1. Naming and Writing Formulas for Molecular Compounds
One milligram of gold is worth only about one cent, but one kilogram of gold is worth approximately $12,500. The correct prefix ( milli- or kilo-) makes quite a difference!
Prefixes are important in chemistry, too. The prefixes in the name of a binary molecular compound tell you its composition.

2. Carbon and oxygen combine to form carbon monoxide (CO) and carbon dioxide (CO2), but these two invisible gases are very different.
Carbon monoxide is an invisible, gaseous compound of carbon and oxygen. It is a toxic product of incomplete burning, such as occurs in automobile engines and faulty furnaces.

3. A prefix tells how many atoms of each element are present in one molecule.
Number
mono- 1
hexa- 6
di- 2
hepta- 7
tri- 3
octa- 8
tetra- 4
nona- 9
penta- 5
deca- 10
carbon monoxide (CO)
carbon dioxide (CO2)

4. Molecular Naming Rules:
Name the elements in the order listed in the formula.
Use prefixes to show the number of each atom.
The suffix of second element is -ide.
No prefix mono- for first element.
CCl4 N2O SO3 H2O

5. Quick Quiz!
1. Which of the following compounds is named INCORRECTLY?
CS2, carbon disulfide
BCl3, boron trichloride
IF7, iodine heptafluoride
PCl5, phosphorus hexachloride

6. Quick Quiz.
2. Which of the following molecular compounds is named INCORRECTLY?
AsCl3, arsenic trichloride
C2O5, dicarbon pentoxide
CF4, carbon tetrafluoride
H3As, hydrogen arsenide

7. Quick Quiz.
3. The correct formula for
tetraphosphorus trisulfide is
P3S4
S3P4
P4S3
S4P3