Lesson 5 – Naming Binary Compounds


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


  • 1 class


1. Describe the difference between a covalent and ionic bond.

2. What is the difference between an ion and an atom?

Background Information:

Binary Compounds

What does this mean? We know that chemically, a compound is made up of 2 or more elements, and compounds form the majority of things around us (the rest being pure elements). What about the word binary? Think of other words that start with the “bi” prefix. Bicycle. A bicycle has 2 wheels. The prefix “bi” means 2. So, a binary compound has 2 elements that make it up.

There is a system to name binary compounds. It’s called the IUPAC (International Union of Pure and Applied Chemistry) rules. Internationally, binary compounds are called the same things. What advantage might this give to scientists and researchers?

Binary Molecular Compounds

Remember, a molecular compound forms when two non-metals share electrons in covalent bonds.

Naming Rules

1. The element closest to the metal section on the periodic table (furthest to the left) is named first

2. Change the ending of the second element to “-ide,” for example: oxygen –> oxide

3. You need to say how many of each element there are. Use a prefix: mono = 1, di = 2, tri = 3, tetra = 4, penta = 5

4. If the first element is singular, you do not need to put “mono” in front.


NO = Nitrogen monoxide

H2O = Dihydrogen monoxide

Binary Ionic Compounds

Remember, these compounds consist of a metal and a non-metal (or a positive and negative ion).

Naming Rules:

1. The metal always goes first.

2. Change the ending of the non-metal to “-ide”

3. If there is more than one ionic charge (ex. iron), show which ionic charge is being used by putting the roman numeral in brackets.


Magnesium chloride

Iron (II) oxide

Writing Chemical Formulas

Molecular Compounds

These ones are easy. Write the symbol of the element, followed by the number indicated by the prefix. Do so again for the second element.

Ex. Sulfur trioxide –>SO3

Ionic Compounds

These are a little more complex, because ionic compounds show the ratio of positive to negative ions, not the amount per molecule. For example, Sodium Chloride (NaCl) has a 1:1 ratio of sodium atoms and chlorine atoms.

However, there is a handy little trick to help you through this. I call this the “swapping” method. 

Step 1: Write out the symbols for your two ions. Write the ionic charges above and to the right of each one (shown in the red and blue circles). In this case, iron is using an ionic charge of 3+, and oxygen has an ionic charge of 2-.

Step 2: Follow the arrows. The ionic charge of the metal moves to the bottom right of the non-metal. The ionic charge of the non-metal moves to the bottom right of the metal. They swap places.

Step 3: Write out your ionic formula. This means that for every 2 iron atoms, you will find 3 oxygen atoms.

Self Check:

1. Write the names for the following molecular compounds:

a) PO5

b) SO2

c) N2O

d) S3I4

2. Write the chemical formulas for the following molecular compounds:

a) Carbon Tetrachloride

b) Diphosphorous moniodide

c) Nitrogen Tribromide

d) Trisulfide Pentafluoride

3. Write the names for the following ionic compounds:

a) LiCl

b) CaBr2

c) Sr3P2

d) Al2O3

4. Write the chemical formulas for the following ionic compounds:

a) Potassium Sulfide

b) Magnesium Oxide

c) Sodium Nitride

d) Beryllium Iodide

5. Molecular compounds can form in many different ways. In fact, two elements, like Nitrogen and Oxygen can form many different compounds. For example, they can form nitrogen monoxide and nitrogen dioxide. However, ionic compounds will always form one way. Sodium and chloride will always form a 1:1 ratio. Why do you think this is so? Explain your thoughts. (Hint: think about the nature of the bonds)

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