Naming Ionic Compounds: Simple Binary, Transition Metal & Polyatomic Ion Compounds

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Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

An important part of dealing with chemical compounds is knowing how to refer to them. Learn to name all ionic compounds, including simple binary compounds, compounds containing transition metals and compounds containing polyatomic ions.

Naming Binary Ionic Compounds

Learning to name ionic compounds is both easy and hard depending on the complexity of the compound. Before we start, though, I just wanted to review a few terms. Remember that positively charged ions are called cations. Negatively charged ions are called anions. An ionic compound is a compound held together by ionic bonds. A simple binary compound is just what it seems - a simple compound with two elements in it.

Binary compounds are easy to name. The cation is always named first and gets its name from the name of the element. For example, K+ is called a potassium ion. An anion also takes its name from its element, but it adds the suffix -ide to it. So Cl- is called a chloride ion; O2- is an oxide ion.

Take the binary compound NaCl. The Na+ is a sodium cation. The Cl- is a chlorine anion, which gets the suffix -ide added to it. When you put them together, it becomes sodium chloride.

Here are some examples for you.

Zn2+ is zinc. S2- is sulfide. Put them together for zinc sulfide (ZnS).

Here's another.

K2O is potassium oxide

Naming Ionic Compounds Containing Transition Metals

If an ionic compound contains a transition metal, use Roman numerals to denote which ion it is
Ionic Compounds With Transition Metals

A transition metal is a metal that can use the inner shell before using the outer shell to bond. These are the elements in the middle of the periodic table, things like zinc and iron and copper. Naming polyatomic ionic compounds that have transition metals in them is also fairly easy. It follows the same naming rules as the simple binary compounds but with an extra rule added in, so you still name the cation first, followed by the anion with the suffix -ide added to the end of it.

The new rule is that transition metals form more than one ion, so this has to be accounted for in the naming. We do this by using Roman numerals to denote which ion it is. The Roman numeral will equal the charge on the ion. For instance, Fe2+ is iron (II). Fe3+ is iron (III).

When compounds are formed with these metals, the different ions still have to be accounted for. If I told you the compound was iron chloride, that wouldn't give you the full story. You wouldn't know if it was iron (II) or iron (III), which means you don't know how many chlorine atoms are in the compound to bond with the iron, since two chlorines would be needed for iron (II) and three for iron (III). If I instead told you that the compound was iron (II) chloride, you would know that it was Fe 2+ in there, which means you have two chlorine atoms bonding with it. The formula would be FeCl2. If I said it was iron (III) chloride, the formula would be FeCl3.

If I told you I had Ag2S, you would know I had silver (I) sulfide. If I gave you FeF2, you would know it was iron (II) fluoride.

Naming Polyatomic Ionic Compounds

A polyatomic ionic compound is a compound made up of a polyatomic ion, which is two or more atoms bonded together, and a metal. Naming polyatomic ions is harder, but doable. First, name the cation, which is just the name of the element. Next name the anion. This gets trickier.

Let's first talk about a polyatomic ionic compound that contains oxygen. If the polyatomic anion contains oxygen, it is called an oxyanion. If it can form more than one form of oxyanion, it gets a suffix of either -ate or -ite. There are some common oxyanions that most people memorize.

Oxyanion Number of forms
ClO 4
NO 2
SO 2

If an oxyanion can form only two different kinds of oxyanions, the name of the ion with the greater number of atoms ends in -ate and the smaller number of atoms ends in -ite.

For example:

NO2- is nitrite, with the ending -ite

NO3- is nitrate, with the ending -ate

SO32- is sulfite, with the ending -ite

SO42- is sulfate, with the ending -ate

If the ion forms four different kinds of oxyanions, then they get a prefix along with their suffix. The first and lowest size ion gets hypo- as its prefix. The largest size ion gets per- as its prefix.

These look like this.

ClO- is hypochlorite. It has the hypo- prefix and -ite suffix because it is the smallest.

ClO2- is chlorite with only the -ite suffix.

ClO3- is chlorate with only the -ate suffix

ClO4- is perchlorate. It has per- for the prefix and -ate for the suffix because it is the largest.

Notice they have the name of the central element plus the suffix or prefix.

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