When we are first learning about elements and compounds, we typically just say the letters out loud. So "LiCl" is said as "el-eye-see-el". But what about when we get to more complex compounds? If you try and say Ca3(PO4)2 out loud as "see-ay-three-pee-oh-four-two" it's a bit of a mouthful.
Chemists have set rules to follow when naming, so when we see Ca3(PO4)2, we just say "calcium phosphate", which is a bit easier. In this article, we will be learning the rules for naming ionic compounds and then applying them.
- This article is about naming ionic compounds
- First, we will cover the basic rules
- Next, we will talk about the naming conventions for polyatomic ions
- Then, we will summarize the rules in a flowchart
- Thereafter, we will practice using these rules
- Lastly, we will cover the basics of naming covalent compounds to see the difference between those rules and those for ionic compounds.
Naming Ionic Compounds Rules
Before we discuss the naming rules for ionic compounds, let's first cover what an ionic compound is.
An ionic compound is a compound where a positively charged ion called a cation and a negatively charged ion called an anion are bonded together in an ionic bond. These bonds are usually between a metal and non-metal
When writing an ionic compound, the cation is written first and the anion is written second. The general rule of naming ionic compounds is pretty simple. The rule is: "name of cation" + "name of anion + -ide". So, for NaCl, it would be sodium chloride. While this is the basic format, there are some other rules we need to follow. One example is a cation that can have several charges. For example, iron (Fe) usually has a charge of +2. So if I said, "iron oxide", I haven't specified the charge for the ion, which makes determining the formula very difficult. Is it FeO or Fe2O3?When a species can have multiple charges (typically a transition metal), we specify the charge using Roman numerals. For example, if I am talking about FeO, I would write "Iron (II) oxide". However, if I was talking about Fe2O3, I would write "Iron (III)" oxide.
While using Roman numerals is the modern way to indicate charge, there is another way to do it.
Instead of writing the charge, we use different suffixes to indicate the charge. This system is not standard, but it is used widely enough to keep an eye out for it.
Here is a table with some common ion names:
Fig.1-Table with some common metallic ion names
Naming Ionic Compounds with Polyatomic Ions
Now, let's talk about the rules for polyatomic ions.
A polyatomic ion is an ion composed of two or more types of atoms
Polyatomic ions can be cations or anions. When it comes to naming compounds with polyatomic ions, we simply just write the name of the ion.
For example, NaNO3 is "Sodium nitrate" since Na is sodium, and the NO3- ion is nitrate.
| Ion | Name | Ion | Name |
| NH4+ | Ammonium | SCN- | Thiocyanate |
| NO3- | Nitrate | ClO4- | Perchlorate |
| SO42- | Sulfate | Cr2O7- | Dichromate |
| OH- | Hydroxide | MnO4- | Permanganate |
| CN- | Cyanide | H3O+ | Hydronium |
| SO32- | Sulfite | CO32- | Carbonate |
Polyatomic ions that contain an element + one or more oxygen are called oxoanions.
The prefix/suffix of the ion name is dependent on the relative number of oxygen, as follows:
- More oxygen: per --root--ate (Ex: perchlorate ClO4-)
- Standard oxygen: root--ate (Ex: chlorate ClO3-
- Less oxygen: root-ite (Ex: chlorite ClO2-)
- Least oxygen: hypo --root-ite (Ex: hypochlorite ClO-)
The naming is in comparison to whatever ion has the -ate ending
For example, SO42- is sulfate, and it has 4 oxygens. However, ClO4- is perchlorate. This is because Sulfur (S) and oxygen only form two ions (SO3- and SO42-), while chlorine (Cl) and oxygen form four ions.
Flow Chart for Naming Ionic Compounds
As a summary of what we've learned, here's a handy flow chart for naming ionic compounds:
Fig.2-Flow chart for naming ionic compounds
Naming Ionic Compounds Practice
Now that we've covered the rule, let's put them to use and look at some examples to help you practice what you have just learned!
