Learning Objectives

write the chemistry formula for a simple ionic compound. Identify polyatomic ions in chemistry formulas.

We have already encountered some chemical recipe for basic ionic compounds. A chemical formulaA concise perform of the facets in a compound and the ratios of these elements. Is a concise perform of the aspects in a compound and the ratios of this elements. To better understand what a chemistry formula means, we must consider how one ionic link is created from the ions.

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Ionic compound exist as alternating positive and negative ions in regular, three-dimensional arrays called crystalsA three-dimensional selection of alternating positive and an adverse ions. (Figure 3.6 "A salt Chloride Crystal"). Together you deserve to see, there room no individual NaCl “particles” in the array; instead, over there is a continuous lattice of alternative sodium and also chloride ions. However, we can use the proportion of sodium ions to chloride ions, to express in the lowest possible whole numbers, together a way of describing the compound. In the situation of sodium chloride, the ratio of sodium ions to chloride ions, express in lowest whole numbers, is 1:1, therefore we usage NaCl (one Na symbol and also one Cl symbol) to represent the compound. Thus, NaCl is the chemical formula for sodium chloride, i m sorry is a concise means of explicate the relative variety of different ions in the compound. A macroscopic sample is written of myriads of NaCl pairs; each pair called a formula unitA collection of oppositely charged ions that compose an ionic compound.. Although the is practically to think the NaCl crystals room composed of individual NaCl units, number 3.6 "A sodium Chloride Crystal" mirrors that no solitary ion is exclusively connected with any kind of other single ion. Each ion is surrounded by ion of opposite charge.

Figure 3.6 A sodium Chloride Crystal


A crystal consists of a three-dimensional selection of alternate positive and an adverse ions. The an exact pattern depends on the compound. A crystal of sodium chloride, presented here, is a collection of alternating sodium and chlorine ions.


In ar 3.2 "Ions", us encountered LiBr and MgO, which are formulas for various other ionic compounds.

The formula for an ionic compound complies with several conventions. First, the cation is written prior to the anion. Since most metals type cations and also most nonmetals form anions, formulas generally list the metal very first and climate the nonmetal. Second, charges are not created in a formula. Remember the in an ionic compound, the component varieties are ions, not neutral atoms, also though the formula does no contain charges. Finally, the proper formula for an ionic compound always obeys the complying with rule: the total positive charge should equal the total an adverse charge. To recognize the ideal formula the any combination of ions, identify how plenty of of every ion is necessary to balance the complete positive and an unfavorable charges in the compound.


This dominance is ultimately based on the fact that issue is, overall, electrically neutral.


By convention, assume that there is only one atom if a subscript is no present. We execute not usage 1 as a subscript.

If us look in ~ the ionic compound consisting of lithium ions and bromide ions, we check out that the lithium ion has actually a 1+ charge and the bromide ion has actually a 1− charge. Just one ion of every is essential to balance these charges. The formula because that lithium bromide is LiBr.

When one ionic compound is formed from magnesium and oxygen, the magnesium ion has a 2+ charge, and the oxygen atom has actually a 2− charge. Return both of these ions have higher charges 보다 the ions in lithium bromide, they still balance each various other in a one-to-one ratio. Therefore, the proper formula for this ionic link is MgO.

Now think about the ionic compound created by magnesium and chlorine. A magnesium ion has a 2+ charge, when a chlorine ion has a 1− charge:

Mg2+ Cl−

Combining one ion of each does not completely balance the confident and negative charges. The easiest way to balance this charges is to i think the existence of two chloride ion for each magnesium ion:

Mg2+ Cl− Cl−

Now the confident and negative charges room balanced. We might write the chemical formula for this ionic compound as MgClCl, however the convention is to use a number subscript once there is more than one ion that a given type—MgCl2. This chemical formula says that there room one magnesium ion and also two chloride ions in this formula. (Do not check out the “Cl2” component of the formula as a molecule that the diatomic elemental chlorine. Chlorine does no exist together a diatomic element in this compound. Rather, it exists as 2 individual chloride ions.) through convention, the lowest totality number proportion is supplied in the recipe of ionic compounds. The formula Mg2Cl4 has balanced charges v the ions in a 1:2 ratio, however it is not the lowest whole number ratio.


By convention, the lowest whole-number ratio of the ions is offered in ionic formulas. There are exceptions for particular ions, such together Hg22+.

Example 3

Write the chemistry formula because that an ionic compound created of each pair of ions.

the sodium ion and also the sulfur ion the aluminum ion and the fluoride ion the 3+ stole ion and also the oxygen ion


To achieve a valence covering octet, sodium creates an ion with a 1+ charge, while the sulfur ion has a 2− charge. 2 sodium 1+ ion are needed to balance the 2− charge on the sulfur ion. Fairly than writing the formula together NaNaS, we shorten the by convention come Na2S. The aluminum ion has actually a 3+ charge, when the fluoride ion formed by fluorine has actually a 1− charge. 3 fluorine 1− ion are necessary to balance the 3+ charge on the aluminum ion. This combination is written as AlF3. Iron can kind two feasible ions, yet the ion with a 3+ charge is specified here. The oxygen atom has actually a 2− charge as one ion. Come balance the positive and an unfavorable charges, we look come the least typical multiple—6: 2 iron 3+ ions will give 6+, while three 2− oxygen ions will give 6−, thereby balancing the as whole positive and an adverse charges. Thus, the formula for this ionic compound is Fe2O3.

