Lesson 6 Molecular Compounds - Naming and Formulas
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| Course: | Science 10 [5 cr] - AB Ed copy 1 |
| Book: | Lesson 6 Molecular Compounds - Naming and Formulas |
| Printed by: | Guest user |
| Date: | Sunday, 7 September 2025, 6:46 PM |
Introduction
How is naming and writing formulas for molecular compounds different from ionic compounds?
B6.1 Carbon dioxide
Molecular elements and molecular compounds are formed when non-metals bond by sharing electrons. Remember that these bonds are called covalent bonds. The system for naming and writing formulas has a slightly different set of guidelines than
the ionic compounds that you just learned about.
Targets
By the end of this lesson, you will be able to
-
write formulas for and name, using IUPAC rules, molecular compounds
-
write formulas for and name, using IUPAC rules, acids
Watch This
Covalent Bonding of Hydrogen, Oxygen, and Nitrogen @ YouTube FuseSchool – Global Education
;
This video reviews the concept of sharing electrons to form a covalent bond by examining the diatoms hydrogen, oxygen, and nitrogen. This will help get you in the right mindset for this lesson.
Molecular Elements
Did you know that oxygen is not a single atom but a molecule of two oxygen atoms bonded together?
You may have noticed that some of the examples of molecules contained just atoms of one element. These are molecular elements. Molecular elements are formed by non-metals when a more stable electron configuration can be achieved by two or more atoms
sharing electrons to attain an octet. There are seven common diatoms that you must memorize!
B6.2 Nurse adjusting oxygen tank for a patient
This concept will become very important when writing balanced chemical reaction equations that you will learn about later in this unit.
In addition to diatoms, there are a few elements that can form molecules with more than two atoms; these are known as polyatomic elements (this is different than polyatomic ions).
In addition to diatoms, there are a few elements that can form molecules with more than two atoms; these are known as polyatomic elements (this is different than polyatomic ions).
| Monatomic elements |
C(s), noble gases, all metals |
| Diatomic elements | H2(g), N2(g), O2(g), F2(g), Cl2(g), Br2(l), I2(s) |
| Polyatomic elements | P4(s), S8(s) |
Did You Know?
“Di” means two (you may recall that “bi” also means two).“Poly” means many.
You need to memorize the seven diatomic elements. Luckily, the diatoms make an interesting pattern on the periodic table. Below are three techniques you can use to help you remember their locations.
- Envision a hockey puck and hockey stick.
- It makes the number “7,” and there are seven diatoms (H is located separately).
- The diatoms are the “gens”—hydrogen, nitrogen, oxygen, and the halogens.
Read This
Please read page 48 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on naming molecular compounds. Remember, if
you have any questions or you do not understand something, ask your teacher! Naming Molecular Compounds
Did you know that the main constituent compound to make glass is sand?
B6.4 Hourglass with sand
Sand and glass are both composed of silicon dioxide (SiO2). Silicon dioxide is a molecular compound and can be recognized as such because it is composed of two non-metallic elements: silicon and oxygen. Notice that the name utilizes the prefix
“di.” This is one of the key differences between ionic nomenclature and molecular nomenclature. In molecular nomenclature, prefixes are used to indicate the number of atoms of each element in the molecule.
| Number of Atoms |
Prefix |
|---|---|
| 1 | mono |
| 2 | dri |
| 3 | tri |
| 4 | tetra |
| 5 | penta |
| 6 | hexa |
| 7 | hepta |
| 8 | octa |
| 9 | nona |
| 10 | deca |
Guidelines for naming molecular compounds:
Note: Usually the element that is located farthest to the left on the periodic table is listed first.
Note: Double vowels are not eliminated; e.g., tetraoxide, triiodide, etc., with the exception for monoxide.
- First element name remains unaltered.
- For the second element in the compound, the name ending changes to “ide.”
- Prefixes are added to indicate the number of atoms of each element in the compound. The one exception is that the prefix “mono” is not used when the first element has one atom. Prefixes are used in all other cases.
- Recall that compound names are not proper nouns, so no capital letters are used.
Note: Usually the element that is located farthest to the left on the periodic table is listed first.
Note: Double vowels are not eliminated; e.g., tetraoxide, triiodide, etc., with the exception for monoxide.
