Atomic Foundations of Matter · Lesson 7 of 14
Naming Covalent Compounds
“A prefix system turns molecular formulae into systematic names while preserving familiar common names.”
• Apply prefixes used for naming binary covalent compounds. • Explain why the second element name ends in ide. • Apply the usual rule for omitting mono on the first element. • Handle vowel changes such as monoxide and pentoxide. • Recognise the special naming treatment of hydrogen first in the formula. • Recognise water and ammonia as common names.
A molecular formula tells us how many atoms of each element occur in a molecule. A systematic name can carry the same information in words. Prefixes such as mono-, di- and tri- indicate the number of atoms represented in the formula.
Naming Covalent Compounds
Prefix System
| Prefix | Number of atoms indicated |
|---|---|
| mono- | 1 |
| di- | 2 |
| tri- | 3 |
| tetra- | 4 |
| penta- | 5 |
| hexa- | 6 |
The first element keeps its regular element name. The second element is modified so that its name ends in -ide. The prefix tells how many atoms of that element are present.
Using Mono
The prefix mono- is usually omitted for the first element but is used for the second element when one atom is represented. This is why CO is carbon monoxide rather than monocarbon monoxide.
Vowel Changes
If a prefix ends with o or a and the following element name begins with a vowel, the final vowel of the prefix may be dropped for smoother pronunciation. Thus mono + oxide becomes monoxide and penta + oxide becomes pentoxide. When the prefix ends with i, the i is retained, giving forms such as dioxide and trioxide.
| Formula | Name |
|---|---|
| CO | Carbon monoxide |
| CO₂ | Carbon dioxide |
| CS₂ | Carbon disulfide |
| PCl₃ | Phosphorus trichloride |
| SF₆ | Sulfur hexafluoride |
| N₂O₄ | Dinitrogen tetroxide |
| N₂O₅ | Dinitrogen pentoxide |
Hydrogen as the First Element
When hydrogen is the first element in the formula, the chapter does not place a numerical prefix before hydrogen. H₂S is therefore named hydrogen sulfide rather than dihydrogen sulfide.
Common Names
Some familiar binary compounds are mainly known by common names. H₂O is called water, even though a systematic pattern might suggest hydrogen monoxide. NH₃ is known as ammonia rather than being routinely called nitrogen trihydride.
Problem
Name N₂O₅ using the prefix rules.
- 1.The first element is nitrogen and there are two nitrogen atoms, so use di-: dinitrogen.
- 2.The second element is oxygen, which becomes oxide.
- 3.There are five oxygen atoms, so use penta-.
- 4.Because penta ends in a and oxide begins with a vowel, the final a is dropped: pentoxide.
- 5.The name is dinitrogen pentoxide.
Quiz
Which statement is a key takeaway from this lesson?
Which additional statement is also a key takeaway from this lesson?
Which further statement is also a key takeaway from this lesson?
Which another statement is also a key takeaway from this lesson?
Which statement correctly applies to the lesson “Naming Covalent Compounds”?
Practice Problems
- Name CO₂, NO₂, SF₆ and PCl₃.
- Explain why CO is carbon monoxide rather than monocarbon monoxide.
- Explain the spelling change from pentaoxide to pentoxide.
- Why is H₂S named hydrogen sulfide in the rule used here?
- State the familiar common names of H₂O and NH₃.
Key Takeaways
• Binary covalent compounds can be named using prefixes that indicate the number of atoms present. • Prefixes such as mono-, di-, tri-, tetra- and penta- represent different numerical quantities. • The second element in the name commonly ends in -ide. • Some compounds continue to be known mainly by familiar common names.