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Course: AP®︎/College Chemistry > Unit 1

Lesson 2: Compounds and ions
Science
AP®︎/College Chemistry
Atoms, compounds, and ions
Compounds and ions
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Polyatomic ions

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How to name ionic compounds containing common polyatomic ions
In this article, we will discuss polyatomic ions. The prefix poly- means many, so a polyatomic ion is an ion that contains more than one atom. This differentiates polyatomic ions from monatomic ions, which contain only one atom. Examples of monatomic ions include Na+, Fe3+, Cl−, and many, many others. This article assumes you have a knowledge of basic monatomic ions as well as the conventions for naming ionic compounds and writing their chemical formulas.
For a review on these topics, see this article on monatomic ions and naming ionic compounds.
Polyatomic ions are everywhere! Chalk is made up of calcium carbonate, CaCO3, which contains calcium cations, Ca2+, and carbonate anions, CO32−, which are polyatomic ions. Image credit: Alice on Flickr, CC BY-NC-ND 2.0

Structure of polyatomic ions

We can think about polyatomic ions by comparing them to monatomic ions. A monatomic ion is an atom that has been ionized by gaining or losing electrons. The ion has a net charge because the total number of electrons is not balanced by the total number of protons in the nucleus. Thus, compared to the neutral atom, we have extra electrons—in the case of a negatively charged anion—or not enough electrons—in the case of a positively charged cation. For example, a neutral chlorine atom has an atomic number of 17, which means it has 17 protons and 17 electrons. The neutral atom will sometimes gain an extra electron to become the chloride anion, Cl−:
             Cl   +   e−   →   Cl−
    17electrons              18electrons
     17protons                17protons
After gaining an electron, the chloride anion has 17 protons and 18 electrons. Since there is one extra electron compared to the number of protons, the ion has a net charge of 1-.
Similarly, we can think of a polyatomic ion as a molecule that has been ionized by gaining or losing electrons. In a polyatomic ion, the group of covalently bonded atoms carries a net charge because the total number of electrons in the molecule is not equal to the total number of protons in the molecule. When drawing Lewis dot structures, the overall charge on a polyatomic ion is equal to the sum of the formal charges on each atom in the ion.
Lewis structure of hydroxide.
The Lewis dot structure for the hydroxide ion. Dots around O indicate three lone pairs, and the line between H and O represents a covalent bond containing two shared electrons. The overall charge on the polyatomic ion is 1-, which is indicated to the upper right outside the brackets. Image credit: Wikimedia Commons, CC BY-SA 3.0
For example, let's consider the polyatomic ion OH−, which is known as hydroxide. On the left we see the dot structure of the hydroxide ion. It contains one oxygen atom and one hydrogen atom. The single line between them represents the covalent bond, which contains two electrons shared between H and O. The dots around O represent lone pairs of electrons. In hydroxide, the oxygen has three lone pairs of electrons, which makes for six lone pair electrons in total.
The net charge on the hydroxide ion is indicated by putting the entire dot structure inside square brackets with the charge on the upper right. We see that hydroxide has a 1- charge, which means the ion has one more electron than there are protons in the nuclei of a hydrogen atom plus an oxygen atom.
Concept check: How many protons and electrons are in a hydroxide ion?

Common polyatomic ions

Polyatomic ions are everywhere! Bicarbonate ions, HCO3−, help maintain the pH level of our blood, while phosphates, PO43−, are extremely important in various metabolic processes. Being familiar with the names, charges, and formulas of the most common polyatomic ions will be helpful for recognizing ionic compounds and predicting their reactivity. The following table lists some of the common polyatomic ions.
FormulaNameFormulaNameHg22+Mercury(I)SCN−ThiocyanateNH4+AmmoniumCO32−CarbonateNO2−NitriteHCO3−Hydrogen carbonate (bicarbonate*)NO3−NitrateClO−HypochloriteSO32−SulfiteClO2−ChloriteSO42−SulfateClO3−ChlorateHSO4−Hydrogen sulfate (bisulfate*)ClO4−PerchlorateOH−HydroxideC2H3O2−(or CH3COO−)AcetateCN−CyanideMnO4−PermanganatePO43−PhosphateCr2O72−DichromateHPO42−Hydrogen phosphateCrO42−ChromateH2PO4−Dihydrogen phosphateO22−PeroxideC2O42−Oxalate
* An informal but commonly used name.
We will be referring to the polyatomic ions in this table for the remainder of the article!

Names and chemical formulas of compounds containing polyatomic ions

You can probably find polyatomic ions in your kitchen! Baking soda is a common name for sodium bicarbonate, NaHCO3, which contains the bicarbonate ion HCO3−. Image credit: pixabay, CC0 1.0
Now that we have a reference for many of the common polyatomic ions, let's look at how to name and write the chemical formulas for compounds that contain them. There are two main things to keep in mind:
  1. If a compound contains more than one polyatomic ion of the same type, we need to place parentheses around the ion's formula before using a subscript to indicate how many ions of that type are in the compound.
  2. The overall charge for the ionic compound must be neutral, which means the sum of the charges from the cations and anions should add up to zero. We can use this rule to figure out the formula of an ionic compound when we know the charge on the anion and the cation. This rule can also be useful for deducing the charge of an ion when the chemical formula for the ionic compound is known.
Let's consider some examples.

