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How to Write Covalent Compound Formulas
 

Writing covalent compounds involves the use of Greek prefixes. They are listed in the table to the right. The prefixes are used to indicate the amount of each atom in the compound. The first element never uses the prefix mono-, but does use all others. If no prefix is used before the first element's name, there is only one of that element. The second element uses all prefixes.

Example #1: phosphorus pentachloride is written PCl
5
Example #2: dihydrogen monoxide is written H
2O

Step by step example: Write the formula for dinitrogen pentasulfide.

Description of Action
Action
1. Look at the first word of the compound's name. Identify the element name. Write the symbol for this element. N
2. If the first name of the compound has a prefix, write the number the prefix refers to as the symbol’s subscript. N2
3. Look at the second word in the compound's name and identify the element. Only the root of the element's name is used so the ending will be different. Write the symbol for the element referred to. N2S
4. Determine the number that the prefix of the second name refers to and write this number after the second symbol as a subscript. N2S5
Covalent Prefixes
1
mono-
2
di-
3
tri-
4
tetra-
5
penta-
6
hexa-
7
hepta-
8
octa-
9
nona-
10
deca-

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