Examples
Work through the following examples to help solidify your understanding of molecular nomenclature. 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.
P—non-metal
Cl—non-metal
Since two non-metals are bonded together, this is a molecular compound and prefixes will be used in the name.
Cl—non-metal
Since two non-metals are bonded together, this is a molecular compound and prefixes will be used in the name.
For the second element in the compound, the name ending changes to end “ide.”
Cl is chlorine; change ending to “ide”—chloride.
Add appropriate prefix for first element.
Only one P atom, so there is no prefix—phosphorous.
Only one P atom, so there is no prefix—phosphorous.
Add appropriate prefix for second element.
«math»«msub»«mi»PCl«/mi»«menclose notation=¨circle¨ mathcolor=¨#FF0000¨»«mn mathcolor=¨#191919¨»3«/mn»«/menclose»«/msub»«/math»
There are three chlorine atoms, so use the prefix “tri”—trichloride
There are three chlorine atoms, so use the prefix “tri”—trichloride
Watch this video to see a teacher work through this example. https://adlc.wistia.com/medias/9hh6atyl0s
Identify the type of compound.
N—non-metal
O—non-metal
Since two non-metals are bonded together, this is a molecular compound and prefixes will be used in the name.
O—non-metal
Since two non-metals are bonded together, this is a molecular compound and prefixes will be used in the name.
For the second element in the compound, the name ending changes to end “ide.”
O is oxygen; change ending to “ide”—oxide.
Add appropriate prefix for first element.
There are two nitrogen atoms, so add the prefix “di”—dinitrogen.
There are two nitrogen atoms, so add the prefix “di”—dinitrogen.
Add appropriate prefix for second element.
There is one oxygen atom, so use the prefix “mono”—monoxide.
There is one oxygen atom, so use the prefix “mono”—monoxide.
Watch this video to see a teacher work through this example. https://adlc.wistia.com/medias/d16kqzmb9m
Identify the type of compound.
Si—non-metal
H—non-metal
Since two non-metals are bonded together, this is a molecular compound and prefixes will be used in the name.
H—non-metal
Since two non-metals are bonded together, this is a molecular compound and prefixes will be used in the name.
For the second element in the compound, the name ending changes to end “ide.”
H is hydrogen; change ending to “ide”—hydride.
Add appropriate prefix for first element.
There are two Si atoms, so used the prefix “di” —disilicon.
There are two Si atoms, so used the prefix “di” —disilicon.
Add appropriate prefix for second element.
There are six hydrogen atoms, so use the prefix “hexa”—hexahydride.
There are six hydrogen atoms, so use the prefix “hexa”—hexahydride.
Watch this video to see a teacher work through this example. https://adlc.wistia.com/medias/arwog1ambt
Identify the type of compound.
S—non-metal
O—non-metal
Since two non-metals are bonded together, this is a molecular compound and prefixes will be used in the name.
O—non-metal
Since two non-metals are bonded together, this is a molecular compound and prefixes will be used in the name.
For the second element in the compound, the name ending changes to end “ide.”
O is oxygen; change ending to “ide”—oxide.
Add appropriate prefix for first element.
Only one S atom, so there is no prefix—sulfur.
Only one S atom, so there is no prefix—sulfur.
Add appropriate prefix for second element.
There are two oxygen atoms, so use the prefix “di”—dioxide.
There are two oxygen atoms, so use the prefix “di”—dioxide.
Watch this video to see a teacher work through this example. https://adlc.wistia.com/medias/6o2pugbjrt
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 naming 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 IUPAC name for each of the following compounds.
P4S7 ClF5 N2O4 O3F2 Si3N4
P4S7 tetraphosphorus heptasulfide CIF5 chlorine pentafluoride N2O4 dinitrogen tetraoxide O3F2 trioxygen difluoride Si3N4 trisilicon tetranitride
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
Acidic Compounds
Did you know that one factor that affects the colour of hydrangea blooms is the acidity of the soil?
Acidic compounds are a special class of compounds with special properties and a unique naming process. Acid-forming compounds can be recognized by their chemical formula: H is written first in the formula, for example, HCl, H2SO4,
H
3PO4.
The process of naming acids is to first identify the name as if the substance was an ionic compound. Then convert that name into an acid name using a pattern.