Example 1: Finding the chemical formula from the compound name

What is the chemical formula for calcium hydroxide?
Calcium is an alkaline earth metal—Group 2 on the periodic table—so it forms ions with a 2+ charge. From our table, we know that hydroxide has the formula OH− and that it carries a 1- charge. We will need two hydroxide ions to exactly cancel the 2+ charge on Ca2+. When writing out the formula, we include parentheses around OH followed by a subscript of 2, to make it clear that there are two hydroxide ions for every Ca2+ cation. Thus, the chemical formula for the compound is Ca(OH)2.
Concept check: Why would CaOH2 be the incorrect formula for calcium hydroxide?

Example 2: Naming ionic compounds that contain polyatomic ions

What is the name of the compound with the formula Ni3(PO4)2?
When naming ionic compounds, it helps to first break down the formula into the cation(s) and the anion(s). In this compound, the cation is based on nickel. Nickel is a transition metal that can form multiple kinds of cations with different charges. That means we will need to figure out the charge on the nickel ion in this specific compound so that we can specify it when naming the ionic compound! Luckily, we know the charge on the anion: phosphate is a polyatomic ion that always has a charge of 3-. Since the overall charge for an ionic compound is zero, we can use the chemical formula and the charge on phosphate to calculate the charge on the nickel ion:
Net charge=0=(# cations×cation charge)+(# anions×anion charge)
If we rearrange this equation, we see that the sum of the charges from the cations must be equal to −1 times the sum of the charges from the anions.
# cations×cation charge=−1×(# anions×anion charge)
We can plug in the known values for the number of cations and anions—from the chemical formula—and the anion charge, then solve for the charge on nickel:
3×cation charge=−1×(2×3−)=6+cation charge=6+3=2+
Now we know the cation in our compound is nickel(II). To name the whole compound, we start with the cation name with the charge included in parentheses using Roman numerals, followed by the anion name.
Therefore, the name of the compound is nickel(II) phosphate.

Example 3: Breaking apart an ionic compound into ions

What are the constituent ions in the ionic compound Ca(MnO4)2?
Specify the formula, charge, and number of each ion.
When analyzing the formula of an ionic compound, we first look for the formulas of familiar ions. In this case, we notice that MnO4−, permanganate, is one of the polyatomic ions listed in our table above. In this compound, we see that we have two ions of permanganate.
Because permanganate carries a 1- charge, this means it is contributing a net charge of 2×(1−)=2− to the ionic compound. Recall that in all ionic compounds, the net charge of the compound must be zero. Therefore, calcium must be present as the cation Ca2+ in order to balance out the 2- net charge from the two MnO4− ions.
To check our reasoning, we can ask ourselves, "Does calcium typically form cations with a 2+ charge?" Because Ca is a Group 2 metal, it will typically lose two electrons to form 2+ cations. Thus our answer is consistent with our knowledge of chemical reactivity, yay!
Therefore, the ionic compound Ca(MnO4)2 contains one Ca2+ cation and two MnO4− anions.
It is often useful to be able to break down an ionic compound into the constituent ions. One example of such a situation is when an ionic compound is dissolved in water for a reaction. Many ionic compounds are water-soluble, and the constituent ions dissociate when in solution. The dissociated ions can then go off and participate in their own chemistry independently of the other ions in the original ionic compound.

Conclusion

Just as ions are formed when neutral atoms gain or lose electrons, a polyatomic ion is formed when a neutral molecule gains or loses electrons. Therefore, a polyatomic ion is a group of covalently bonded atoms that carries a net charge due to the fact that the total number of electrons in the molecule is not equal to the total number of protons in the molecule. In the Lewis dot structure of a polyatomic ion, the sum of the formal charges on all the atoms must equal the net charge on the ion.
Being familiar with the most common polyatomic ions will be helpful for recognizing ionic compounds and predicting their reactivity. While learning all the polyatomic ions can seem daunting, there are patterns to the formulas, names, and charges of many ions. These patterns can be learned, so you don't have to simply memorize all the ions.
Bonus: Once you know the polyatomic ions, you can play the game, "Find the polyatomic ion," which involves looking for examples of compounds that contain polyatomic ions in everyday life. We gave examples of baking soda and chalk in this article. Can you find more? Hint: check out the ingredients list of your lotion, shampoo, or toothpaste. Feel free to post others in the comments below!

Try it!

Problem 1
What is the chemical formula for lithium hydrogen phosphate?
Choose 1 answer:

Problem 2
What is the name of the compound with the chemical formula (NH4)2S ?
Choose 1 answer:

Problem 3
What are the constituent ions in the ionic compound FeCr2O7?
Specify the formula, charge, and number of each ion.
Choose 1 answer:

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