Lesson 6 Molecular Compounds - Naming and Formulas
Completion requirements
Writing Formulas for Molecular Compounds
Emissions from vehicles have been shown to have negative effects on public health and the natural environment. What is in those emissions?
B6.5 Emissions from a vehicle
Many vehicle emissions are molecular compounds, such as carbon monoxide, carbon dioxide, nitrogen monoxide, nitrogen dioxide, and several sulfur oxide compounds. In this part of the lesson, you are going to learn how to write chemical formulas for molecular
compounds.
When writing formulas, the prefix indicates the number of atoms of that element and is written as a subscript after the element symbol. Remember that a subscript of 1 is not shown (similar to math, in that you would write x not 1x)
When writing formulas, the prefix indicates the number of atoms of that element and is written as a subscript after the element symbol. Remember that a subscript of 1 is not shown (similar to math, in that you would write x not 1x)
| Example 1 |
Example 2 |
|---|---|
 |
 |
Another guideline to remember is that unlike ionic formulas, molecular formulas are not reduced.
Work through the following examples to solidify your understanding of writing formulas for molecular compounds.
Examples
Each example has a video to go with it. To play the video, click on the play icon next to the example.
Identify the type of compound.
iodine—non-metal
fluoride—non-metal
Since two non-metals are bonded together, this is a molecular compound and prefixes will be used to determine subscripts in the formula.
fluoride—non-metal
Since two non-metals are bonded together, this is a molecular compound and prefixes will be used to determine subscripts in the formula.
Write the symbol for the first element; include a subscript to indicate the number of atoms. The subscript is determined by looking at the prefix.
iodine
No prefix means one atom.
I
No prefix means one atom.
I
Write the symbol for the second element; include a subscript to indicate the number of atoms. The subscript is determined by looking at the prefix.
heptafluoride
“Hepta” means seven fluorine atoms; “7” is written as a subscript after “F.”
F7
“Hepta” means seven fluorine atoms; “7” is written as a subscript after “F.”
F7
Watch this video to see a teacher work through this example. https://adlc.wistia.com/medias/wnrm70ivpz
Identify the type of compound.
nitrogen—non-metal
oxide—non-metal
Since two non-metals are bonded together, this is a molecular compound and prefixes will be used to determine subscripts in the formula.
oxide—non-metal
Since two non-metals are bonded together, this is a molecular compound and prefixes will be used to determine subscripts in the formula.
Write the symbol for the first element; include a subscript to indicate the number of atoms. The subscript is determined by looking at the prefix.
tetranitrogen
“Tetra” means four nitrogen atoms “4” is written as a subscript after “N.”
N4
“Tetra” means four nitrogen atoms “4” is written as a subscript after “N.”
N4
Write the symbol for the second element; include a subscript to indicate the number of atoms. The subscript is determined by looking at the prefix.
hexaoxide
“Hexa” means six oxygen atoms; “6” is written as a subscript after “O.”
O6
“Hexa” means six oxygen atoms; “6” is written as a subscript after “O.”
O6
Watch this video to see a teacher work through this example. https://adlc.wistia.com/medias/9tppiknh7y
Identify the type of compound.
phosphorus—non-metal
iodide—non-metal
Since two non-metals are bonded together, this is a molecular compound and prefixes will be used to determine subscripts in the formula.
iodide—non-metal
Since two non-metals are bonded together, this is a molecular compound and prefixes will be used to determine subscripts in the formula.
Write the symbol for the first element; include a subscript to indicate the number of atoms. The subscript is determined by looking at the prefix.
diphosphorus
“Di” means two phosphorus atoms; “2” is written as a subscript after “P.”
P2
“Di” means two phosphorus atoms; “2” is written as a subscript after “P.”
P2
Write the symbol for the second element; include a subscript to indicate the number of atoms. The subscript is determined by looking at the prefix.
tetraiodide
“Tetra” means four iodine atoms; “4” is written as a subscript after “I.”
I4
“Tetra” means four iodine atoms; “4” is written as a subscript after “I.”
I4
Watch this video to see a teacher work through this example. https://adlc.wistia.com/medias/9z6xatgemj
Some molecular compounds do not follow the simple rules we are learning here, such as
In future chemistry classes, you will look at some of these naming systems in more detail. In the meantime, you should be familiar with the formulas for these common substances.
C2H2—acetylene
NH3—ammonia
C2H5OH—ethanol
C 6H12O6—glucose
H2O2—hydrogen peroxide
NH3—ammonia
C2H5OH—ethanol
C 6H12O6—glucose
H2O2—hydrogen peroxide
H2S—hydrogen sulfide
CH4—methane
C3H8—propane
C12H 22O11—sucrose
H2O—water
CH4—methane
C3H8—propane
C12H 22O11—sucrose
H2O—water
In future chemistry classes, you will look at some of these naming systems in more detail. In the meantime, you should be familiar with the formulas for these common substances.
Read This
Please read pages 48 and 49 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on writing formulas for molecular compounds.
Remember, if you have any questions or you do not understand something, ask your teacher! Practice Questions
Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice
questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.- Provide the correct formula for each of the following compounds.
iodine pentaoxide disulfur decafluoride tetranitrogen monoxide pentasulfur hexanitride dibromine trioxide
iodine pentaoxide IO5 disulfur decafluoride S2F10 tetranitrogen monoxide N4O pentasulfur hexanitride S5N6 dibromine trioxide Br2O3