The process of naming acids is to first identify the name as if the substance was an ionic compound. Then convert that name into an acid name using a pattern.
| Solutions of Compounds Named |
Classical Acid Name |
|---|---|
| hydrogen ___ide | hydro____ic acid |
| hydrogen ___ate | ____ic acid |
| hydrogen ___ite | ____ous acid |
Note: Acids that contain phosphorus have “or” added in for the acid name, and acids that contain sulfur have “ur” added in for the acid name.
| Formula | Solutions of Compounds Named |
Classical Acid Name |
|---|---|---|
| HCl | hydrogen chloride | hydrochloric acid |
| H3PO4 | hydrogen phosphate | phosphoric acid |
| H2SO3 | hydrogen sulfite | sulfurous acid |
Examples
Work through the following examples to gain a better understanding of acid nomenclature. 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.
Hydrogen written first and bonded to a non-metal indicates it is an acid-forming compound.
Provide a name for the compound based on previously learned rules.
hydrogen fluoride
Using the rules for naming acids, determine the classical acid name.
hydrofluoric acid
Watch this video to see a teacher work through this example. https://adlc.wistia.com/medias/kujlvc0jq7
Identify the type of compound.
Hydrogen written first and bonded to a polyatomic indicates it is an acid-forming compound.
Provide a name for the compound based on previously learned rules.
hydrogen perchlorate
Using the rules for naming acids, determine the classical acid name.
perchloric acid
Watch this video to see a teacher work through this example. https://adlc.wistia.com/medias/7j29kacbdi
Identify the type of compound.
Hydrogen written first and bonded to a polyatomic indicates it is an acid-forming compound.
Provide a name for the compound based on previously learned rules.
hydrogen chlorite
Watch this video to see a teacher work through this example. https://adlc.wistia.com/medias/uxeo1bv4wa
The acids you have studied thus far are all inorganic acids; however, there are also organic acids. The formulas of organic acids can be recognized by the ending “–COOH.” Examples:
ethanoic acid, also called acetic acid, and as a dilute solution called vinegar
CH3COOH
methanoic acid, also known as formic acid—the substance found in bee strings and ants
HCOOH
ethanoic acid, also called acetic acid, and as a dilute solution called vinegar
CH3COOH
methanoic acid, also known as formic acid—the substance found in bee strings and ants
HCOOH
To determine the formula of an acid, use the pattern to convert the acid name into an ionic name and write a proper formula taking charges on the ions into consideration.
Examples
Work through these examples to gain a deeper understanding of writing formulas for acids. Each example has a video to go with it. To play the video, click on the play icon next to the example.Using the rules for naming acids, determine the compound name.
nitrous acid = hydrogen nitrite
Write the ion symbols.
«math»«msup»«mi mathvariant=¨normal¨»H«/mi»«mo»+«/mo»«/msup»«mo»§#160;«/mo»«msubsup»«mi»NO«/mi»«mn»2«/mn»«mo»-«/mo»«/msubsup»«/math»
Determine the lowest common multiple of the two ions.
The lowest common multiple between 1 and 1 is 1.
Watch this video to see a teacher work through this example. https://adlc.wistia.com/medias/crfhnia1mo
Using the rules for naming acids, determine the compound name.
carbonic acid = hydrogen carbonate
Write the ion symbols.
«math»«msup»«mi mathvariant=¨normal¨»H«/mi»«mo»+«/mo»«/msup»«mo»§#160;«/mo»«mo»§#160;«/mo»«msubsup»«mi»CO«/mi»«mn»3«/mn»«mrow»«mn»2«/mn»«mo»-«/mo»«/mrow»«/msubsup»«/math»
«math»«msup»«mi mathvariant=¨normal¨»H«/mi»«mo»+«/mo»«/msup»«mo»§#160;«/mo»«mo»§#160;«/mo»«msubsup»«mi»CO«/mi»«mn»3«/mn»«mrow»«mn»2«/mn»«mo»-«/mo»«/mrow»«/msubsup»«/math»
Determine the lowest common multiple of the two ions.
The lowest common multiple between 1 and 2 is 2.
Determine the factor to multiple each ion by.
