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
, , , 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.
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,
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.
For example, let's consider the polyatomic ion
, 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 and . The dots around 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,
, help maintain the pH level of our blood, while phosphates, , 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.
* 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
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:
- 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.
- 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
and that it carries a 1- charge. We will need two hydroxide ions to exactly cancel the 2+ charge on . When writing out the formula, we include parentheses around followed by a subscript of 2, to make it clear that there are two hydroxide ions for every cation. Thus, the chemical formula for the compound is .
Concept check: Why would
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
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:
If we rearrange this equation, we see that the sum of the charges from the cations must be equal to
times the sum of the charges from the anions.
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:
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
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
, 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
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 in order to balance out the 2- net charge from the two ions.
To check our reasoning, we can ask ourselves, "Does calcium typically form cations with a 2+ charge?" Because
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
contains one cation and two 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.
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!
Want to join the conversation?
- Where are the 10 electrons in the hydroxide? wouldn't it have 8, 6 in lone pairs and two in the covalent bond?(50 votes)
- Do not think about the dot structure for this.
Now, we know that oxygen has 8 protons & 8 electrons. And we also know that hydrogen has 1 electron.
The extra electron or the minus one sign at the top left of a hydroxide ion also contributes to the total number of electrons in the ion.
How we get this extra electron is,in the water molecule, H-O-H, one of the hydrogen's break their bond with the central oxygen atom, to become a proton, meaning they lose their only electron to oxygen & become a H+ ion. This happens due to ionisation of water molecules.
Now what is left behind is a H+ ion with no electrons & only a single proton and an OH- ion with an extra electron that was donated by the hydrogen.
Now, coming back to the question, the OH- ion will have: 1 hydrogen with 1 electron, 1 oxygen with 8 electrons and 1 extra electron. When you add these up, you get, 1 + 8 + 1 = 10 electrons in total in a hydroxide ion.
Hope this helps.(95 votes)
- how are polyatomic ion formed?(43 votes)
- Polyatomic ions are formed through covalent bonds, most especially coordinate covalent bonds.(53 votes)
- How many polyatomic ions are there in total (accurately)?(10 votes)
- Practically unlimited, even though there are some that are more and some that are less common. There is nothing preventing you from making more polyatomic ions(27 votes)
- I am totally confused with the anions and cations and how to do them and all that basically I have no clue can some one explain it a little simpler please! Any help would be greatly appreciated
- Cations are positively charged, with less electrons than the number of protons (overall positive charge).
Anions are negatively charged, with more electrons than protons (overall negative charge).
Hope this helps! :)(20 votes)
- The last hint of the last question contains a typo, the ion must be iron (ll) to balance out the 2- charge of Dichromate. It shows 2+ charge on Dichromate.(13 votes)
- Why is mercury (and oxygen) considered polayatomic? Does it bond to itself?(10 votes)
- Mercury and oxygen aren't considered polyatomic ions by themselves, but in the mercury (I) Hg2+2 and peroxide ions O2-2 you indeed have a bond between the same atoms.(7 votes)
- Is there an error in problem 1? Isn't it supposed to be Li_2(HPO_4) instead of Li_2HPO_4?(2 votes)
- You don’t have to put parentheses around a polyatomic ion if there is only one of them in the formula unit.
Thus, you write LiH₂PO₄ but Ca(H₂PO₄)₂.(17 votes)
- Why are these things called polyatomic ions?
At the top of the article, it says poly- means many, but atomic? These things are made mostly of ions, are they not?
Why aren't they called polyionic ions?(3 votes)
- Polyatomic ions are groups of atoms covalently bonded together, but the group of atoms as a whole has a charge. They aren't made of multiple ions.(8 votes)
- In the paragraph about poly atomic ions, does all the atoms in the ion need to be negative or positive for the entire to be that charge?(4 votes)
- No, the charge on the ion is the sum of all the + and - charges on all the atoms in the ion.
Usually, only one atom has the charge and the other atoms have no charge.
At this stage, all you have to know is the overall charge on the ion.(8 votes)
- Very basic question. If Cl gains 1 electron isn't it now fully satisfied having 8 electrons in its outer shell and hence no likely to interact with another atom but rather be content as a noble gas?(3 votes)
- But now it has a negative charge, so it's going to attract a positive charge, right? That's a big difference versus noble gases, which are neutrally charged when they have their full valence shell.
And that's how you get salt: a Cl- ion finds an Na+ ion and you have NaCl.(6 votes)