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⚙️Learning Objectives
- Write correct names and formulas for ionic compounds containing polyatomic ions.
Some ions consist of groups of atoms bonded together that carry 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, the nitrate ionhas one nitrogen atom and three oxygen atoms with an overall charge of 1−. The formula of the nitrate ion is written as NO3−. A list of the most common polyatomic ions appears in Table \(\PageIndex{1}\). Since these ions are regularly used throughout the rest of this text, it generally a good practice to memorize this list of ions.
Writing Formulas for Ionic Compounds Containing Polyatomic Ions
Writing formulas for ionic compounds containing polyatomic ionsinvolves the same steps as for a binary ionic compound. The cation is written first in the chemical formula followed by the anion. Considerpotassium nitrate. Potassium is located in Group IA of the periodic table, so it forms ions with a 1+ charge, K+. As shown above in Table \(\PageIndex{1}\), the formula of the nitrate ion is NO3–. Since the charges of the two ionsare already balanced, the formula for potassium nitrate may be written as KNO3.
When more than one polyatomic ion is required to balance charge, the formula of the polyatomic ion isenclosed in parentheses followed bya subscript that indicates the quantity of that polyatomic ion required to balance the charge. This means that the formula of calcium nitrate is written as Ca(NO3)2, since calcium is located in Group IIA and forms ions with a 2+ charge, Ca2+,and will require two nitrate ions, NO3–, to balance the charge of a single calcium ion.
It should be emphasized that enclosing a polyatomic ion in parentheses is only done when more than one is required to balance charge. Parentheses are not included when only one polyatomic ion is required to balance charge. That is why the formula of potassium nitrate is written as KNO3and not as K(NO3).
✅Example \(\PageIndex{1}\):Writing Chemical Formulas
Write the chemical formulas for:
- copper(I) carbonate
- aluminum acetate
- magnesium hydroxide
Solution
| copper(I) carbonate | aluminum acetate | magnesium hydroxide | ||||
|---|---|---|---|---|---|---|
| Cu+ | CO32– | Al3+ | C2H3O2– | Mg2+ | OH– |
| Cu+ Cu+ | CO32– | Al3+ | C2H3O2– C2H3O2– C2H3O2– | Mg2+ | OH– OH– |
| 2+ balances 2– | 3+ balances 3– | 2+ balances 2– | |||
| Cu2CO3 | Al(C2H3O2)3 | Mg(OH)2 | |||
Writing Namesfor Ionic Compounds Containing Polyatomic Ions
The reverse process is used when writing namesfor ionic compounds containing polyatomic ions. Each ion is named in the order that they appear in the chemical formula. The most important part is to recognize that a polyatomic ion is present in the compound. Na3PO4would be called sodium phosphate, since it contains the sodium ion, Na+, and the phosphate ion, PO43–. Mg(ClO3)2would be called magnesium chlorate, since it contains the magnesiumion, Mg2+, and the chlorateion, ClO3–.
✅Example \(\PageIndex{2}\): Writing Chemical Names
Write the name for each compound.
- (NH4)2S
- FeSO4
- Fe2(SO4)3
Solution
- NH4+is the ammonium ion. S= sulfur. The root name of sulfur is sulf-. The nonmetal has a suffix of -ide. (NH4)2Sis ammoniumsulfide.
- SO42–is the sulfate ion. Fe is a transition element that forms ions with multiple charges. Since only one iron ion isneeded to balance the charge of onesulfate ion, it must have a 2+ charge. An iron ion with a 2+ charge is called the iron(II) ion. FeSO4is iron(II) sulfate.
- SO42–is the sulfate ion. Fe is a transition element that forms ions with multiple charges. Since twoiron ions are needed to balance the charge of threesulfate ions, it must have a 3+ charge. An iron ion with a 3+ charge is called the iron(III) ion. Fe2(SO4)3is iron(III) sulfate.