«math»«mtable columnspacing=¨0px¨ columnalign=¨right center left¨»«mtr»«mtd»«msup»«mi mathvariant=¨normal¨»H«/mi»«mo»+«/mo»«/msup»«mo»§#160;«/mo»«mo»§#215;«/mo»«mo»§#160;«/mo»«mn»2«/mn»«mo»§#160;«/mo»«/mtd»«mtd»«mo»=«/mo»«/mtd»«mtd»«mo»§#160;«/mo»«mn»2«/mn»«mo»+«/mo»«/mtd»«/mtr»«mtr»«mtd»«msubsup»«mi»CO«/mi»«mn»3«/mn»«mrow»«mn»2«/mn»«mo»-«/mo»«/mrow»«/msubsup»«mo»§#160;«/mo»«mo»§#215;«/mo»«mo»§#160;«/mo»«mn»1«/mn»«mo»§#160;«/mo»«/mtd»«mtd»«mo»=«/mo»«/mtd»«mtd»«mo»§#160;«/mo»«mn»2«/mn»«mo»§#8211;«/mo»«/mtd»«/mtr»«/mtable»«/math»
This will create a neutral compound.
The ratio of «math»«/math»«math»«msup»«mi mathvariant=¨normal¨»H«/mi»«mo»+«/mo»«/msup»«mo»§#160;«/mo»«mi»to«/mi»«mo»§#160;«/mo»«msubsup»«mi»CO«/mi»«mn»3«/mn»«mrow»«mn»2«/mn»«mo»-«/mo»«/mrow»«/msubsup»«/math» is 2:1.
This will create a neutral compound.
The ratio of «math»«/math»«math»«msup»«mi mathvariant=¨normal¨»H«/mi»«mo»+«/mo»«/msup»«mo»§#160;«/mo»«mi»to«/mi»«mo»§#160;«/mo»«msubsup»«mi»CO«/mi»«mn»3«/mn»«mrow»«mn»2«/mn»«mo»-«/mo»«/mrow»«/msubsup»«/math» is 2:1.
Watch this video to see a teacher work through this example. https://adlc.wistia.com/medias/0008hosa3e
Using the rules for naming acids, determine the compound name.
hydroiodic acid = hydrogen iodide
Determine the lowest common multiple of the two ions.
The lowest common multiple between 1 and 1 is 1.
Watch this video to see a teacher work through this example. https://adlc.wistia.com/medias/1bbj6hmocw
Read This
Please read pages 64 and 65 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on writing formulas and naming acidic
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.- Fill in the missing information in this chart.
Acid-Forming Compound Name of Acid-Forming Compound Classical Acid Name HBr HClO nitric acid hydrosulfuric acid
Acid-Forming Compound Name of Acid-Forming Compound Classical Acid Name HBr hydrogen bromide hydrobromic acid HClO hydrogen hypochlorite hypochlorous acid HNO3 hydrogen nitrate nitric acid H2S hydrogen sulfide hydrosulfuric acid
Conclusion
Is this shampoo pH balanced?
B6.7 Content label
By learning how to name and write formulas for compounds, you should be gaining the confidence that will allow you to start making informed choices about products based on their ingredient list. You should have a strong understanding of the rules used to name molecular compounds and acidic compounds, as well as being able to translate the names into chemical formulas. These skills will be required in Section 3.
Online Practice Quiz
This quiz will give you practice on naming and writing formulas for molecular compounds. If you get a question incorrect, feedback is provided to point out your error. The quiz can also be accessed at https://www.chem.purdue.edu/gchelp/nomenclature/covalent_2009.htmQuiz
2.4 Assignment
Unit 2 Assignment Lessons 5-8
It is now time to complete the Lesson 6 portion of 2.4 Assignment. This assignment has two parts.
- Part 1 Written Portion: Select the preferred document type from the options below. Download and save the assignment on your desktop (or documents folder).
PDF Document - Open and print this saved document.
- Record your responses in the appropriate textboxes for the Lesson 6 portion.
- When you have completed the assignment, scan it and save it on your desktop (or documents folder).
- Once you have completed the written portion of your assignment, click on the button below to go to the submission page.
Written Portion Submission Page - Part 2 Online Portion: It is now time to complete the Lesson 6 questions of the online portion of this assignment. Click on the button below to go to the online questions of this assignment.
Online Questions
This assignment is worth ___% of your final grade.