How to Write Formulas for Binary Molecular Compounds

• Look at the first word of the name of the molecular compound.

• Identify the prefix. The prefix represents the number of atoms of that element in the compound. Here is a list of prefixes and their meanings.

mono- = 1 hexa- = 6di- = 2 hepta- = 7tri- = 3 octa- =8tetra- = 4 nona- = 9 penta- = 5 deca- = 10

SPECIAL CASE – For the first element, you could find an element without a prefix. In this case, there is only one of this element in the compound.

• Identify the element. Using your periodic table, find the symbol for that element.

• For the first part of the formula, you will write the symbol followed by the subscript to represent the number of the first element in the compound.

• Look at the second word of the name.

• Identify the prefix. The prefix represents the number of atoms of that element in the compound.

• To identify the element, you will want to drop the –ide ending that has been added and look for a nonmetallic element that has a similar root.

• After the element symbol and subscript for the first element, write the subscript after the symbol for the second element.

1

Let’s Look at an Exampledinitrogen tetroxide

• Look at the first word of the name of the molecular compound.

dinitrogen

• Identify the prefix.

dinitrogen

Di- means 2.

• Identify the element. Using your periodic table, find the symbol for that element.

• For the first part of the formula, you will write the symbol followed by the subscript to represent the number of the first element in the compound.

N2

2

Let’s Look at an Exampledinitrogen tetroxide

• Look at the second word of the name. tetroxide

• Identify the prefix. The prefix represents the number of atoms of that element in the compound. Here is a list of prefixes and their meanings.

tetroxide

Sometimes, the “a” at the end of a prefix will be dropped if the element’s name begins with a vowel.

Tetra means 4.

• To identify the element, you will want to drop the –ide ending that has been added and look for a nonmetallic element that has a similar root.

oxide

• After the element symbol and subscript for the first element, write the subscript after the symbol for the second element.

N2O4

Oxygen or O

Hydrogen, the yellow

elements, and the green

elements are all nonmetals.