Lesson 6 – Polyatomic Ions


  • to predict chemical formulas
  • to be able to write the names of ionic and molecular compounds


  • 1 class


1. What is a binary compound? Give an example of one.

2. How do molecular compounds form? What types of elements form molecular compounds?

3. How do ionic compounds form? What types of elements form ionic compounds?

4. Are the following compounds ionic or molecular?

Background Information:

Remember, binary compounds only have 2 elements in them. It can be a bond between a metal and a non-metal (ionic), or a bond between two non-metals. However, many compounds can have more than 2 elements. We call these compounds polyatomic (poly = many).

A polyatomic ion is a group of atoms that have a covalent bond. BUT they still have an overall negative or positive charge.

These are not a compound, because compounds are overall neutral.

Ex: NO3 nitrate

Nitrate dot diagram

You can see from the dot diagram that covalent bonds form between the oxygens and the nitrogen. However, the nitrogen and the oxygen on the bottom left don’t have a full outer valence! Instead of becoming a neutral compound, this group becomes a charged particle that seeks out the last electron needed to fill that valence. It becomes a polyatomic ion!

What type of element gives away electrons?

This ion will form an ionic bond with a metal, and rip away the metal’s valence electron in order to complete the valence.

Polyatomic ions are a mixture of both ionic and covalent bonds!

More information, including a nice table of polyatomic ions, can be found here. Notice that polyatomic ions have their own names, they don’t follow the IUPAC rules discussed in lesson 5, although they do have a pattern.

You can write chemical formulas of compounds that have a polyatomic ion just the same way you would write the chemical formula of a binary ionic compound.


1. Write the name of the metal first.

2. Write the name of the polyatomic ion. DO NOT change the ending to “-ide”

Ex. CaCO3

Ca = Calcium

CO3 = Carbonate

The compound is called: Calcium Carbonate

Chemical Formulas:

This is the same as making an ionic compound. Polyatomic ions are considered one entity. If you need more than one polyatomic ion, you must put brackets around the entire ion.

Ex 1: Ammonium Nitrate

1. Put the symbol of the first thing listed (note here, the first entity is another polyatomic ion! This is ok, as long as it is positively charged).

2. Put the symbol of the polyatomic ion.

3. Check the ion charged, and use the swapping method.

Ammonium = NH4+

Nitrate = NO3

Their ionic charges are equal. For every one ammonium, you have one nitrate, so we don’t need to write the one’s in.

It becomes: NH4 NO3

Ex. 2: Aluminum Nitrate

1. Put the symbol of the metal down

2. Put the symbol of the polyatomic ion

3. Check ion charges, and use the swapping method. Use brackets to keep the polyatomic ion together.

Aluminum = Al3+

Nitrate = NO3

Aluminum has 3 electrons in needs to give up. That means, for every 1 aluminum, you need 3 nitrates in order to make the compound neutral.

It becomes: Al(NO3)3

*Note* See what the brackets mean? There are 3 nitrogens and 9 oxygens in this compound!

Aluminum Nitrate diagram


1. Identify the binary compounds in this list:

LiCl, Sodium hydrogen carbonate,  CaI2, H2O, NaNO3

2. Check if these chemical formulas are correct. If they are not, correct them. Lastly, write the name of each formula, both correct and incorrect.

a) NaSO4

b) CaS

c) Al3O2

d) (NH4)3N

3. These are compounds with transition metals (can use more than one ion charge). Which form of the metal ion is being used? Write the name of the compound, use brackets and roman numerals (I, II, III, IV) to show which metal ion to use.

a) FeF3

b) PtS

c) Cu2O

d) MnO2

4. These compounds have been improperly named. Fix the name, and state why it was wrong.

a) Fe2O3, Diiron Trioxide

b) KCl, Potassium (I) Chloride

c) NiNO3, Nickel Nitrogen Oxide

d) SO2, Sulfur (IV) Oxide

Need more practice? Try these naming worksheets: Naming worksheet 2, Naming worksheet 3 from this General Chemistry website.

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One Response to “Lesson 6 – Polyatomic Ions”

  1. many thanks for sharing

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