Name the following ionic compounds:
a) Na2O b) Al(OH)3 c) CaSO4 d) CuI e) (NH4)2CO3
a) Both Na and O are monoatomic. While there are two sodium (Na) atoms, polyatomic only refers to multiple types of atoms, not multiples of one. Sodium has one possible charge (+1), so the name of this compound is:
"Sodium oxide"
b) While aluminum is monoatomic, OH is polyatomic. Looking at our chart OH is called "hydroxide". Aluminum only has one charge (+3), so the name of this compound is:
"Aluminum hydroxide"
c) Like with the previous example, we have a cation with only one possible charge (calcium, which is +2), and a polyatomic anion. The name of SO4 is sulfate, so the name of this compound is:
"Calcium sulfate"
d) Both of our ions are monoatomic, however, copper (Cu) can have multiple charges. Iodine (I) has a charge of -1 (all halogens/group 17 have -1 charges), so copper should have a charge of +1 to balance. Since copper can have multiple charges, we need to indicate the charge with a Roman numeral. Therefore, the name of the compound is:
"Copper (I) iodide"
If we were to follow the common naming system, the name would be:
"Cuprous iodide"
e) Here, both of the ions are polyatomic, so we just combine the names of the polyatomic ions. Therefore, the name of this compound is:
"Ammonium carbonate"
Now that we've named a few compounds, let's do the reverse and write the formula to the name:
Write the chemical formula that corresponds to the name of the ionic compound:
a) Lithium chloride b) Sodium perchlorate c) iron (II) iodide d) aluminum carbonate
a) When we write formulas from the name, it's important to know the common charges of elements. Lithium (Li) has a charge of +1, and chlorine (Cl) has a charge of -1. Since it would take one of each to balance the charges, the formula is:
LiCl
b) Perchlorate doesn't follow the "name+-ide" formula, which tells us that it is a polyatomic ion. The formula for perchlorate is ClO4-. Sodium (Na) has a charge of +1, so there is a 1:1 of cation to anion for charge balance. This means the formula is:
NaClO4
c) Iodine (I) has a charge of -1, while we are told that iron (Fe) has a charge of +2. This means that we need two iodine to balance the charge of iron, so the formula is:
FeI2
d) Carbonate is a polyatomic ion, whose formula is CO32-. Aluminum's common charge is +3. This means we need 2 aluminum atoms per 3 carbonate molecules to balance out the charge. Therefore, the formula is:
Al2(CO3)2
As an aside, pay close attention to the suffixes of the polyatomic ions. It can be easy to mix up words like nitrite (NO2-) and nitrate (NO3-).
Naming Ionic and Covalent Compounds
Let's finish off by looking at how covalent compounds are named.
Covalent compounds are compounds containing two or more non-metals bonded by a covalent bond,
When naming simple (two-element) covalent compounds, we follow similar rules: 1) The first element is simply its name 2) The second element is its name + -ide.Looks just like ionic compounds, right? However, there is another step that sets these two apart
3) Write the numbered prefix to specify the number of atoms
-If there is only one of the first element, the "mono" is left out
Below is a list of these prefixes:
| Number of atoms | Prefix | Number of atoms | Prefix |
| 1 | mono- | 6 | hexa- |
| 2 | di- | 7 | hepta- |
| 3 | tri- | 8 | octa- |
| 4 | tetra- | 9 | nona- |
| 5 | penta- | 10 | deca- |
Here are some examples:
ClF3 - Chlorine trifluoride
N2O5- Dinitrogen pentoxide
SF6- Sulfur hexafluoride
Pretty simple right? The main difficulty here is remembering what is ionic and what is covalent. An easy trick is to look at your periodic table.
Any compounds that are made of one element on the left side of the table (excluding hydrogen) and one on the right side is ionic. Since species on the left are metals and on the right past the metalloids or "staircase" elements (B, Si, Ge,As, Sb,Te) are non-metals.
Compounds that are made up of only "right-side" elements (and hydrogen) are covalent compounds.
Naming Ionic Compounds - Key takeaways
- An ionic compound is a compound where a positively charged ion called a cation and a negatively charged ion called an anion are bonded together in an ionic bond. These bonds are usually between a metal and non-metal
- The general rule of naming ionic compounds is pretty simple. The rule is:"name of cation" + "name of anion + -ide"
- For cations with multiple possible charges, we write the charge in Roman numerals
- For polyatomic ions, we write the name of the ion (no -ide for anions)
- For covalent compounds, the steps are:
- The first element is simply its name
- The second element is its name + -ide
- Add numbered prefixes to specify the number of atoms (mono- is not included for the first element)
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