Oxoanions — Overview & Nomenclature

One of the most common classes of polyatomic ions is oxoanions. Sometimes you will see oxoanions called oxyanions. Oxoanions are anions formed when an element forms a covalent bond with oxygen. Oxoanions must have covalent bonds. The alkali and alkaline-earth metals form ionic bonds with oxygen. They do not form oxoanions. The other groups of the periodic table do from oxoanions. We know of transition metal, metalloid, nonmetal, and halogen oxoanions. The molecular formula for an oxoanion is $E_{x}$$O_{y}^{z-}$. In this formula, E is an element. The O is oxygen. The x and y are the numbers of atoms in the molecule. The z is the charge of the oxoanion.

Naming Oxoanions

One of the most difficult parts of learning the oxoanions is naming them. Here are the rules for naming oxoanions.

1. The most common oxoanion has the suffix -ate. Unfortunately, knowing the most common oxoanion is something you have to memorize.
2. If you remove one oxygen from the -ate oxoanion, you change the suffix to -ite.
3. If you add one oxygen to the -ate oxoanion, you add the prefix per-.
4. If you remove one oxygen from the -ite oxoanion, you add the prefix hypo-.

Let's look at the chlorine series as an example.

1. The most common chlorine oxoanion is chlorate. Chlorate has the formula $Cl$$O_{3}^{-}$.
2. Removing one oxygen from chlorate is chlorite. Chlorite has the formula $Cl$$O_{2}^{-}$.
3. Adding one oxygen to chlorate is perchlorate. Perchlorate has the formula $Cl$$O_{4}^{-}$.
4. Removing one oxygen from chlorite is hypochlorite. Hypochlorite has the formula $Cl$$O^{-}$.

What Oxoanions form?

Not all elements form a series of oxoanions. Below are some general guidelines. The guideline will help you to determine which elements form a series of oxoanions.

1. The elements in periods three and four, only form one oxoanion. Because it is the only one, it has the -ate suffix. For example, there is only borate and carbonate. Sadly for Star Wars fans, carbonite does not exist.
2. Nitrogen only forms nitrate and nitrite. Both are very common.
3. Sulfur and phosphorous both form a series of oxoanions. The sulfate and phosphate series are common.
4. The halogens can from the full series of oxoanions. See the above example of chlorine.
5. Many of the transition metals can form series of oxoanions. When looking at the transition metals remember to make sure the bond is covalent. A few of the transition metals form ionic bonds with oxygen.
6. The transition metals do not always follow the naming conventions. Again, unfortunately, you have to memorize the exceptions.

Chemistry of the Oxoanions

You may have wondered why some oxoanions exist. For example, we would not expect oxygen and chlorine to react. Both are diatomic elements. Oxygen has a stable double bond. Chlorine only has a single bond. But, chlorine has a high electronegativity. Chlorine's single bond is stable. The reaction of oxygen and chlorine gas is endothermic. To make the halogen oxoanions, you need to add energy. Many of the reactions need an additional catalyst.

Most of the oxoanions obey the octet rule. Some are exceptions to the octet rule. All the oxoanions have a negative charge. When we are applying the octet rule we have to consider the negative charge. Some of the atoms will fill their valence shell with the extra electron. The best way to know which atom has the extra electron is to draw the lewis dot diagram. Because of the negative charge they often bond with the alkali, alkaline-earth, and transition metals. When bonded to a metal they form salts. Hydrogen ions can also balance the negative charge. When an oxoanion is bonded to hydrogen it is an oxoacid. Many of the known strong acids are oxoanion acids. For example, sulfuric acid and phosphoric acid have the oxoanions sulfate and phosphate.

In an oxoanion series, the oxidation state of the central atom always changes by two. The change of two is because oxygen is added or removed. Oxygen always has a negative two oxidation state. The oxoanions are strong oxidizing agents. In a redox reaction, they will give their electrons. Because they are strong oxidizing agents, some oxoanions are used for household cleaners. For example, bleach is sodium hypochlorite. A bottle of bleach even has a warning about it being a strong oxidizer.

(Video) How to Memorize The Polyatomic Ions - Formulas, Charges, Naming - Chemistry

by The Organic Chemistry Tutor

You need to know the polyatomic ions. But, they don't have a standard naming system. What can you do? Unfortunately, you have to memorize them. The oxoanions are particularly challenging to remember. In this video, The Organic Chemistry Tutor will help you learn all the polyatomic ions! He does a great job explaining the patterns in the oxoanions. They appear first in the video. But, if you need help with the other polyatomic ions, keep watching.