). Because of this, it is most most likely an ionic compound. (In truth, it is ionic.) In contrast, the compound NO2 consists of two facets that are both nonsteels (nitrogen, from team 5A, and also oxygen, from group 6A). It is not an ionic compound; it belongs to the category of covalent compounds that we will certainly examine in Chapter 4 "Covalent Bonding and Simple Molecular Compounds". Also note that this combination of nitrogen and oxygen has no electrical charge mentioned, 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 view the formula Ba(NO3)2, you might identify the “NO3” component as the nitprice 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 component of the formula, Ba, is actually the Ba2+ ion, via the 2+ charge balancing the all at once 2− charge from the 2 nitrate ions. Therefore, this compound is likewise ionic.


Example 5

Identify each compound as ionic or not ionic.

Na2O PCl3 NH4Cl OF2

Solution

Sodium is a metal, and also oxygen is a nonmetal; therefore, Na2O is supposed to be ionic. Both phosphorus and chlorine are nonmetals. Because of this, PCl3 is not ionic. The NH4 in the formula represents the ammonium ion, NH4+, which shows that this compound is ionic. Both oxygen and fluorine are nonmetals. Because of this, OF2 is not ionic.

Skill-Building Exercise

Identify each compound as ionic or not ionic.


N2O


FeCl3


Looking Closer: Blood and Seawater

Science has actually long recognized that blood and also seawater have actually similar compositions. After all, both liquids have actually ionic compounds dissolved in them. The similarity may be even more than mere coincidence; many researchers think that the initially creates of life on Earth occurred in the oceans.

See more: Which Of These Is Not A Characteristic Shared By All Mammals?

A closer look, yet, reflects that blood and also seawater are fairly different. A 0.9% solution of sodium chloride approximates the salt concentration discovered in blood. In comparison, seawater is principally a 3% sodium chloride solution, over 3 times the concentration in blood. Here is a compariboy of the amounts of ions in blood and seawater:


Ion Percent in Seawater Percent in Blood
Na+ 2.36 0.322
Cl− 1.94 0.366
Mg2+ 0.13 0.002
SO42− 0.09
K+ 0.04 0.016
Ca2+ 0.04 0.0096
HCO3− 0.002 0.165
HPO42−, H2PO4− 0.01

Most ions are more plentiful in seawater than they are in blood, through some important exceptions. Tright here are far even more hydrogen carbonate ions (HCO3−) in blood than in seawater. This difference is significant because the hydrogen carbonate ion and also some associated ions have actually an essential function in controlling the acid-base properties of blood. (For even more information on the acid-base properties of blood, see Chapter 10 "Acids and also Bases", Section 10.5 "Buffers".) The amount of hydrogen phosphate ions—HPO42− and also H2PO4−—in seawater is incredibly low, yet they are existing in higher quantities in blood, where they additionally impact acid-base properties. Another remarkable distinction is that blood does not have considerable amounts of the sulfate ion (SO42−), but this ion is present in seawater.


Concept Review Exercises


What indevelopment is included in the formula of an ionic compound?


Why do the chemical formulas for some ionic compounds contain subscripts, while others perform not?