Skill-Building Exercise

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

the calcium ion and the oxygen ion

the 2+ copper ion and the sulfur ion

the 1+ copper ion and also the sulfur ion

Table 3.1 some Polyatomic Ions

surname Formula
ammonium ion NH4+
acetate ion C2H3O2− (also written CH3CO2−)
carbonate ion CO32−
chromate ion CrO42−
dichromate ion Cr2O72−
hydrogen lead carbonate ion (bicarbonate ion) HCO3−
cyanide ion CN−
hydroxide ion OH−
nitrate 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 dominion for constructing formulas because that ionic link containing polyatomic ions is the exact same as for formulas include monatomic (single-atom) ions: the positive and an unfavorable charges need to balance. If an ext than one of a specific polyatomic ion is needed to balance the charge, the entire formula because that the polyatomic ion must be attached in parentheses, and also the number subscript is placed outside the parentheses. This is to present that the subscript applies to the whole polyatomic ion. An example is Ba(NO3)2.

Example 4

Write the chemical formula for an ionic compound composed of every pair that ions.

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


Potassium ions have actually a charge of 1+, when sulfate ions have a charge of 2−. We will require two potassium ion to balance the charge on the sulfate ion, so the ideal chemical formula is K2SO4. Calcium ions have actually a charge of 2+, when nitrate ions have a charge of 1−. We will need two nitrate ions to balance the charge on each calcium ion. The formula because that nitrate must be attached in parentheses. Thus, we compose Ca(NO3)2 together the formula for this ionic compound.

Skill-Building Exercise

Write the chemistry formula for an ionic compound created of every pair of ions.

the magnesium ion and the lead carbonate ion

the aluminum ion and also the acetate ion

Recognizing Ionic Compounds

There room two means to identify ionic compounds. First, compounds in between metal and nonmetal aspects are generally ionic. Because that example, CaBr2 contains a metallic element (calcium, a team 2A metal) and also a nonmetallic element (bromine, a team 7A nonmetal). Therefore, the is most most likely an ionic compound. (In fact, it is ionic.) In contrast, the compound NO2 contains two elements that space both nonmetals (nitrogen, from group 5A, and oxygen, from group 6A). It is no an ionic compound; that belongs come the classification of covalent compound that us will study in chapter 4 "Covalent Bonding and simple Molecular Compounds". Also note that this mix of nitrogen and oxygen has no electric charge specified, so that 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 might recognize the “NO3” component as the nitrate ion, NO3−. (Remember the the convention for composing formulas because that ionic compounds is not to incorporate the ionic charge.) This is a clue the the other component of the formula, Ba, is in reality the Ba2+ ion, through the 2+ fee balancing the overall 2− fee from the 2 nitrate ions. Thus, this compound is additionally ionic.

Example 5

Identify every compound together ionic or not ionic.

Na2O PCl3 NH4Cl OF2


sodium is a metal, and also oxygen is a nonmetal; therefore, Na2O is supposed to it is in ionic. Both phosphorus and also chlorine room nonmetals. Therefore, PCl3 is no ionic. The NH4 in the formula to represent the ammonium ion, NH4+, which shows that this compound is ionic. Both oxygen and also fluorine space nonmetals. Therefore, OF2 is no ionic.

Skill-Building Exercise

Identify every compound together ionic or not ionic.



Looking Closer: Blood and also Seawater

Science has actually long well-known that blood and also seawater have comparable compositions. ~ all, both liquids have actually ionic compounds liquified in them. The similarity may be much more than only coincidence; countless scientists think the the an initial forms that life ~ above Earth arose in the oceans.

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A closer look, however, reflects that blood and seawater are quite different. A 0.9% equipment of sodium chloride approximates the salt concentration discovered in blood. In contrast, seawater is principally a 3% salt chloride solution, over three times the concentration in blood. Here is a compare of the quantities of ion in blood and also 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 abundant in seawater 보다 they are in blood, with some vital exceptions. There are far an ext hydrogen carbonate ion (HCO3−) in blood 보다 in seawater. This difference is far-reaching because the hydrogen lead carbonate ion and some related ions have actually a vital role in regulating the acid-base properties of blood. (For much more information on the acid-base nature of blood, check out Chapter 10 "Acids and Bases", section 10.5 "Buffers".) The amount of hydrogen phosphate ions—HPO42− and also H2PO4−—in seawater is really low, but they are existing in higher amounts in blood, whereby they also impact acid-base properties. One more notable difference is the blood does not have far-reaching amounts that the sulfate ion (SO42−), but this ion is existing in seawater.

Concept evaluation Exercises

What details is consisted of in the formula of one ionic compound?

Why perform the chemical formulas for part ionic compounds contain subscripts, when others carry out not?