4.10: Naming Binary Molecular Compounds

NAMING COVALENT COMPOUNDS

Naming binary (two-element) covalent compounds is similar to naming simple ionic compounds. The first element in the formula is simply listed using the name of the element. The second element is named by taking the stem of the element name and adding the suffix -ide. A system of numerical prefixes is used to specify the number of atoms in a molecule. Table $$\PageIndex{1}$$ lists these numerical prefixes. Normally, no prefix is added to the first element’s name if there is only one atom of the first element in a molecule. If the second element is oxygen, the trailing vowel is usually omitted from the end of a polysyllabic prefix but not a monosyllabic one (that is, we would say “monoxide” rather than “monooxide” and “trioxide” rather than “troxide”).

Table $$\PageIndex{1}$$: Numerical Prefixes for Naming Binary Covalent Compounds
Number of Atoms in Compound Prefix on the Name of the Element
1 mono-*
2 di-
3 tri-
4 tetra-
5 penta-
6 hexa-
7 hepta-
8 octa-
9 nona-
10 deca-
*This prefix is not used for the first element’s name.

Let us practice by naming the compound whose molecular formula is CCl4. The name begins with the name of the first element—carbon. The second element, chlorine, becomes chloride, and we attach the correct numerical prefix (“tetra-”) to indicate that the molecule contains four chlorine atoms. Putting these pieces together gives the name carbon tetrachloride for this compound.

Example $$\PageIndex{2}$$

Write the molecular formula for each compound.

1. chlorine trifluoride
2. phosphorus pentachloride
3. sulfur dioxide
4. dinitrogen pentoxide

Solution

If there is no numerical prefix on the first element’s name, we can assume that there is only one atom of that element in a molecule.

1. ClF3
2. PCl5
3. SO2
4. N2O5 (The di- prefix on nitrogen indicates that two nitrogen atoms are present.)

Exercise $$\PageIndex{2}$$

Write the molecular formula for each compound.

1. nitrogen dioxide
2. dioxygen difluoride
3. sulfur hexafluoride
4. selenium monoxide

a. NO2

O2F2

SF6

SeO

Because it is so unreactive, sulfur hexafluoride is used as a spark suppressant in electrical devices such as transformers.

Example $$\PageIndex{3}$$

Write the name for each compound.

1. BrF5
2. S2F2
3. CO

Solution

1. bromine pentafluoride
2. disulfur difluoride
3. carbon monoxide

Exercise $$\PageIndex{3}$$

Write the name for each compound.

1. CF4
2. SeCl2
3. SO3

carbon tetrafluoride

selenium dichloride

sulfur trioxide

For some simple covalent compounds, we use common names rather than systematic names. We have already encountered these compounds, but we list them here explicitly:

• H2O: water
• NH3: ammonia
• CH4: methane

Methane is the simplest organic compound. Organic compounds are compounds with carbon atoms and are named by a separate nomenclature system that we will introduce in Section 4.6 "Introduction to Organic Chemistry".

Concept Review Exercises

1. How do you recognize a covalent compound?
2. What are the rules for writing the molecular formula of a simple covalent compound?
3. What are the rules for naming a simple covalent compound?

1. A covalent compound is usually composed of two or more nonmetal elements.
2. It is just like an ionic compound except that the element further down and to the left on the periodic table is listed first and is named with the element name.
3. Name the first element first and then the second element by using the stem of the element name plus the suffix -ide. Use numerical prefixes if there is more than one atom of the first element; always use numerical prefixes for the number of atoms of the second element.

Key Takeaways

• The chemical formula of a simple covalent compound can be determined from its name.
• The name of a simple covalent compound can be determined from its chemical formula.

Exercises

1. Identify whether each compound has covalent bonds.

1. NaI
2. Na2CO3
3. N2O
4. SiO2
2. Identify whether each compound has covalent bonds.

1. C2H6
2. C6H5Cl
3. KC2H3O2
4. Ca(OH)2
3. Identify whether each compound has ionic bonds, covalent bonds, or both.

1. Na3PO4
2. K2O
3. COCl2
4. CoCl2
4. Identify whether each compound has ionic bonds, covalent bonds, or both.

1. FeCl3
2. Fe(NO3)3
3. (NH2)2CO
4. SO3
5. Which is the correct molecular formula—H4Si or SiH4? Explain.

6. Which is the correct molecular formula—SF6 or F6S? Explain.

7. Write the name for each covalent compound.

1. SiF4
2. NO2
3. CS2
4. P2O5
8. Write the name for each covalent compound.

1. CO
2. S2O3
3. BF3
4. GeS2
9. Write the formula for each covalent compound.

1. iodine trichloride
2. disulfur dibromide
3. arsenic trioxide
4. xenon hexafluoride
10. Write the formula for each covalent compound.

1. boron trichloride
2. carbon dioxide
3. tetraphosphorus decoxide
4. germanium dichloride
11. Write two covalent compounds that have common rather than systematic names.

12. What is the name of the simplest organic compound? What would its name be if it followed the nomenclature for binary covalent compounds?

1. no
2. yes
3. yes
4. yes

2.

1. yes
2. yes
3. yes
4. yes
1. both
2. ionic
3. covalent
4. ionic

4.

1. ionic
2. both
3. covalent
4. covalent
1. SiH4; except for water, hydrogen is almost never listed first in a covalent compound.

6. SF6; the less electronegative atom (S) is written first
1. silicon tetrafluoride
2. nitrogen dioxide
3. carbon disulfide
4. diphosphorus pentoxide

8.

1. carbon monoxide
2. disulfur trioxide
3. boron trifluoride
4. germanium disulfide
1. ICl3
2. S2Br2
3. AsO3
4. XeF6
10.
1. BCl3
2. CO2
3. P4O10
4. GeCl2
1. H2O and NH3 (water and ammonia) (answers will vary)
2. CH4; carbon tetrahydride