Oxyanions
You may have noticed that most of the polyatomic ions in Table \(\PageIndex{1}\)carrya negative charge and containthe element oxygen. Anions that contain the element oxygen are called oxyanions. Many oxyanions belong to a series where the number of oxygen atoms in the ion varies, as shown in Table \(\PageIndex{2}\).
| Name | Formula |
|---|---|
| chlorateion | ClO3− |
| chlorite ion | ClO2− |
| nitrate ion | NO3− |
| nitrite ion | NO2− |
| sulfate ion | SO42− |
| sulfiteion | SO32− |
Note that oxyanions may end with either an -ate or -ite suffix.In other words, if you encountera compound (or polyatomic ion) that ends with-ate or -ite, it will always contain oxygen. To be clear, simplycontaining oxygen doesn't mean the compound will always end with an -ate or -ite suffix. We have seen examples of oxides, where oxygen is part of a binary compound such as lithium oxide, Li2O, and hydroxides, where oxygen is part of the polyatomic ion OH–.
When comparing oxyanions that end with -ateand -ite, the ion that ends with-atewillalways have one more oxygen than the corresponding ion that ends with -ite.Theionin the seriesthat ends with-ate is the one found in Table \(\PageIndex{1}\): The Most Common Polyatomic Ions. This is because the -ate form is more commonly found in nature than the -ite form. It is also the reason why memorizing the form that ends with -ate is recommended rather than the one that ends with -ite. Furthermore, if you learn the one that ends with -ate, the ion that ends with -itecan easily be obtained, since it always has one less oxygen than the -ate form.
It should also be pointed out that just because the -ite form always has one less oxygen than the -ate form doesn't mean that an -ite form is possible. Even though the carbonate ion, CO32–, is quite common, there is no such thing as the carbonite ion. Some of you reading this might be saying right now, "Wait a minute! I remember hearing about carbonite in Star Wars!" That's true. However, Star Wars is science fiction and may have even been the reason for the choice of using the word carbonite...it doesn't really exist!
✅Example \(\PageIndex{3}\)
- What is the formula of zinc sulfite?
- Write the chemical name of Sr(NO2)2?
- If the bromate ion has a formula of BrO3–, what is the formula of the bromite ion?
Solution
- From Figure 5.5.1, Zn is one of the three transition elements that has a fixed charge. Zinc ions have a charge of 2+, Zn2+. The formula of the sulfite ion is SO32–, which has one less oxygen atom than the sulfate ion. The charge onZn2+balances the charge onSO32–, so the formula of zinc sulfite is ZnSO3.
- Sr2+is theformula of the strontium ion. NO2–is the formula of the nitrite ion, which has one less oxygen than the nitrate ion.Sr(NO2)2is strontium nitrite.
- The -ite form always contains one less oxygen atom that the ion with an -ate suffix. If the bromate ion is BrO3–,then the bromite ion has a formula ofBrO2–.
✏️Exercise \(\PageIndex{1}\)
Write the formula for each:
- silver cyanide
- calcium bicarbonate
- lithiumchlorite
- nickel(III) sulfate
- phosphite ion
- Answer A
- AgCN
- Answer B
- Ca(HCO3)2
- Answer C
- LiClO2
- Answer D
- Ni2(SO4)3
- Answer E
- PO33–
✏️Exercise \(\PageIndex{2}\)
Write the name ofeach:
- Na2SO3
- Cd(C2H3O2)2
- (NH4)3PO4
- KOH
- Pb(NO3)4
- Answer A
- sodium sulfite
- Answer B
- cadmium acetate
- Answer C
- ammonium phosphate
- Answer D
- potassium hydroxide
- Answer E
- lead(IV) nitrate
Additional Polyatomic Ions
There are many other polyatomic ions that may be encountered in this text besides the ones already listed. Table \(\PageIndex{3}\) is not an all-inclusive list of additional polyatomic ions that exist, but rather ions you mayexpect to see in this text. Ions are groupedtogether if they are a part of a series.
This pageis shared under a CK-12 license and was authored, remixed, and/or curated byMelissaAlviar-Agnew,Henry Agnew, and Lance S. Lund (Anoka-Ramsey Community College). Original source:https://www.ck12.org/c/chemistry/.
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