Writing the formula of a molecular compound follows the same process as writing its name. Simply use the prefixes to identify how many of each element are present in that compound and then write that number as a subscript. The most common prefixes are shown in this table.
| Number of atoms | Prefix |
|---|---|
| 1 | mono- |
| 2 | di- |
| 3 | tri- |
| 4 | tetra- |
| 5 | penta- |
| 6 | hexa- |
| 7 | hepta- |
| 8 | octa- |
| 9 | nona- |
| 10 | deca- |
Look at this example
Dihydrogen monoxide (DHMO) might sound like a dangerous chemical, but it is actually a common substance used in everyday life. Use what you learned about prefixes to determine the formula for the “DHMO” molecule, dihydrogen monoxide.
The formula for dihydrogen monoxide is H2O (aka water). This compound contains two atoms of hydrogen (the prefix “di-” means two) and one atom of oxygen (the prefix “mono-” means one).
Tips for Naming Molecular Compounds:
When using these prefixes, keep in mind that the first element in the formula never uses the prefix “mono-.” For example, the compound NO is named nitrogen monoxide. Notice the first element does not use the “mono-“ prefix.
Also, when the final letter in the prefix begins with the letter a or the letter o, drop the final letter in the prefix so the name flows better. For example, monoxide flows better and is easier to say than mono-oxide.
Let's Practice
Complete this activity to practice writing the names and formulas of molecular compounds. For each formula, identify the name of that molecule and for each name, identify the formula of that molecule. Click each molecule to check your answer.
If you need a periodic table, click below to open an interactive periodic table or to download a PDF.
NF3
The name of NF3 is nitrogen trifluoride.
P4S5
The name of P4S5 is tetraphosphorus pentasulfide.
B6Si
The name of B6Si is hexaboron monosilicide.
dichlorine pentoxide
The formula of dichlorine pentoxide is Cl2O5.
Notice how the “a-“ of “penta-“ is dropped because oxide begins with the letter o.
sulfur tetrachloride
The formula of sulfur tetrachloride is SCl4.
phosphorus triiodide
The formula of phosphorus triiodide is PI3.
